Roof system typesRoof systems and materials generally are divided into generic classifications: low slope and steep slope. Low slope roofing includes water impermeable, or weatherproof, types of roof membranes installed on slopes less than or equal to 3:12 (14 degrees). Steep slope roofing includes watershedding types of roof coverings installed on slopes exceeding 3:12 (14 degrees). 

Low-slope roofing 

There are five generic classifications of low slope roof membranes or systems. Click on a roof system for more information.

Built-up roof (BUR) membranes

Metal panel roof systems for low-slope applications Polymer-modified bitumen sheet membranes Single-ply membranes

o Thermoplastic membranes (e.g., PVC, TPO)o Thermoset membranes (e.g., EPDM)

Spray polyurethane foam-based (SPF) roof systems

Most low-slope roof membranes have three principal components:

Weatherproofing layer or layers — the weatherproofing component is the most important element because it keeps water from entering a roof assembly.

Reinforcement — reinforcement adds strength, puncture resistance and dimensional stability to a membrane.

Surfacing — surfacing is the component that protects the weatherproofing and reinforcement from sunlight and weather. Some surfacings provide other benefits such as increased fire resistance, improved traffic and hail resistance, and increased solar reflectivity.

With some roof membranes, a component may perform more than one function. 

Steep-slope roofing 

There are six generic classifications of steep slope roof coverings. Click on a roof covering type for more information.

Asphalt shingles Clay tile and concrete tile Metal roof systems for steep-slope applications Slate Wood shakes and wood shingles Synthetic

Steep-slope roof systems typically are composed of individual pieces or components installed in shingle fashion. Steep-slope roof assemblies typically consist of three primary parts:

Roof deck — a roof deck is the structural substrate and usually is a wood-based material such as plywood or oriented strand board (OSB).

Underlayment — underlayment provides temporary protection until a roof covering is installed and provides a secondary weatherproofing barrier. Sometimes underlayment is referred to as "felt" or "paper."

Roof covering — the roof covering is the external watershedding material.

Built-up roof (BUR) membranesClick on a topic for more information.

Material and system description Material standards Warranties

Material and system description 

Built up roof membranes, referred to by the acronym BUR, have been in use in the U.S. for more than 100 years. These roof systems are commonly referred to as "tar and gravel" roofs. BUR systems generally are composed of alternating layers of bitumen and reinforcing fabrics that create a finished membrane. The number of plies in a cross section is the number of plies on a roof: The term "four plies" denotes a four ply roof membrane construction. Sometimes, a base sheet, used as the bottommost ply, is mechanically fastened. Built up roofs generally are considered to be fully adhered if applied directly to roof decks or insulation. 

The reinforcing fabrics also are called roofing felts or ply sheets. Roofing felts are reinforced with either glass-fiber mats or organic mats. Felts are produced in a standard width of 36 inches and metric width of about one meter. 

The bitumen typically used in BUR roof systems is asphalt, coal tar or cold-applied adhesive. The asphalt or coal tar is heated in a kettle or tanker and then applied by mop or mechanical spreader. Asphalt is a petroleum product refined from crude oil; coal tar is derived from the distillation of coal. Cold-applied adhesives typically are solvent-based asphalts that don't have to be heated in a kettle or tanker. 

Surfacings for built up roof systems include aggregate (such as gravel, slag or mineral granules), glass-fiber or mineral surfaced cap sheets, hot asphalt mopped over the entire surface, aluminum coatings or elastomeric coatings. 

Photo of an aggregate-surfaced BUR 

A roof system composed of a built up roof membrane with two or three plies and a polymer-modified

bitumen membrane cap sheet is commonly referred to as "hybrid" system. NRCA considers this type to be a polymer modified bitumen membrane system. 

Material standards 

NRCA does not make any recommendations about which product or manufacturer to use; however, NRCA does recommend that bitumens, felts and mats used for BUR systems meet standards established by ASTM International. 

The following commonly referenced ASTM International standards apply to materials used in the construction of BUR systems:

ASTM D312, "Standard Specification for Asphalt Used in Roofing" ASTM D450, "Standard Specification for Coal Tar Pitch Used in Roofing, Dampproofing and

Waterproofing" ASTM D1863, "Standard Specification for Mineral Aggregate Used on Built Up Roofs" ASTM D2178, "Standard Specification for Asphalt Glass Felt Used in Roofing and

Waterproofing" ASTM D2824, "Standard Specification for Aluminum Pigmented Asphalt Roof Coatings,

Nonfibered, Asbestos Fibered, and Fibered without Asbestos" ASTM D3909, "Standard Specification for Asphalt Roll Roofing (Glass Felt) Surfaced With

Mineral Granules" ASTM D4601, "Standard Specification for Asphalt Coated Glass Fiber Base Sheet Used in

Roofing" ASTM D4990, "Standard Specification for Coal Tar Glass Felt Used in Roofing and

Waterproofing"

Warranties 

When purchasing a new roof system, there will be two warranties to consider. First, there will be the manufacturer's warranty. In general, these warranties cover defects in the manufacture of the roof membrane. Please read NRCA's consumer advisory bulletin addressing roofing warranties for more information. Once the project is complete, be sure the contractor provides you with a certificate for your records. 

Second, the roofing contractor will provide you with a warranty covering his workmanship. Typically, this will cover installation and related issues. The warranty should contain what items are covered and what will void them. Many contractors offer one year or two years of coverage; however, there is no industry standard. 

Metal panel roof systems for low-slope applicationsClick on a topic for more information.

Metal panel roof system description Roof deck Underlayment Vapor retarders, insulation and ventilation Material standards Warranties

Metal panel roof system description 

There is only one category of metal roof systems used in low-slope applications — structural metal panel. Structural metal panel roof systems can be used for low slope roofs because of their hydrostatic, or water barrier, characteristics. It is important to note structural metal panel roof systems

can be used for steep slope roof assemblies, too. 

Most structural metal panel roof systems are designed to resist the passage of water at laps and other joints, as sealant or anti capillary designs can be used in the seams. Structural metal panel roof systems possess strength characteristics that allow them to span supporting members. 

Example of a structural metal panel roof system 

Roof deck 

Structural metal panel roof systems are installed over a large variety of substrates. There are two general categories of substrates: one is continuous or closely spaced decking that provides solid support for the metal roof panel, and the other is composed of spaced structural supports (such as purlins) where the metal panels must span between supports. Most structural metal panels are used over spaced structural supports without being supported by a solid roof deck. 

