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FirewiseConstructionDESIGN AND MATERIALS


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FirewiseConstructionDESIGN AND MATERIALS

Peter Slack

First edition printing, 1999Revised printing, 2000

To obtain copies of this publication contact theColorado State Forest Service at970-491-6303 or online at csfs.colostate.edu


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About the Author Peter Slack of Boulder, Colorado was a practicing architect for 26years, until his untimely death in June 2000. Peters practice includedmany homes and other buildings in the Interface. His design empha-sized the integration of re-resistive elements with other importantdesign principles such as proper site development for limited impact,low energy and water consumption, and the use of appropriate,resource conserving materials.Peter was a re ghter and a of cer in a high-risk mountain re districtfor 19 years. He speci cally worked with wildland re suppression andmitigation issues for much of that time. After ghting Boulder Countys

two major interface res, Black Tiger, 1988 and Olde Stage, 1990, Peterparticipated in Boulder Countys WHIMS Program (Wild re HazardIdenti cation and Mitigation System). FEMA funded the WHIMS pro -gram as a result of those res. This publication is developed from alecture on rewise construction that Peter presented for several years.

AcknowledgementsThis publication combines Peters professional knowledge as an

architect and builder in the Interface with his experience as a re ght -er for 19 years. Added to his experiences is the wealth of informationand experience so generously given by the re ghting community. Mostof all, it was their assistance in learning to visualize re in its environ -ment and around our buildings that facilitated the creation of theseillustrations.The following people contributed to this pamphlet by providing awealth of information.

Dr. Claire Hay, consultant, Wild re Interface GroupMark Mulinex, Wildland Fire Coordinator, City of Boulder Fire Dept.Mike Tombolato, chief, Cherryvale Fire Protection DistrictThe many members of the Boulder County WHIMS program, whoover many years have developed, to this date, one of the most compre-hensive and systematic approaches to understanding the hazards ofwildland re to homes in the Interface.

The following people helped make this publication possible. They wereresponsible for choosing the author and providing additional technicaldetails and editing:Frank C. Dennis, Colorado State Forest ServiceFred Sibley, Of ce of Emergency Management, State of Colorado

The following people assisted in the production of this document:Karen Gerhardt, Westerly Design, layout and design

Chris White, Wild re Mitigation Coordinator, Boulder CountyGillans Engineering, Colorado, editing, second printing

Jill Croft Slack, editing and support

Firewise Construction

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Table of Contents 1. Introduction: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

What is the Wildland Urban Interface? Fire suppression and increased fuels How can we protect our buildings? Fire intensity and duration related to the re resistance of a house

Evaluating re hazards

2. Fire Behavior: Fuels, Topography and Weather . . . . . . . . . . . . . 5 Wildland res and burning structures

Vegetation is the fuel for wildland res Fire duration and fuel Fire behavior and slope Fire behavior, ignition of fuels: mechanisms of heat transfer Convective lifting indirect: convective heating radiant heating Direct contact or impingement Weather3. Building Site Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Topography and vegetation: re behavior and intensity

AspectDangerous topographic features: areas of more intense re behavior

Natural barriers and buffer zones How this affects building location and design decisions Site design and modi cations to the forest: developing a defensible space

4. Building Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Simple vs. complex forms Aspect ratio Vents, eaves, sof ts and decks

Decks5. Building Materials and Components . . . . . . . . . . . . . . . . . . . . . 25

Ratings Roo ng Siding Windows and glass

Doors6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 References and additional information

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Firewise Construction Introduction

This publication provides homeown-ers and builders in the Wildland UrbanInterface with design and buildingtechniques that can offer more protec-tion from wildland or forest res.The Federal Emergency ManagementAgency (FEMA), the Colorado State

Forest Service and the Colorado Of ceof Emergency Management funded thisproject.

What is the Urban WildlandInterface?The Urban Wildland Interface, or

Interface, is any area where man-madebuildings are built close to or withinnatural terrain and ammable vegeta -tion, where high potential for wildland

res exists.

During the past few decades, popula-tion growth in the Interface hasincreased. Subdivisions and other high-density developments have createda situation where a wildland re caninvolve more buildings than any amountof re equipment can possibly protect.

Fire suppression andincreased fuelsThe past 100 years of wildland re andsuppression has created more vegeta-tion for fuel.As population in the Interface hasincreased, so too has the dif cultyof protecting that population fromwildland res. When res occur in the

Interface, we put them out to preventthe destruction of homes. This createsa problem because forests have histori-cally depended on re to maintain good

health. Fire thins trees and brush andeliminates dead material. By suppress-ing res to protect our homes andpopulation, we have interfered with thisnatural process. Since natural res arenow infrequent, vegetation density hasincreased, which provides more fuel for

res. When res do occur, the denservegetation burns with more intensity,and the re is more destructive anddangerous.

How can we protect ourbuildings?

This publication offers a two-partapproach to the problem:

1. Build more re-resistive structuresand

2. Reduce the hazards of forest fuels.

If we consider the speci c needs ofInterface structures, we can combinedesign elements and constructionmaterials to build more re-resistivestructures. Our goal is to createbuildings that can either resist re ontheir own, or at least make it easier for

re ghters to protect structures safely.We recognize that building a reproofstructure, as we do in an urban setting,can be prohibitively expensive. Thispublication discusses how to considera combination of cost effective strate-gies that increase the probability of abuilding surviving a wildland re.

1. Introduction

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Firewise ConstructionIntroduction

Solutions to problems in the Interfacedepend on a two-part approach: Makeour buildings more re resistive andmanage the surrounding wildlands. Ifwe leave the surrounding wildland in itscurrent state, we need to build struc-tures that are nearly reproof. Fireproofstructures are far too expensive tobuild. Conversely, trying to providea defensible space large enough for atypical, combustible structure may notbe practical or desirable. Choosing thebest combination of these two strate-gies for a particular site requires a basicunderstanding of wildland re behavior.

Another goal of this publication is to

give the homeowner and builder abetter understanding of how buildingsin the Interface ignite during a wildland

re. With this information they canmake better choices when consideringbuilding techniques and materials.When reading this publication keepin mind that re is only one ofmany considerations during buildingconstruction. We are not suggestingthat any one technique is absolutelynecessary, or that you cannot usealternate materials or design elements.

Rather, we want to show you howan awareness of the unique issuesfacing Interface buildings can directyou toward a more comprehensivesolution in the design process. Somedesign elements and materials may helpmitigate re hazards; and some maynot. It is possible, however, to compen-sate for less appropriate re protectionchoices and meet design goals.

Fire intensity and durationrelated to the re resistanceof a houseHow re resistive should a house be?The answer to this question dependson the re intensity, (how hot the reburns), and the re duration, (how longthe re will last a your site). If the rehazard is low to moderate, only a fewprecautions may be needed. If the rehazard is high or very high, most, or all,of the strategies we describe may beneeded.In Colorado, generally any areasurrounded by natural vegetation faces

some hazard due to wildland res. Inmountainous regions between eleva-tions of 5,000 and 10,000 feet, hazardis increased due to topography andincreased vegetation density. The nextsection discusses this in more detail.

Combustible house with no

defensible space

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Firewise Construction Introduction

Evaluating re hazardsA good way to determine the speci chazard rating at a site is to look at a

re hazard map or study located at thecounty building or land use department.The Colorado State Forest Service oryour local re protection district mayalso have information. If this informa-tion is not immediately available, usethis short evaluation to determine asites hazard level.

Note: We refer to this hazard ratingthroughout this publication withrespect to design and material elementsin building design.This short evaluation is based onthe Wildland Home Fire Risk Meterdeveloped by the National Wild reCoordinating Group (www.nwcg.com).

Slope Score

Level: 00 - 10 110 - 20 220 - 30 330+ 4

Vegetation water, bare rock, irrigated lawn 0

grass, shrub, less than 2 feet tall, no trees 1grass, shrub, less than 4 feet widely dispersed trees 2dense young shrubs, no dead wood or trees 2many trees, touching, some grass and brush 3dense shrubs with some trees 3thick, tall grass 3dense evergreen trees with grass and shrubs 4dense mature shrub with dead branches 4

After selecting the appropriate slope and vegetationscores, add them together to determine the hazardrating.

Scores Hazard Rating0 01 - 2 low3 - 4 medium5 - 6 high7 - 8 very high

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Firewise ConstructionIntroduction

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Firewise Construction Fire Behavior

Wildland res and thenature of burning structuresWildland res have been studied ingreat detail to help predict re behavior.Anticipating the intensity, duration andmovement of a wildland re is veryimportant for both re ghter safety,

and as the basis of tactical decisionsmade during the suppression of a re.

Understanding re behavior, especiallyits intensity and duration at a buildingsite, will help homeowners and buildersdecide how re-resistive a house needsto be.

Three factors affect wildland rebehavior:

1. The fuel for the re. The type,continuity and density of the sur-rounding vegetation provides fuelto keep the re burning.

2. The topography of the site. The steepness of slopes and otherland features affects the re behav -ior.

3. The weather. Wind and humidityaffect each re.

Vegetation is the fuel for wildland resThe type and density of a speci c plantdetermines how it will burn. Not allvegatation burns the same way. Somevegetation almost never burns; othersburn at different times of the year; andsome can burn almost anytime.

2. Fire Behavior: Fuels, Topographyand Weather

Deciduous trees and bushes: Treessuch as aspen, cottonwood andmountain ash; bushes such asmountain maple and dwarf lilacusually burn only during severedroughts.

Bushes, such as the Gambel oak,serviceberry and sage, can burneither in the fall when leaves havechanged or dropped, or when thereis an extended dry period.

Evergreen trees with resinous sap:Pines, spruce and rs can burn anytime of year. They usually burn dur-ing extended dry weather or highwind events.

Evergreen bushes: Cedar and juni-per can also burn any time of yearwhen conditions are dry.

Grasses. Grasses can burn any time ofthe year and only need a short dryperiod before they are receptiveto re. Grass is re resistive onlywhen it is very green or a goodsnow cover exists.