Underlayment 

Underlayment (or "felt paper" as it is frequently called) is installed over the roof deck before the application of a metal panel roof system. An underlayment performs two primary functions: it provides temporary weather protection until the metal panel roof is installed, and it provides a secondary weatherproofing barrier if moisture infiltrates the metal roof panels. 

Underlayments typically are not used with structural metal panel roof systems when intermittent supports are used to carry the roof systems. However, if there is a continuous or closely spaced roof deck, NRCA recommends an underlayment be installed. 

Asphalt saturated, nonperforated organic felts are among the most common underlayments used for metal roof systems; they commonly are designated as Type 15 and Type 30 or referred to as No. 15 and No. 30, which are reflective of a once used pound per square weight designation. The terms Type I and Type II now are used within the industry in lieu of No. 15 or No. 30, respectively. 

If an underlayment is to be installed, NRCA recommends a minimum of one layer of No. 30 asphalt-saturated felt applied horizontally in shingle fashion on roof decks having a slope of 4:12 (18 degrees) or more. For roof decks having slopes of 3:12 (14 degrees) up to 4:12 (18 degrees), a minimum of two layers of No. 30 asphalt-saturated underlayment should be applied horizontally in shingle fashion. 

In locations where the average temperature for January is 30º F or less, NRCA suggests installation of an ice-dam protection membrane. An ice-dam protection membrane generally is a self-adhering polymer-modified bitumen membrane. 

An ice dam protection membrane should be applied starting at a roof's eaves and extending upslope a minimum of 24 inches from the exterior wall line of a building. For slopes less than 4:12 (18 degrees), a minimum of 36 inches is recommended. See Figure 1. 

Figure 1 - Example of ice damming 

NRCA also recommends a slip sheet be installed over the underlayment for metal panel roof systems. A slip sheet is a layer of smooth building paper, such as rosin-sized or unsaturated building paper. Its purpose is to protect the underlayment from damage, as the panels can adhere to and tear the underlayment. 

Vapor retarders, insulation and ventilation 

Condensation should be expected to develop on the undersides of metal roof panels. Careful consideration should be paid to vapor retarder, insulation and ventilation issues. Because every building is in some way unique, building owners and designers may need to consult moisture-control specialists. 

Material standards 

NRCA does not make any recommendations about which product or manufacturer to use; however, NRCA does recommend that metal roof systems meet standards established by ASTM International.

ASTM E1514, "Standard Specification for Structural Standing Seam Steel Roof Panel Systems" ASTM E1637, "Standard Specification for Structural Standing Seam Aluminum Roof Panel

Systems"

Warranties 

When purchasing a new roof system, there will be two warranties to consider. First, there will be the manufacturer's warranty. In general, these warranties cover defects in the manufacture of the roof covering. In the case of metal panel roofs, manufacturers tend to cover only the metal finishes or coatings. Please read NRCA's consumer advisory bulletin addressing roofing warranties for more information. Once the project is complete, be sure the contractor provides you with a certificate for your records. 

Second, the roofing contractor will provide you with a warranty covering his workmanship. Typically, this will cover installation and related issues. The warranty should contain what items are covered and what will void them. Many contractors offer one year or two years of coverage; however, there is no industry standard. 

Polymer-modified bitumen sheet membranesClick on a topic for more information.

Material and system description Material standards Warranties

Material and system description 

Polymer-modified bitumen or modified bitumen (MB) sheet membranes were developed in Europe in the early 1960s and have been in use in the U.S. since the mid 1970s. Polymer-modified roof membranes are composed of reinforcing fabrics that serve as carriers for the hot polymer-modified bitumen as it is manufactured into a roll material. MB roof system membranes are composed of multiple layers, much like BUR membranes. MB roof systems typically are installed as a two-ply system and almost always are fully adhered. 

There are two types of MB roofing membranes:

SBS polymer-modified bitumen membranes commonly are installed in hot moppings of asphalt (similar to BUR systems) or cold adhesive. Some SBS modified membranes are self adhering; that is, they contain an adhesive backing.

APP polymer-modified bitumen membranes typically are heat-welded or torch-applied. Consumers should be cautioned that NRCA does not recommend torch-applying a modified bitumen membrane sheet directly to a wood deck.

Generally, APP modifiers impart a "plasticized" quality to asphalt, and SBS modifiers impart a "rubberized" quality to asphalt. MB membranes and EPDM, a thermoset membrane, often are confused by consumers because of colloquialisms used by roofing contractors. MB and EPDM membranes are sometimes called "rubber roofs." 

Surfacings for MB membranes include aggregate surfacing, mineral surfacing, metal foil-laminate surfacing and smooth liquid-applied surfacing. 

Photo of a mineral-surfaced MB roof system 

A roof system composed of a built up roof membrane with 2 plies or 3 plies and a polymer-modified bitumen membrane cap sheet is commonly referred to as "hybrid" system. NRCA considers this type to be a polymer modified bitumen membrane system. 

Material standards 

NRCA does not make any recommendations about which MB products or manufacturer to use; however, NRCA does recommend that MB meet standards established by ASTM International. 

APP polymer modified bitumen products:

ASTM D6222, "Standard Specification for Atactic Polypropylene (APP) Modified Bituminous Sheet Materials Using Polyester Reinforcements"

ASTM D6223, "Standard Specification for Atactic Polypropylene (APP) Modified Bituminous Sheet Materials Using a Combination of Polyester and Glass Fiber Reinforcements"

SBS polymer modified bitumen products:

ASTM D6162, "Standard Specification for Styrene Butadiene Styrene (SBS) Modified Bituminous Sheet Materials Using a Combination of Polyester and Glass Fiber Reinforcements"

ASTM D6163, "Standard Specification for Styrene Butadiene Styrene (SBS) Modified Bituminous Sheet Materials Using Glass Fiber Reinforcements"

ASTM D6164, "Standard Specification for Styrene Butadiene Styrene (SBS) Modified Bituminous Sheet Materials Using Polyester Reinforcements"

ASTM D6298, "Standard Specification for Fiberglass Reinforced Styrene Butadiene Styrene (SBS) Modified Bitumen Sheets with a Factory Applied Metal Surface"

Warranties 

When purchasing a new roof system, there will be two warranties to consider. First, there will be the manufacturer's warranty. In general, these warranties cover defects in the manufacture of the roof membrane. Please read NRCA's consumer advisory bulletin addressing roofing warranties for more information. Once the project is complete, be sure the contractor provides you with a certificate for your records. 