Fire duration and fuelFire duration is how long a re willburn at a particular site. The type offuel and its density determines a res

duration. For example, grass is a lightfuel. It will burn in less than ve minutesand produce relatively less heat thanheavier fuels would produce. Mediumfuels, such as brush, burn ve to 10minutes with more heat. Large treesare considered heavy fuels because theyburn from 10 minutes to over an hour

with the most heat.Understanding this is very importantto determine how long a house must

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Firewise ConstructionFire Behavior

resist a re. Different building materialscan resist re for different time periods.

Fire behavior and slopeSlope is the angle of the groundrelative to the horizon. It is commonlymeasured in either degrees or as apercent. Slope topography shows thesteepness of the slope and the shape ofthe land.

The steeper the slope, the more quicklya re moves and the hotter it burns.For example, a re will spread twiceas fast on a 30 percent slope than itwill on level ground.This means that a house locatedon a steep slope needs more reresistance.

Fire behavior, ignition offuels: mechanisms of heattransfer As a re burns, it releases hot gas andair from the combustion of burningvegetation or buildings. These gasesmove up the slope, drying and preheat-ing any vegetation in the res path.The re also releases large amountsof radiant energy, like that of the sun,which also heats and dries the fuels.Once ames make contact with theseplants, they ignite more easily. This inturn speeds up the rate at which the

re moves and increases its intensity.Look more closely at the mechanismsof re and how re ignites a buildingby studying three categories of heattransfer:1. indirect convective heating

and lifting

2. indirect radiant3. direct contact or impingement.

Slope chart

45 = 100%

22.5 = 50%

9 = 20%4.5 = 10%

Slope: Angle=% Grade

Convective and radiant energy from a re

Radiant energy

Convective energy

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Firewise Construction Fire Behavior

Convective liftingFire produces hot gases that rise andcarry partially burned substances andsmoke into the atmosphere. During awildland re this atmospheric effect canbe very strong, even causing its ownwind as cooler air rushes in to replacethe rising hot air.

Convective vertical air currents canalso lift burning materials or embers,called rebrands, and carry themhorizontally for long distances from the

re.Once out of the rising air currents,

rebrands fall back to the groundand onto horizontal surfaces suchas combustible roofs, decks and dryvegetation around a house. This effect,called spotting, can be very widespread.Firebrands often travel hundreds oreven thousands of feet in front of theactual re.

Indirect: Convective HeatingThe same hot air and gasses that dryand preheat vegetation do the samething to a building, making any combus-tible materials ready to ignite once the

re gets closer.

Indirect Convective

Convective lifting

Convection: Heat from rerises causing strong vertical air

currents

Firebrands, transported by convective lifting, create spot res

Super heated air & gases dry & heat

all fuels, both vegetation & structure

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Firewise ConstructionFire Behavior

Indirect: radiant heatingBuildings can be preheated, even igniteand burn, from the transfer of heat byradiant energy from the re. This issimilar to sunlight heating objects, but

re heats only in the infrared portionof the light spectrum. Radiant heattransfers on a straight line of sight andcan be reduced by barriers.

Vertical surfaces, such as siding, canignite from this effect well before re

actually reaches the building. Largeheavy fuels, once ignited, burn withhigh temperatures that amplify radiantenergy, creating more potential forignition through heat transfer.

Direct contact orimpingementContinuous and abundant fuels likethose found in unmanaged vegetationareas provide a direct path for a re tocontact a building. Creating defensiblespace and fuel breaks around a buildingis speci cally intended to reduce thiseffect.

Weather Weather is a major factor affecting

re behavior and is, of course, highlyvariable in terms of time, intensity andlocation. During extended periods oflow moisture, the possibility of wild reincreases. Weather can also increaseand intensify re behavior when there

is low humidity and high winds.Colorados re season is highly variable.Typically, winter and spring have few

wild res; summer and fall have more

wild res. However the period betweenwinter and spring, after the snow hasmelted but the vegetation has not yetgreened, is often a period of high reoccurrence.

Colorado typically has 50 to 100 daysa year of critical re weather whensevere wild res are possible. More re days occur at lower elevationswhile fewer re days occur at higherelevations.

Fire directly impinging on a house

Radiant heating

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Firewise Construction Building Site Location

Topography and vegetation:re behavior and intensity

The location of a structure willin uence the intensity and durationof the re to which it is exposed. Asdiscussed in the re behavior section,we know at any location how intense

a re will be; how long it will be there;and how fast it will travel, based on thesurrounding topography and vegetation.

When choosing a site location ordetermining the level of re resistancea building requires, the builder orhomeowner should be aware of howthe local vegetation and topographicvariations affect re behavior.

AspectAspect is the direction that a sitesslope faces. Vegetation varies widelybetween the extremes of south facingand north facing slopes.

South slopes tend to have the leastvegetation in an area because theyquickly dry out and have less availablemoisture for plants. Since there is lessfuel on south facing slopes, re burnswith less intensity than on other slopeswith more fuel.

East and west slopes generally havemore vegetation than south slopes.They are more prone to drying out inthe summer when the sun is high in thesky. Fire potential increases on theseslopes during the summer season.

North slopes typically have the mostdense vegetation because there is more

water available for plants. The highermoisture content of the vegetation onnorth slopes means that res occurthere less frequently. However, when

res do occur, they burn with moreintensity because there is more fuel.

3. Building Site Location

Top of the hill

East slope aspect, moderate vegetation

Topo or contour lines

West slope aspect, moderate vegetation

South slope aspect, least vegetation

Aspect of slope

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Firewise ConstructionBuilding Site Location

Dangerous topographicfeatures: areas of moreintense re behavior

Variations of topographic features suchas valleys, ridges, canyons and saddlescan be dangerous areas that furtherintensify or attract a re.

A valley, as a concave form, tends tocollect and concentrate winds. Thismeans that a wildland res intensityincreases as it moves through a valley.

If the valley is narrow with steep sides,such as a canyon, this effect is morepronounced.

When a valley crosses a ridge it createsa saddle between the higher partsof that ridge. Like a valley, saddleswill channel, intensify and speed upa re. These areas tend to be builtupon because they offer some shelterand often at areas. It is important torecognize that saddles are natural repaths where re will travel rst, andwith more intensity.

Ridges experience more windprimarily because they are elevated

above the surrounding land. When are moves up a slope toward a ridge, it

gathers speed and intensity.

As the wind crosses a ridge it usuallyhas a leeward eddy where the windrolls around and comes up the leewardside, exposing both sides of thestructure to wind and re. There areusually no areas on ridges to provideprotection from the re.

Saddle, low area on a ridge

Ridge with wind exposure

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Leeward eddy


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Natural barriers andbuffer zonesSome physical features will reduce

re behavior and can be used toslow, reduce or de ect a re. Someexamples of these bene cial barriers

are natural rock outcroppings, wetlands,streams, lakes and deciduous treestands, (aspen, cottonwood, etc). Takeadvantage of these features by placinga building so that the natural barrieris between the building and the antici-pated path of a re.

How this affects buildinglocation and design decisionsOn large parcels of land consider thesephysical features when choosing the

nal location of a building. Many otherfactors such as privacy, views, access

and aesthetic values will also effect sitelocation decisions. Fire is just one ofthese factors. Whether or not re is

the primary consideration will dependon how high the re hazard is in thearea.

On smaller parcels there may be onlyone suitable building location. The sitesphysical features will determine the

probable re intensity and dictate whatcombination of site modi cations andre-resistive construction is necessary

to prevent the building from igniting.

Site: House located relativeto natural features that bufferagainst re

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Pond

Path

Aspen grove

Rock outcrop


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Site design and modi ca -tions to the forest: develop-

ing a defensible spaceAfter evaluating the re hazard ratingof a site, develop a plan to managethe surrounding forest and defen-sible space. This is the rst part of aour two-part stategy to build a re-resistive structure. Defensible spaceis the area around a building that hasbeen signi cantly modi ed to reducea wild res intensity just enough toprevent the re from igniting the house.The defensible space will also allow

re ghters to more safely defend thehouse. It can also help prevent a house

re from spreading to surrounding

vegetation.A diagram of the features at a buildingsite would show that moving away from

the building out into the wildland, thefeatures gradually shift from man-made

to more natural elements. We dividethis gradation into zones. Develop-ing a defensible space plan requires aninventory of the existing site featuresand analysis of how hazardous theyare. Man-made elements are landscap-ing features such as masonry walls,patios, footpaths and driveways. Thesefeatures create barriers and bufferzones.

The area next to the building (Zone1-A) should contain primarily noncom -bustible surfaces. Any planting in thiszone should be only deciduous, well-trimmed and irrigated. Ground coversshould be owerbeds and cut grass.

Moving away from the building, thenext area (Zone 1-B) can have more

Defensible space

Defensible Space

Zone 1

Segments

Zone 2 Zone 3

A B C

Masonry wall

Masonry patioFoot path

Landscaped areasonly isolated trees

Trees have adequateseparations & arelimbed to 10 feet

Trees are prunedof dead materials

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landscaping and less man-made surfaces.Vegetation should still be deciduoustrees, bushes and grass can be native,but they must be kept trimmed to

fewer than 6 inches tall.Moving farther away from the buildingto (Zone 1-C) the landscaping shouldchange from introduced deciduousplants to natural vegetation, includingevergreens. These trees or bushesshould be far apart and well maintainedby trimming.

In Zone 2 the landscape is entirelynatural vegetation that is intenselymanaged or modi ed. Trim deadmaterial from natural vegetation closestto the buildings. Prune all limbs to 10feet above the ground. Thin trees sothat a minimum of 10 feet separates

the tree crowns.Moving vegetation farther away fromthe building into Zone 3, the forestmanagement gradually becomes lessintensive and subtler. Tree limbs needto be pruned only 4 to 5 feet above

the ground. Tree crowns can be closertogether.

Remember, the more intensive andwide-ranging modi cations you make inthe defensible space, the less the needfor re-resistive materials and buildingdesign. Conversely, fewer modi cationsto the surrounding wildland increasethe need to use re-resistive materialsand design for the building. These twostrategies work together to achieve thegoal of building a rewise structure thatdoes not burn when wild res occur.