Second, the roofing contractor will provide you with a warranty covering his workmanship. Typically, this will cover installation and related issues. The warranty should contain what items are covered and what will void them. Many contractors offer one year or two years of coverage; however, there is no industry standard. Single-ply membranesSingle-ply membranes are factory-manufactured sheet membranes. They generally are categorized as either thermoplastic or thermoset. Thermoplastic materials can be repeatedly softened when heated and hardened when cooled. Thermoset materials solidify, or "set," irreversibly after heating. Single ply membranes commonly are referred to by their chemical acronyms, such as ethylene propylene diene terpolymer (EPDM). 

Single-ply sheet membranes are produced using one of three manufacturing processes: calendering, extruding or spread coating. The membranes may contain reinforcement layers. Common reinforcements for single ply membranes include polyester fabrics or scrims, glass fiber, or a felt or fleece backing. 

A finished sheet's thickness typically is referred to as mil thickness; 1 mil equals 0.001 inch. Common mil thicknesses for these sheet membranes range from 30 mils to 60 mils. 

Single-ply membranes can be installed fully adhered, mechanically attached or held down with ballast. Most single-ply roof systems do not receive surfacings. 

In many instances, a combination of attachment methods are used to secure a roof system. For instance, an insulation may be mechanically attached to the substrate with the roof membrane fully adhered to the insulation. 

Spray polyurethane foam-based (SPF) roof systemsClick on a topic for more information.

Material and system description Material standards Warranties

Material and system description 

SPF-based roof systems are constructed by mixing and spraying a two-component liquid that forms the base of an adhered roof system. SPF can be installed in various thicknesses to provide slope to drain or meet a specified thermal resistance (R-value). A protective surfacing is then applied to the foam to provide protection from the elements. 

The first component of an SPF-based roof system is rigid, closed cell, spray polyurethane foam insulation. The foam is composed of two components: isocyanate and polyol. Transfer pumps are used to get the components to a proportioning unit that properly meters the two at a one to one ratio and heats and pumps them through dual hoses. The components are mixed at the spray gun, which is used to apply them to a substrate. 

The second component, the protective surfacing, typically is a spray applied elastomeric coating, though hand and power rollers can be used. The protective surfacing also can be a membrane, such as a fleece backed thermoset single ply membrane. The purpose of the surfacing is to provide weatherproofing, protect the foam from UV exposure, provide protection from mechanical damage and assist with the fire-resistant characteristic of the roof system. 

The generic types of coatings used on SPF-based roof systems include:

Acrylic Butyl Rubber

Hypalon™ Silicone Aromatic Polyurethane Elastomer Aliphatic Polyurethane Elastomer Modified Polyurethane Elastomer

Mineral granules or sand may be broadcast into a coating to provide increased surface durability and aesthetic value. 

The chemicals used to produce polyurethane foam compounds are manufactured or blended to perform in various temperature ranges. Manufacturers of spray polyurethane foam based roof systems offer materials in different reaction profiles. Profile change is created by altering the chemical ingredients, usually with a catalyst, to compensate for ambient temperature changes. Conditions such as wind speed, sunlight, surface moisture, humidity and temperature of the substrate can affect the reaction of the polyurethane foam. 

Example of an SPF-based roof system 

Material standards 

NRCA does not make any recommendations about which SPF products or manufacturer to use; however, NRCA does recommend that SPF products meet standards established by ASTM International.

ASTM C1029, "Specification for Spray Applied Rigid Cellular Polyurethane Thermal Insulation" ASTM D6083, "Specification for Liquid Applied Acrylic Coating Used in Roofing" ASTM D6694, "Standard Specification for Liquid Applied Silicone Coating Used in Spray

Polyurethane Foam Roofing Systems" ASTM D6947, "Standard Specification for Liquid Applied Moisture Cured Polyurethane Coating

Used in Spray Polyurethane Foam Roofing System" ASTM D7425, "Standard Specification for Spray Polyurethane Foam Used for Roofing

Applications"

Warranties 

When purchasing a new roof system, there will be two warranties to consider. First, there will be the manufacturer's warranty. In general, these warranties cover defects in the manufacture of the roof membrane. Please read NRCA's consumer advisory bulletin addressing roofing warranties for

more information. Once the project is complete, be sure the contractor provides you with a certificate for your records. 

Second, the roofing contractor will provide you with a warranty covering his workmanship. Typically, this will cover installation and related issues. The warranty should contain what items are covered and what will void them. Many contractors offer one year or two years of coverage; however, there is no industry standard. 

Asphalt shinglesClick on a topic for more information.

Material description Roof deck Underlayment Fasteners Flashings Material standards Warranties

Material description 

Asphalt shingles are composed of: a base material, either organic felt or glass-fiber mat, that provides support for the weather-resistant components and gives a shingle strength; asphalt and fillers; and surfacing material, generally in the form of mineral granules, that provides protection from impact and UV degradation and improves fire resistance. 

The most common form of asphalt shingles are strip shingles. They are rectangular, the most prevalent size being 12 inches wide by 36 inches long. Metric shingles are 13-1/4 inches by 39-3/8 inches. Strip shingles most frequently have three tabs that are exposed along the length of the shingle for visual effect and are called 3 tab strip shingles. 

Shingles may be produced in a single layer or two or more layers. The latter generally are known aslaminated strip shingles, or architectural shingles, and they have a three dimensional appearance. 

Both 3-tab asphalt shingles and laminated asphalt shingles contain a strip of factory applied adhesive that is activated by the sun's heat after installation and seals each shingle to the next course. The seal strip also provides much of a shingle's resistance to wind uplift. Shingles with factory-applied adhesive have a strip of clear polyester film applied to each shingle to prevent the sealing strips from bonding the shingles together when packaged. When the shingles are installed, the self-sealing strips will not align with the plastic film strips and will bond to adjacent shingles. For this reason, the plastic film strips do not have to be removed. 

Roof deck 

NRCA recommends that asphalt shingles be applied over continuous or closely spaced wood plank decking or wood decking. The most common materials used for roof decks are plywood or oriented strand board (OSB). When plywood is used, NRCA recommends the use of a minimum 15/32 thick or 1/2 inch nominal exterior-grade plywood for 16-inch rafter spacings and 5/8 inch nominal thickness for

24-inch rafter spacings. For OSB, NRCA recommends using a minimum 15/32 inch thick or 1/2 inch nominal exterior-grade OSB for 16-inch rafter spacings. 