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Fi i C i B ildi D i


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Firewise Construction Building Design

Eddy currents hold heatand burning embers

So far we have discussed elementaryre behavior and how to manage the

wildlands surrounding an Interfacebuilding. The second part of ourapproach to building re-resistivestructures is learning about appropriatedesign and material choices.

Simple vs. complexformsSimple building forms have less surfacearea relative to the volume of thebuilding. Complex building formshave much more surface area relativeto volume. Simple building forms areless expensive to build, more energyef cient and easier to protect fromwildland res. There is simply lessexterior surface to protect.Complex forms not only increasethe surface area of the structure, but

also create shapes that trap the resheat. These areas are called heat traps.Transitions between vertical surfacesand horizontal surfaces, inside cornersbetween two walls or abrupt intersec-tions of different solid planes formpockets where wind velocity drops andeddies form.

Parapet walls, solar collectors, roofsintersecting walls, roof valleys anddecks are examples of heat traps.These forms cannot be avoided, andtheir locations require much moreattention to re-resistive materials.

When wind speed decreases burningembers falls most often at the locationsdescribed above.Roofs are very susceptible to rebrandsin a wind driven re.

A simple root form such as a hip orstraight gable is best. Complicatedroofs with intersecting planes andvalleys form dead air pockets and eddy

currents. The use of complicated formsfurther highlights the importance of atruly re-resistive roof.

4. Building Design

Flow of heated air

Projecting walls

Building

Heat traps around walls

Heat traps around roofs

Flow ofheated air

Wind carryingburning embers

Solar Collector

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Firewise ConstructionBuilding Design


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Aspect ratioAspect ratio is the ratio between theeast-west axis and the north-south axis.In Colorados climate it is generallybetter to have a structure that is longeron the east-west axis than the north-south axis. Such a structure has a morefavorable energy relationship with the

re. On a at site the direction of are is somewhat unpredictable, but it

will generally be determined by thepredominant winds and fuel.

The probable re path is more easilypredicted on sloping sites. Fire can beexpected to approach up the slope. Oneast and west facing slopes, placing thebuilding on the longer east-west axisworks well for both energy and reconsiderations. The building presentsits widest side to the winter sun and itsnarrowest side to the

re path.

Remember, a building can contradictthese principles. In that case thebuilding will require more re-resistivebuilding materials and componentswhen simple forms and optimum aspect

ratios cannot be used.

Vents, eaves, sof ts anddecksBuilding a re-resistive house can becompared to building a watertight roof.

One little hole in the roof allows waterto leak in, and it doesnt matter howwell the job was done on the rest ofthe roof, it failed and damage occurred.Small building elements like sof tsand vents can be the weak link ina re. An otherwise re-resistivehouse is damaged or destroyed

because re found a way inthrough these areas.

VentsVents are required by the building codeto prevent accumulation of water vapor.All crawl spaces under wood oorsare required to have ventilation. Onesquare foot of vent is required for

climate and can gain the bene ts of thesuns passive solar heat.With regard to re, if a house presentsits widest exterior in the direction fromwhich a re is likely to come, it will bemore vulnerable. More re-resistivematerials and components are neededon the side that faces the oncoming

Intersecting planescreate traps

Simple form Complicated formRoof forms

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every 150 square feet of oor area.Since these vents are typically locatednear the ground, care should be takento not have any combustible vegetation

immediately next to them.Vents located on the downhill sideof the house should have landscapingelements like stone patios or wallsthat block the direct path of the re.Building codes typically allow alterna-tives to traditional vents. In some caseslouvered vents are permitted. Thesecan be closed when moisture is nota problem. (Fire season is usually thedry season.) Mechanical ventilationwith intakes and exhaust located awayfrom the ground or other vulnerablelocations can also be used.All attic spaces and roof cavities arerequired to have ventilation. Onesquare foot of vent is required forevery 300 square feet. of roof. (Seeeaves and sof ts on page 18.) In bothcases the vents should be made ofmetal with wire screen material thathas 1/4 inch or smaller openings.

Fire and hot airwith burningembers

Vent: metallouver andmetal screen

Intended

normal airow forventilation

Crawl space ventilation

No combustiblematerials in this area

Wall or other object tobuffer from re

Exterior

wallMetallouveredvent withmetalscreen

Normal airow for

ventilationHot air

and gasesfrom re

Attic Vent

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Building Design

Eaves and sof tsThe extension of the roof beyond theexterior wall is the eave. This archi-tectural form is particularly prone toignition. As re approaches the building,the exterior wall de ects the hot airand gasses up into the eave. If theexterior wall is combustible this effectis ampli ed.

The solution is to cover the eave with asof t. If the sof t is applied directly tothe rafter eave, it forms a sloping sof t.This still makes a pocket that can trap

re.

A better detail is to form a at sof tthat allows the building to more readilyde ect re outward.The sof t material should be at least3/4 inch plywood in low re hazard

areas, noncombustible in moderate andhigh areas, and one-hour rated materialin very high hazard areas.