Caution should be exercised when roof decks are constructed out of the following materials:

Oriented strand board (OSB): NRCA is concerned with potential fastener-holding problems and dimensional stability because of the effects of moisture where OSB and other nonveneer products are used as roof decking.

Preservative-treated wood: Many roofing material manufacturers recommend wood roof decks be constructed with wood that has been treated with a nonoil preservative pressure treatment or with nontreated air- or kiln-dried lumber.

Fire-retardant-treated wood: Because of the deterioration of some fire-retardant-treated wood panels caused by premature fire retardant activation caused by heat history in service, the use of fire-retardant-treated wood panel decks should be carefully considered.

Underlayment 

Underlayment (or "felt paper" as it is frequently called) is installed over the roof deck before the application of asphalt shingles. An underlayment performs two primary functions: it provides temporary weather protection until the asphalt shingles are installed, and it provides a secondary weatherproofing barrier if moisture infiltrates the asphalt shingles. 

It is not uncommon for it to rain after the contractor installs underlayment but before he installs the asphalt shingles. The underlayment gets wet and becomes wrinkled. If the wrinkling isn't severe enough to affect the shingle installation (i.e. the wrinkling won't telegraph through the shingles and they won't appear buckled or wavy once installed), the underlayment probably can remain in place. The effects of wrinkling also will be minimized by using heavier weight shingles. 

Asphalt saturated, nonperforated organic felts are among the most common underlayments used for shingles; they commonly are designated as Type 15 and Type 30 or referred to as No. 15 and No. 30, which are reflective of a once used pound per square weight designation. The terms Type I and Type II now are used within the industry in lieu of No. 15 or No. 30, respectively. 

Another type of underlayment is a synthetic underlayment. It is characterized as being lightweight, water-resistant and less likely to wrinkle; having high tear strength; and being easy to walk on—even when wet. Theoretically, the product may be left exposed to the elements for extended periods of time. Although synthetic underlayments and their purported attributes seem appealing, there are significant issues to consider before using them. To date, there are no applicable ASTM standards for these products. Many synthetic underlayments do not meet current building code requirements, so manufacturers need to obtain a code evaluation report for code compliance. And use of these underlayments may void some manufacturers' material warranties for certain roof coverings (such as with asphalt shingles). 

NRCA recommends a minimum of one layer of No. 15 asphalt-saturated felt applied horizontally in shingle fashion on roof decks having a slope of 4:12 (18 degrees) or more. For roof decks having slopes of 3:12 (14 degrees) up to 4:12 (18 degrees), a minimum of two layers of No. 15 asphalt-saturated underlayment should be applied horizontally in shingle fashion. It should be noted that one layer of No. 30 asphalt-saturated underlayment is not the same as two layers of No. 15. 

In locations where the average temperature for January is 30º F or less, NRCA suggests installation of an ice-dam protection membrane. An ice-dam protection membrane generally is a self-adhering polymer-modified bitumen membrane. 

An ice dam protection membrane should be applied starting at a roof's eaves and extending upslope a minimum of 24 inches from the exterior wall line of a building. For slopes less than 4:12 (18 degrees), NRCA recommends a minimum of 36 inches. See Figure 1. 

Figure 1 - Example of ice damming 

Fasteners 

NRCA recommends roofing nails be 11-gauge or 12-gauge galvanized steel or the equivalent corrosion-resistant roofing nails. Nail heads should be low-profile, smooth and flat. Shanks should be barbed or otherwise deformed for added pull-out strength. Nails should be long enough to penetrate through all layers of roofing materials and extend through the underside of the roof deck or penetrate at least ¾ inch into wood plank or board decks. 

Flashings 

Flashings for asphalt shingle roof systems fall into four categories: perimeter edge metal, penetrations, valleys and vertical surfaces. See Figure 2.

Perimeter edge metal: Depending on the severity of climate, anticipated rainfall and freeze-thaw cycling, the use of perimeter edge metal should be considered.

Penetrations: Plumbing soil stacks, exhaust vents and pipes are flashed into asphalt shingle roof systems with some type of flat flange that extends around a penetration and is installed under shingles on the upslope of a flange.

Valleys: Valleys that are called "open valleys" are typically lined with sheet metal. Vertical surfaces: When a roof system abuts a vertical surface, there are four types of flashing

commonly used: apron, step, cricket (or backer) and counterflashing.

Figure 2 - Basic sheet metal flashing components 

Apron, step and cricket flashings require some form of counterflashing to cover and protect the top edges from water intrusion. In many cases, the wall covering or cladding material acts as counterflashing. When this does not occur, a metal counterflashing mounted to the vertical surface should be installed. See Figures 3, 4 and 5 for examples. 

Figure 3 - Example of metal counterflashing inset in masonry mortar joint 

Figure 4 - Example of through-wall metal counterflashing embedded in masonry mortar joint 

Figure 5 - Example of surface-mount metal counterflashing 

Material standards 

NRCA does not make any recommendations about which shingle product or manufacturer to use; however, NRCA does recommend asphalt shingles meet standards established by ASTM International.

Organic asphalt shingles should meet ASTM D225, "Standard Specification for Asphalt Shingles (Organic Felt) Surfaced With Mineral Granules."

Fiberglass asphalt shingles should meet ASTM D3462, "Standard Specification for Asphalt Shingles Made from Glass Felt and Surfaced with Mineral Granules."

Warranties 

When purchasing a new roof system, there will be two warranties to consider. First, there will be the manufacturer's warranty. In general, these warranties cover defects in the manufacture of the roof covering. The period of coverage can range from 20 years to a lifetime. Please read NRCA's consumer advisory bulletin addressing roofing warranties for more information. Once the project is complete, be sure the contractor provides you with a certificate for your records. 

Second, the roofing contractor will provide you with a warranty covering his workmanship. Typically, this will cover installation and related issues. The warranty should contain what items are covered and what will void them. Many contractors offer one year or two years of coverage; however, there is no industry standard. 

SlateClick on a topic for more information.