Open eavetraps heat

Exteriorwall

Hot airand gasesfrom re

Open eave with no sof t

Class A roof

Sloped sof tencloses eave

Hot air

and gasesfrom re

Roofventallowssomeheat in

Open eave with sof t

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g g

Vents for roof ventilation are oftenfound in the sof t. Placing vents inthese locations creates a perfectpath for re to enter the roof

structure. If the vent must be in thislocation it is better to place it fartherfrom the wall and closer to the fascia.The vent can also be placed in thefascia or near the lower edge of theroof.

Class A roof

Vent locatedin the fascia

Non combustiblesof t

Hot air and gasesare de ected outaway from thebuilding

Fully enclosed sof t with isolated vent

DecksDecks are a very popular and well-used part of the house, especially inmountainous terrain. Because theyprovide elevation above the terrainand surrounding vegetation, they offera better view. They also supply atareas for walking on otherwise slopingterrain.The problem is that most decks are

highly combustible structures. They arethe ultimate heat traps. Their shapetraps hot gasses from an approaching

re. Decks often face downhill towardsa res most likely approach up a slope.

Conventional deck in a re

Space under deck traps heatcoming from re

Wood deck

Wood deckingon wood joists

Wood railing

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Decks are built perfectly to burn,almost as easily as wood stacked in a

replace. All the components of a deck; joists, decking and railings, are madeof only 2 inch thick wood with a highsurface-to-volume ratios.When re approaches, the woodquickly dries out and heats up. Iignitioncan occur very easily when either theradiant energy from the re gets hotenough or a burning ember lands on it.

Ignition of decks

Conventional wood decks are socombustible that when wildland reapproaches, the deck often ignitesbefore the re gets to the house.Sometimes unburned vegetationexists between the house and the re,demonstrating that the deck was more

ammable than the vegatation.

2 x 8 top rail

4 x 4 post

2 x 4 rail

2 x 4 decking

2 x 8 joist

Conventional deck construction detail

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Isolate the deck from there with a patio and a wall

In low and moderate re areas, it maybe suf cient to isolate the deck fromthe fuels and re by building a noncom -bustible patio and wall below it. Thepatio will assure that no combustiblematerials are below the deck. The wallwill act as a shield, de ecting both theradiant and convective energy of the

re.

Heavy timber constructionIn moderate hazard areas the use ofheavy timber construction is accept-able. Like log siding, heavy timber iscombustible but so thick that it burnsvery slowly.Minimum thickness for a heavy timber

deck is 6 inches for the posts andstructural members and 3 inches forthe decking and rails. This type ofconstruction can be used with a patiobelow for additional protection.

Conventionalwood deck

Stone or concretepatio below

Masonry or concretewall below and outin front isolates deckfrom re

Deck with a patio and a wall below

3 x 6 top rail

6 post

3 decking

3 x 8 joist

6 x 10 beam

Heavy timber deck

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Fire-resistive deckconstructionIn the highest re hazard areas,consider noncombustible surfaces and

re-resistive building materials for adeck. Wood frame construction ispermitted, but change the surface tononcombustible or one-hour ratedmaterialsTo build this type of surface, place awaterproof membrane over the top ofthe deck. This allows the use of re-resistive sof t materials, which cannottolerate moisture. The most commonmaterials are cement ber panels ormetal (noncombustible), or gypsum(noncombustible and one-hour rated).

Cover the membrane with decking.One suggestion is plastic wood which

has low combustibility; it will burnbut only very slowly. Better yet, use1 to 2 inches of concrete or stone.This surface is re-proof and protectsthe deck from air-born rebrands.However, this covering requires that thestructure be strengthened to supportthe additional weight.

Posts and railings can be economicallybuilt from steel. Wood posts nearthe ground can have stone, brick, ornoncombustible coverings. A popularbaluster design is steel wire, but thisis expensive. Steel pipe, usually 1 to 2inches in diameter, is very economicaland easy to work with. Square steelshapes can look like traditional woodrailings.

Steel postand railing

Metal ashing

Vent in fascia

Cement board

2 concrete

Rubbermembrane

Wood joist

Gypsum sof t

Fire-resistive deck construction detail

22



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Fully enclosed decksThe best design is to convert the deckto a solid form by fully enclosing it. Thiscompletely eliminates the heat trap.

This form also complies with the newUrban/Wildland Interface code (1997).

In the photo above, the deck is overthe garage. It has a metal railing with

heavy timber posts and concrete deck.

Convective energyfrom re encounterssolid re resistiveform and is de ectedupward

Solid railing,re resistive

Fully enclosed solid deck,waterproof and re resistantdeck surface

Fully enclosed solid deck

23



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24

Firewise Construction Building Materials and Components


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RatingsWhen discussing building materialsand components we make frequentreferences to ratings. Through testingvarious national organizations provideratings or evaluations for the re resis -tivity of materials or building assem-

blies. A building assembly is a combina-tion of materials forming a componentof a building such as a roof or wall. Theratings are in the following categories:

Combustible or noncombustibleClasses: A (best), B, and C

Time: 20 minute, one-hour,two-hour and four-hour

The organizations that provide theseratings are: the International Confer-ence of Building Of cials (ICBO)through its publication, the UniformBuilding Code (UBC); Also a foundingmember of the International CodeCouncil (ICC) through its publica -tion the International Building Code(IBC);The American Society for Testingand Materials (ASTM); the Underwrit -ers Laboratory (UL); and the NationalFire Protection Association (NFPA).