Material description Roof deck Underlayment Fasteners Flashings Material standards Warranties

Material description 

Roofing slate is a dense, durable, naturally occurring material that is essentially nonabsorbent. Two properties of slate are cleavage and fracture. It has natural cleavage, which permits it to be easily split in one direction. Fracture, usually occurring at right angles to the cleavage, is called the grain. Roofing slate commonly is split so the length of the slate runs in the direction of the grain. The surface texture of slate after being split for commercial use derives from the characteristics of the rock from which it was quarried. Some slate splits to a smooth, practically even surface, while other yields a surface that is rough and uneven. 

The color of slate is determined by its chemical and mineral composition. Because these factors differ in various regions, roofing slate can be obtained in a variety of colors. In addition, exposure to weather causes slate to change color. The degree of change varies depending on the slate. Slate exhibiting minimal color change is known as "permanent" or "unfading" slate. Slate that shows a more marked color change is known as "weathering" slate. Between unfading slate and weathering is "semi weathering" slate. 

Example of a slate roof system 

There are several classifications for slate roof systems. The first is standard slate, which refers to slate that generally is from 3/16 inch (5 mm) to 1/4 inch (6 mm) thick with uniform length. The category

"standard smooth" refers to standard slate that has a relatively smooth surface, in comparison with "standard rough" or just "rough". Rough slate has a rougher texture and generally is available in thicker pieces. Finally, there is "graduated/textural slate," which is designed with varying lengths and thicknesses and generally is rougher than standard slates. 

Roof deck 

NRCA recommends slate be applied over continuous or closely spaced wood decking. When plywood is used, NRCA recommends the use of a minimum 5/8 thick nominal exterior-grade plywood. 

Caution should be exercised when roof decks are constructed out of the following materials:

Oriented strand board (OSB): NRCA is concerned with potential fastener-holding problems and dimensional stability because of the effects of moisture where OSB and other nonveneer products are used as roof decking.

Preservative-treated wood: Many roofing material manufacturers recommend wood roof decks be constructed with wood that has been treated with a nonoil preservative pressure treatment or with nontreated air- or kiln-dried lumber.

Fire-retardant-treated wood: Because of the deterioration of some fire-retardant-treated wood panels caused by premature fire retardant activation caused by heat history in service, the use of fire-retardant-treated wood panel decks should be carefully considered.

Underlayment 

Underlayment (or "felt paper" as it is frequently called) is installed over the roof deck before the application of slate. An underlayment performs two primary functions: it provides temporary weather protection until the slate is installed, and it provides a secondary weatherproofing barrier if moisture infiltrates the slate roof covering. Many slate roofs have outlived the underlayment felts over which they were installed. Therefore, an underlayment's service life should be comparable to the design service life of the slate roof covering. 

Asphalt saturated, nonperforated organic felts are among the most common underlayments; they commonly are designated as Type 15 and Type 30 or referred to as No. 15 and No. 30, which are reflective of a once used pound per square weight designation. The terms Type I and Type II now are used within the industry in lieu of No. 15 or No. 30, respectively. 

Another type of underlayment is a synthetic underlayment. It is characterized as being lightweight, water-resistant and less likely to wrinkle; having high tear strength; and being easy to walk on—even when wet. Theoretically, the product may be left exposed to the elements for extended periods of time. Although synthetic underlayments and their purported attributes seem appealing, there are significant issues to consider before using them. To date, there are no applicable ASTM standards for these products. Many synthetic underlayments do not meet current building code requirements, so manufacturers need to obtain a code evaluation report for code compliance. 

NRCA recommends a minimum of one layer of No. 30 asphalt-saturated felt applied horizontally in shingle fashion on roof decks having a slope of 8:12 (34 degrees) or more. Where weather conditions are severe and hard wind-driven rains are common, NRCA recommends a minimum of two layers of No. 30 asphalt-saturated felt applied horizontally in shingle fashion. For roof slopes of 4:12 (18 degrees) to 8:12 (34 degrees), a minimum of two layers of No. 30 asphalt-saturated felt are recommended as long as standard-size slate with 3-inch minimum headlap is used. NRCA does not recommend installing slate roof systems on roof slopes less than 4:12 (18 degrees). 

In locations where the average temperature for January is 30º F or less, NRCA suggests installation of an ice-dam protection membrane. An ice-dam protection membrane generally is a self-adhering polymer-modified bitumen membrane. 

An ice dam protection membrane should be applied starting at a roof's eaves and extending upslope a

minimum of 24 inches from the exterior wall line of a building. For slopes less than 4:12 (18 degrees), a minimum of 36 inches is recommended. See Figure 1. 

Figure 1 - Example of ice damming 

Fasteners 

NRCA suggests the use of copper slating nails for slate. NRCA does not recommend unprotected black-iron and electroplated nails. NRCA recommends nails for standard-sized slate are sharp-point, 3/8 inch large flat head, copper-wire slating nails. Nails should be long enough to penetrate through all layers of roofing materials and extend through the underside of the roof deck or penetrate at least ¾ inch into wood plank or board decks. All roofing slate should have a minimum of two nails, however, slate subject to high-wind conditions and/or thicker slate should be fastened with four nails. 

Flashings 

Flashings for slate roofs fall into four categories: perimeter edge metal, penetrations, valleys and vertical surfaces. See Figure 2.

Perimeter edge metal: Depending on the severity of climate, anticipated rainfall and freeze-thaw cycling, the use of perimeter edge metal should be considered.

Penetrations: Plumbing soil stacks, exhaust vents and pipes are flashed into slate roof systems with some type of flat flange that extends around a penetration and is installed under shingles on the upslope of a flange.

Valleys: Valleys that are called "open valleys" are typically lined with sheet metal. Vertical surfaces: When a roof system abuts a vertical surface, there are four types of flashing

commonly used: apron, step, cricket (or backer) and counterflashing.

Figure 2 - Basic sheet metal flashing components 

Apron, step and cricket flashings require some form of counterflashing to cover and protect the top edges from water intrusion. In many cases, the wall covering or cladding material acts as counterflashing. When this does not occur, a metal counterflashing mounted to the vertical surface should be installed. See Figures 3, 4 and 5 for examples. 

Figure 3 - Example of through-wall metal counterflashing inset in masonry mortar joint 

Figure 4 - Example of metal counterflashing embedded in masonry mortar joint 

Figure 5 - Example of surface-mount metal counterflashing 

Material standards 

NRCA does not make any recommendations about which tile or manufacturer to use; however, NRCA does recommend clay tile roof coverings meet standards established by ASTM International.

ASTM C406, "Standard Specification for Roofing Slate"

Warranties 

When purchasing a new roof system, there will be two warranties to consider. First, there will be the manufacturer's warranty. In general, these warranties cover defects in the manufacture of the roof covering or in the case of slate, failure in the slate itself. Please read NRCA's consumer advisory bulletin addressing roofing warranties for more information. Once the project is complete, be sure the contractor provides you with a certificate for your records. 

Second, the roofing contractor will provide you with a warranty covering his workmanship. Typically, this will cover installation and related issues. The warranty should contain what items are covered and what will void them. Many contractors offer one year or two years of coverage; however, there is no industry standard. 

Clay tile and concrete tileClick on a topic for more information.

Material description Roof deck Underlayment Securement methods Flashings Material standards Warranties

Material description 

Clay tile is produced by baking molded clay into tile. The density of the clay is determined by the length of time and temperature at which it is heated. Tiles may be glazed and also may have surface texture treatments applied. As a result, there are a wide variety of tile profiles, styles, finishes and colors available. In addition, there may be separate accessory tiles—matched to each field tile design—of various shapes designed for use on ridges, hips, hip intersections and gable ends. Installation methods depend on the nature of the tile being installed; that is, whether it is two piece, one piece, interlocking or flat. 

Example of a clay tile roof 

Concrete tiles are made of portland cement, sand and water in varying proportions. The material is mixed and extruded on molds under high pressure. The exposed surface of a tile may be finished with cementitious material colored with synthetic oxide additives. The tiles are cured to reach the required strength. They generally have lugs on their undersides for anchoring to batten strips. There are additional waterlocks or interlocking ribs on the longitudinal edges that impede movement and prevent water infiltration. 

As with clay tile, there are a wide variety of profiles, styles, finishes and colors available. Color may be added to the surface of a tile or dispersed throughout (color through). Special texture may be added in

surface treatment. Each type of tile roof system may make use of separate ridge, hip, hip intersection, gable end and finial accessory tiles of various shapes in addition to field tiles. 

Roof deck 

NRCA recommends tile roof systems be applied over continuous wood decking. When plywood is used, NRCA recommends the use of a minimum 5/8 thick nominal exterior-grade plywood. 

Caution should be exercised when roof decks are constructed out of the following materials:

Oriented strand board (OSB): NRCA is concerned with potential fastener-holding problems and dimensional stability because of the effects of moisture where OSB and other nonveneer products are used as roof decking.

Preservative-treated wood: Many roofing material manufacturers recommend wood roof decks be constructed with wood that has been treated with a nonoil preservative pressure treatment or with nontreated air- or kiln-dried lumber.

Fire-retardant-treated wood: Because of the deterioration of some fire-retardant-treated wood panels caused by premature fire retardant activation caused by heat history in service, the use of fire-retardant-treated wood panel decks should be carefully considered.

For some types of tile roofs, batten and counter-batten systems are used to hang tile that has head lugs. Battens are spaced according to the length and exposure of the specified tile. Battens should be fastened to the deck with 8d common, galvanized, corrosion-resistant nails at approximately 12 inches on center. 

Underlayment 

Underlayment (or "felt paper" as it is frequently called) is installed over the roof deck before the application of tile. An underlayment performs two primary functions: it provides temporary weather protection until the tile is installed, and it provides a secondary weatherproofing barrier if moisture infiltrates the tile roof covering. Many tile roofs have outlived the underlayment felts over which they were installed. Therefore, an underlayment's service life should be comparable to the design service life of the tile roof covering. 

Asphalt saturated, nonperforated organic felts are among the most common underlayments; they commonly are designated as Type 15 and Type 30 or referred to as No. 15 and No. 30, which are reflective of a once used pound per square weight designation. The terms Type I and Type II now are used within the industry in lieu of No. 15 or No. 30, respectively. 

Another type of underlayment is a synthetic underlayment. It is characterized as being lightweight, water-resistant and less likely to wrinkle; having high tear strength; and being easy to walk on—even when wet. Theoretically, the product may be left exposed to the elements for extended periods of time. Although synthetic underlayments and their purported attributes seem appealing, there are significant issues to consider before using them. To date, there are no applicable ASTM standards for these products. Many synthetic underlayments do not meet current building code requirements, so manufacturers need to obtain a code evaluation report for code compliance. 

NRCA recommends a minimum of one layer of No. 30 asphalt-saturated felt applied horizontally in shingle fashion on roof decks having a slope of 10:12 (40 degrees) or more. For roof decks having slopes of 4:12 (18 degrees) up to 10:12 (40 degrees), a minimum of two layers of No. 30 asphalt-saturated underlayment should be applied horizontally in shingle fashion. NRCA does not recommend installing tile roof systems on roof slopes less than 4:12 (18 degrees). 

In locations where the average temperature for January is 30º F or less, NRCA suggests installation of an ice-dam protection membrane. An ice-dam protection membrane generally is a self-adhering polymer-modified bitumen membrane. 

An ice dam protection membrane should be applied starting at a roof's eaves and extending upslope a minimum of 24 inches from the exterior wall line of a building. For slopes less than 4:12 (18 degrees), NRCA recommends a minimum of 36 inches. See Figure 1. 

Figure 1 - Example of ice damming 

Securement methods 

Many different types and combinations of securement methods are used for the various types of roof tile. To select a method of securement, many conditions need to be considered: wind, deck type, seismic considerations, slope, building codes, local practices and manufacturer recommendations. Fasteners should be made of noncorrosive materials that will remain serviceable in the intended environment for the roof's design life.

Nails—Nailing is the most common method. NRCA recommends that roofing nails be 11- gauge or 12-gauge galvanized steel or the equivalent corrosion-resistant roofing nails. Nail heads should be low-profile, smooth and flat. Shanks should be barbed or otherwise deformed for added pull-out strength. Nails should be long enough to penetrate through all layers of roofing materials and extend through the underside of the roof deck or penetrate at least 3/4 inch into wood plank or board decks.

Wire tie and strapping systems—Hanging tile with wire is used on non-nailable decks, insulated decks or where fastening through metal flashings needs to be avoided.

Clips—Nose or butt clips sometimes are used in conjunction with other attachment methods in high-wind or seismic areas.

Lug-hung-tile—Many types of tile have formed undersides near their heads that may be hung over a batten. Lug hanging tile usually is used in combination with other securement methods.