The difference between a non-combus-tible material and a rated materialor assembly is the surface resistanceto ignition versus the protectionafforded the building behind it. A goodexample of a non-combustible materialis metal roo ng and siding. Metal isnon-combustible, but an excellent

conductor of heat. If the re remainspresent long enough, the heat will beconducted through the metal and ignitethe material behind it. An example of

a re-rated assembly is wood sidingapplied over gypsum sheathing. Thisassembly is rated as one hour. Thesurface can ignite, but the building isprotected from the re for one hour.The importance of this is thedifference between intensity of reand duration of re as described inthe re behavior section.Most ratings are for commercialbuildings in urban settings, but someapply to residential structures. Forexample, the wall between a garage anda house must be rated as one-hour reresistive. The door between the garageand the house must have a C labelrated for 20 minutes with an automaticcloser.Material ratings for the wildland

re environment have been directlyaddressed by the I.C.B.O, through asubsidiary, the International Fire CodeInstitute, Fire Service Division and

its publication, the Urban WildlandInterface Code and N.F.P.A. Standard299. These publications also addressother issues covered in this publica-tion such as access, utilities and watersupplies for re suppression. Much ofwhat is contained in this publication isbased on or refers to these publica-

tions.

5. Building Materials and Components

25

Firewise ConstructionBuilding Materials and Components

h d h h d l d l l


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Roo ngRoo ng is one of the most importantways to protect a house from wildland

re. As shown earlier, when wildland

res become more intense, the loftedrebrands become a signi cant cause of

the re spread. Since most roo ng hasa rough surface and numerous cracks,it can trap wind blown embers and

rebrands. In all major Interface res,houses thousands of feet from the rehave been observed with burning roofs.

Wood shakes and shinglesSimply put, wood shakes and shinglesare made perfectly to burn. They arealmost like kindling. They are thin,1/2 to 1 inch thick, with a very roughsurface and many cracks. When awood roof burns it also lofts burningembers, contributing to the spread of

re. Another important characteristicof wood roofs is that they dry out inColorados dry climate.

A cedar roof can be modi ed to bere-resistive. Pressure treatment with

chemicals can change wood shingles toa class B or C roof. Chemically treatedcedar roofs built with a gypsum under-layment can have a class A assemblyrating. However, many doubt that thetesting conditions for these shinglesmatched Colorado s climate of low

humidity, high winds, elevated ultravioletradiation and extreme temperaturevariations.The use of wood shakes in the

Colorado region is diminishing, notbecause of the re risk they pose,but because of the unavailability ofinsurance coverage for damage dueto hail and high winds. Cost wise, haillosses in Colorado are 10 times greaterthan re losses.

Asphalt shinglesAsphalt shingles are probably the mosteconomical way to roof a building,especially in terms of dollars spent peryears of guaranteed life. Conventionalmineral reinforced asphalt shingles havebeen around for more than 60 years.

They are normally guaranteed for 10to 20 years, and usually have a class Crating.

Mineral reinforced shingles havegradually been replaced by berglassreinforced asphalt shingles. These offer

guarantees of 20 to 40 years and are aclass A material. They are available inmany colors and textures and can evenimitate wood or slate shingles.

26


M l h d hi l Fib hi l


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Metal: sheet and shinglesMetal roo ng has always been availablein sheet form in many colors. It usuallyhas standing seams or ribs. The most

common metal roof is galvanized steelwith factory-applied paint (usually atwo-part epoxy type, not too differentfrom automobile paint).

Metal roo ng is also available as animitation wood shingle. This product ismade by stamping a texture and shapeon the metal and then applying theappropriate color. This imitation is sogood that at a distance of 100 feet ormore it is dif cult to tell the differencebetween it and a wood shingle.

The advantage of metal roo ng, bothat and stamped shingle, is that it is

non-combustible, durable and verylightweight. It requires a gypsumunderlayment in order to have a classA assembly rating, but that is onlynecessary in high or very high rehazard situations. Guarantees start at20 years and go to 50 years.In addition to galvanized steel withpaint, metal roo ng is also availablein aluminum with paint, stainless steeland copper. These tend to be moreexpensive but also last longer.

Fibercement shinglesThese shingles are made of cementand berglass, or cement and wood.Like the metal shingle, they are made

to imitate a wood shingles texture,shape and color. The cement in theseproducts is altered with polymers tomake it less brittle. These productsare also noncombustible but requirean underlayment for a class A assemblyrating.

Membrane roofsThese materials include both rubberand hot applied, bituminous saturatedmineral felt for at roofs. Thesematerials are marginally combustiblebut are most often used with othercovering systems like concrete. It can

be applied over a gypsum underlay-ment for a class A assembly rating.Guarantees are only in the 10 to 20year range, but these products can beconsidered permanent when coveredwith concrete.