Bedding Tile—Laying tile in a bed of mortar of foam adhesive is common in some areas of North America where freeze/thaw conditions are not encountered. Bedding often is used in combination with other securement methods.

Flashings 

Flashings for tile roofs fall into four categories: perimeter edge metal, penetrations, valleys and vertical surfaces. See Figure 2.

Perimeter edge metal-depending on the severity of climate, anticipated rainfall and freeze-thaw cycling, the use of perimeter edge metal should be considered.

Penetrations-plumbing soil stacks, exhaust vents and pipes are flashed into tile roof systems with some type of flat flange that extends around a penetration and is installed under shingles on the upslope of a flange.

Valleys-valleys that are called "open valleys" are typically lined with sheet metal.

Vertical surfaces-when a roof system abuts a vertical surface, there are four types of flashing commonly used: apron, step, cricket (or backer) and counterflashing.

Figure 2 - Basic sheet metal flashing components 

Apron, step and cricket flashings require some form of counterflashing to cover and protect the top edges from water intrusion. In many cases, the wall covering or cladding material acts as counterflashing. When this does not occur, a metal counterflashing mounted to the vertical surface should be installed. See Figures 3,4 and 5 for examples. 

Figure 3 - Example of metal counterflashing inset in masonry mortar joint 

Figure 4 - Example of through-wall metal counterflashing embedded in masonry mortar joint 

Figure 5 - Example of surface-mount metal counterflashing 

Material standards 

Material standards NRCA does not make any recommendations about which tile or manufacturer to use; however, NRCA does recommend clay tile roof coverings meet standards established by ASTM International. 

ASTM C1167, "Standard Specification for Clay Roof Tiles" ASTM C1492, “Standard Specification for Concrete Roof Tile†�

Warranties 

When purchasing a new roof system, there will be two warranties to consider. First, there will be the manufacturer's warranty. In general, these warranties cover defects in the manufacture of the roof covering. Please read NRCA's consumer advisory bulletin addressing roofing warranties for more information. Once the project is complete, be sure the contractor provides you with a certificate for your records. 

Second, the roofing contractor will provide you with a warranty covering his workmanship. Typically, this will cover installation and related issues. The warranty should contain what items are covered and what will void them. Many companies offer one year or two years of coverage; however, there is no industry standard. 

Wood shakes and wood shinglesClick on a topic for more information.

Material description Roof deck Underlayment and interlayment Fasteners Flashings Material standards Warranties

Material description 

Wood shakes and wood shingles are manufactured from western red cedar, cypress, pine and redwood trees. Shakes are split from logs and reshaped by manufacturers for commercial use. They are thicker at the butt end than shingles; generally one or both surfaces are split to obtain a textured effect. A split and resawn shake has a split face and sawn back. A taper sawn shake has a natural taper and is sawn on both sides. Wood shingles are sawn on both sides and have an even taper and uniform thickness. When applied to shingles, the industry terms "Perfection" and "Royal" mean 18 inch and 24 inch lengths, respectively. 

Cedar shakes and cedar shingles are available pressure treated with fire retardants and chemical preservatives for increased fire resistance and to prevent premature rot and decay in some climates. 

Pine shakes are made from southern yellow pine and are taper sawn. They also are available pressure treated with preservatives to protect against decay and insects. Interlayment felts are required for pine shakes. 

Roof deck 

Wood roof systems may be applied over continuously or closely spaced wood decking or over a spaced, sometimes referred to as "skipped," sheathing. Solid roof decking or sheathing should be used in areas of the roof deck where an ice dam protection membrane is required. 

The most common materials used for roof decks are plywood or oriented strand board (OSB). When plywood is used, NRCA recommends the use of a minimum 15/32 thick or ½ inch nominal exterior-grade plywood for 16-inch rafter spacings and 5/8 inch nominal thickness for 24-inch rafter spacings. For OSB, NRCA recommends a minimum 15/32 inch thick or ½ inch nominal exterior-grade OSB for 16-inch rafter spacings. 

Caution should be exercised when roof decks are constructed out of the following materials:

Oriented strand board (OSB): NRCA is concerned with potential fastener-holding problems and dimensional stability because of the effects of moisture where OSB and other nonveneer products are used as roof decking.

Preservative-treated wood: Many roofing material manufacturers recommend wood roof decks be constructed with wood that has been treated with a nonoil preservative pressure treatment or with nontreated air- or kiln-dried lumber.

Fire-retardant-treated wood: Because of the deterioration of some fire-retardant-treated wood panels caused by premature fire retardant activation caused by heat history in service the use of fire-retardant-treated wood panel decks should be carefully considered.

Underlayment and interlayment 

Asphalt saturated, nonperforated organic felts are among the most common underlayments used for wood shakes and wood shingles; they commonly are designated as Type 15 and Type 30 or referred to as No. 15 and No. 30, which are reflective of a once used pound per square weight designation. The terms Type I and Type II now are used within the industry in lieu of No. 15 or No. 30, respectively. 

When underlayment (or "felt paper" as it is frequently called) is specified, No. 15 or No. 30 asphalt-saturated, nonperforated felt should be applied shingle fashion on roof decks having a slope of 4:12 (18 degrees) or more. NRCA does not recommend using wood shakes and wood shingles on slopes less than 4:12 (18 degrees). 

In the case of wood shakes, these sheets are produced in 18 inch (450 mm) widths as "interlayment" felts; that is, they are applied between courses of wood shakes rather than directly over a substrate. See Figure 1. 

Figure 1 - Example of a wood shake roof system with underlayment and interlayment 

In locations where the average temperature for January is 30º F or less, NRCA suggests installation of an ice-dam protection membrane. An ice-dam protection membrane generally is a self-adhering polymer-modified bitumen membrane. 

An ice dam protection membrane should be applied starting at a roof's eaves and extending upslope a minimum of 24 inches from the exterior wall line of a building. For slopes less than 4:12 (18 degrees), NRCA recommends a minimum of 36 inches. See Figure 2. 

Figure 2 - Example of ice damming 

Fasteners 

Wood roofing may be attached to a roof deck with noncorroding, galvanized steel or stainless steel nails or noncorroding metal staples. A minimum of two fasteners should be used to attach each shake or shingle. Nails should be long enough to penetrate through all layers of roofing materials and extend through the underside of the roof deck or penetrate at least ¾ inch into wood plank or board decks. 

Flashings 

Flashings for wood roof systems fall into four categories: perimeter edge metal, penetrations, valleys and vertical surfaces.

Perimeter edge metal: Depending on the severity of climate, anticipated rainfall and freeze-thaw cycling, the use of perimeter edge metal should be considered.

Penetrations: Plumbing soil stacks, exhaust vents and pipes are flashed into wood roof systems with some type of flat flange that extends around a penetration and is installed under shingles on the upslope of a flange.

Valleys: Valleys that are called "open valleys" are typically lined with sheet metal. Vertical surfaces: When a roof system abuts a vertical surface, there are four types of flashing

commonly used: apron, step, cricket (or backer) and counterflashing.

Apron, step and cricket flashings require some form of counterflashing to cover and protect the top edges from water intrusion. In many cases, the wall covering or cladding material acts as counterflashing. When this does not occur, a metal counterflashing mounted to the vertical surface should be installed. See Figures 3, 4 and 5 for examples. 

Figure 3 - Example of through-wall metal counterflashing inset in masonry mortar joint 

Figure 4 - Example of through-wall metal counterflashing embedded in masonry mortar joint 

Figure 5 - Example of surface-mount metal counterflashing 

Material standards 

NRCA does not make any recommendations about which shake or shingle products or manufacturers to use. There are no ASTM standards for wood roof coverings; however there are standards for grading. Cedar Shake and Shingle Bureau (CSSB) Standard CSSB-97 contains grading rules for wood shakes and wood shingles. NRCA recommends that cedar shakes and cedar shingles be a minimum No. 1 grade, which requires 100 percent edge grain, clear heartwood and no face defects. 

Warranties 

When purchasing a new roof system, there will be two warranties to consider. First, there will be the manufacturer's warranty. In general, these warranties cover failures in the roof covering product. Please read NRCA's consumer advisory bulletin addressing roofing warranties for more information. Once the project is complete, be sure the contractor provides you with a certificate for your records. 

Second, the roofing contractor will provide you with a warranty covering his workmanship. Typically, this will cover installation and related issues. The warranty should contain what items are covered and what will void them. Many contractors offer one year or two years of coverage; however, there is no industry standard. 

SyntheticClick on a topic for more information.

Material description Material standards

Material description 

"Synthetic" as it pertains to steep-slope roofing materials refers to manufactured products that replicate asphalt shingles, concrete tile, clay tile, metal panels, slate, wood shakes and wood shingles.

Synthetic roof coverings contain recycled plastic and/or rubber as a key ingredient. These products have been available since 1993. 

There are some advantages to using synthetic roof coverings when compared to their traditional counterparts. Synthetic slate, or "fake slate," for example, weighs substantially less than natural slate. The reduction in weight allows synthetic slate to be installed over conventional roof decks. Some synthetic products purport to be hail-, mold- and algae-resistant. Several synthetic cedar shake and cedar shingle manufacturers claim a labor savings, because fire-retardants or anti-algae coatings do not have to be applied to the product. 

Despite the benefits, there are some significant drawbacks. Synthetic roof coverings are relatively new and there isn't a proven track record about their performance. Most synthetic products are manufactured with dyes or coloring agents and it is unknown whether these products will fade because of ultra-violet exposure and weathering. It also is unknown whether these products will become more brittle or less flexible over time. And most important, model building codes do not recognize any synthetic roof coverings. You need to check with your local building department before installing these products. Because of these reasons, caution should be exercised when using synthetic roof covering products. 

Material standards 

Currently, there are no ASTM International standards for synthetic steep-slope roof covering products. 

Steep Slope RoofingThe slope of a roof is often referred to as the pitch. The slope, or pitch, of the roof is determined by the vertical rise in inches for every horizontal twelve inch (12") length (called the "run"). A roof with x rise/12 run slope means that for every 12 inches horizontally (run), it rises x inches.

Generally, roofing types and roofing products are typically divided up into two primary categories, steep slope roofing and low slope roofing.  Sometimes steep slope roofing is also referred to as “residential roofing” due to the fact that most single family homes in the U.S. are constructed with some sort of pitch or slope to the roof.  Just as steep slope roofing is often referred to as residential roofing, the term low slope roofing is oftentimes synonymous with “commercial roofing”.  Again, this is due to the fact that, predominantly, commercial buildings have “flat” roof designs. If we were to classify roofing slopes more specifically, the list below contains the common roof slopes and the terms which classify them.  However, in general terms, low slope roofs (commercial roofs) are those below 2/12 or 3/12 while steep slope roofing would consists of roof pitches above either 2/12 or 3/12 and higher.       Flat Roof: 2:12      Low Slope: 2:12-4:12      Conventional Slope Roof: 4:12-9:12      Higher Slopes:  9:12 - 20:12      Steep Slope: 21:12 and higher

Roof slope is a very important aspect and it is considered the primary factor in roof design. The slope of a roof has an effect on the interior volume of a building, the drainage, the style, and the material used for covering. For example, if you notice water collecting on the roof the problem is probably related to the slope. The style is affected too because the framing of the roof changes the slope.

Steep slope roofing products are generally more visually appealing because they are critical aesthetic components for residential construction where the roof can consist of 40% of the exterior visual appearance of a home.  Steep slope roofing also generally lasts longer than low slope roofing because the systems shed water much more efficiently and generally are subject to less direct U/V activity.

Residential Low Slope RoofingResidential low slope roofing can generally be divided up into two broad categories:  larger flat roofed applications (low slope) such as residential condominium complexes, row homes, and town houses and smaller applications such as residential porch and garage transitions from the main home where the slope of the roof changes dramatically from steep slope to low slope on the same structure.

For the larger residential low slope roofing applications, CertainTeed has a host of products and systems available including BUR (Built-Up Roofing) systems, APP Modified Bitumen systems, and SBS Modified Bitumen systems, including many products that feature CertainTeed’s CoolStar surfaces that meet California Title 24 requirements and Energy Star and may qualify for federal tax rebates when installed as a residential low slope roofing product.

For the smaller residential low slope roofing applications such as garage and porch transitions, CertainTeed offers an excellent solution in our Flintlastic SA, self-adhering SBS modified bitumen roofing system.  Flintlastic SA products feature easy and clean application and also have granulated surfaces with colors that coordinate with CertainTeed shingles providing the homeowner with a complete and coordinated solution for roof designs that transition from steep to low slope.