Concrete shingles and tile,slate shingles, clay tileThese products provide the best re-resistive roof, but they are expensive.They are 1 inch thick, heavy (10 poundsper square foot), non-combustible,class A rated and usually come with50 year guarantees. Concrete shinglesare manufactured to look like woodshingles. When having a tile roofinstalled, pay careful attention to theclosure of the round openings of thetiles at the edge of the roof.

27


E t i ll idi g sheathing they offer the most economi


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Exterior walls: sidingThe exterior walls of a building aremost affected by radiant energy fromthe re and, if there is not enough

defensible space provided, by the directimpingement of the re.

Wood panels and boardsWood panels and boards are the mostcommon and economical forms ofsiding, but they are readily combustible.This siding is usually not very thick,1/2inch to 3/4 inch, and will burn throughto the structure behind it in less than10 minutes. A one-hour rating can beachieved by adding gypsum sheathingbehind the siding. However, thisaddition is of limited value because thebuilding can still ignite, and the re canspread to other parts of the buildingsuch as the eaves above the exteriorwall or the windows.

Fiber cement panels, boardsand shinglesThese products are non-combustible,but they may not be rated and mayneed gypsum sheathing to achieve aone-hour rating. These materials arevery economical and cost just a littlemore than wood products. When theseproducts are applied with the gypsum

sheathing they offer the most economi-cal way to side a house that will resistalmost all re hazard conditions. Thesematerials are virtually permanent

on a vertical surface and come witha 50 year guarantee, but they needto be painted. Some can even take astain with satisfactory results. Theseproducts are available with texturesmolded to imitate wood grain.

Metal: galvanized steel,aluminum, boards, panels and shinglesLike their counterparts in roo ng, theseproducts are available in either atsheets with seams, a stamped board orshingle that imitates a wood product.They are factory painted with two-partepoxy paint and usually have a 50 year

guarantee. Unlike the ber cementproduct, the paint on this product isa part of the guarantee; thus, it is analmost permanent, no-maintenancematerial. It is noncombustible, but likeother metal products needs a gypsumsheathing to achieve a one-hour rating.

Real StuccoReal stucco, as base material, is 3/4inch to 1 inch thick cement and gypsum.The stucco is applied in two or threecoats with metal mesh reinforcing.The color is integrated into the nalcoat and thus lasts a very long time.Guarantees are 10 to 20 years. It isboth a non-combustible and one-hourrated material, which makes it a verygood material for high hazard areas.Real stucco tends to be expensive andis also prone to cracking if not appliedabsolutely correctly.

28


Synthetic stucco exterior Concrete synthetic stone


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Synthetic stucco, exteriorinsulating nish system (EIFS)

This product is a 1/8 inch thick acryliccement nish on berglass mesh. This isapplied to the tap surface 1 to 2 inchesof expanded polystyrene (EPS). Thecolor, like real stucco, is in the cementcoat and thus lasts a long time. This isthe preferred way to do stucco becauseit takes less labor and is thereforecheaper. The foam insulation isolatesthe stucco nish from the building,which virtually eliminates cracking.The surface is noncombustible andhas no rating by itself. This product isinteresting in a re because it signi -cantly delays a re due to the insula -tion quality of the rigid foam and thefact that the system does not ignite; it

actually fails and falls away. In moderateto high re hazard situations thisproduct will work well. It can, likeother products, obtain a one-hourrating with gypsum sheathing, whichshould be used in a very high rehazard area.

Heavy timber or logconstructionThis wood product has a minimumthickness of 6 inches for framemembers and exterior siding, and

Concrete synthetic stoneThese products are cast concrete withintegral color forming the texture andshape of the stone being imitated. They

are modular shapes that have consis-tent dimensions with at backs, keepinglabor costs down.

Synthetic stone is reinforced with ber -glass and steel mesh, making it veryresistant to cracking. It is fully non-noncombustible and is usually rated as aone-hour material.

Brick, stone and block These materials are both permanentand reproof. Ratings are usually twohours. These are the best products touse in regard to re resistivity but arethe most expensive.

3 inches fordecking and steps.Heavy timber isrecognized bybuilding codes asa separate re-resistive category.

Even though heavy timber is combus -tible, the low surface-to- volume ratiocauses it to burn very slowly. Thismakes it very appropriate for mediumand high re risk situations.

29


Windows and Glass percent of the heat is transmitted to


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Windows and GlassWindows are one of the weakest partsof a building with regard to re. Theyusually fail before the building ignites,

providing a direct path for the re toreach the building interior.

Glass failureGlass provides only a partial barrierto re and only for a short time. Itfractures in the presence of heat. In thecase of a wildland re, this will happen

in about ve minutes. Glass de ectsmost of the convective energy, but notthe radiant energy of the re.

Convective energy is hot air andgasses. About 70 percent of the heatis de ected by window glass; about 20percent of the heat is absorbed; and 10

percent of the heat is transmitted tothe interior of the building.Radiant energy from a re is infraredlight energy, like the energy we experi-

ence from the sun. Most radiant energyfrom a re, 60 percent, is transmittedthrough the glass to the interior of thebuilding; about 20 percent is re ected;and about 20 percent is absorbed bythe window glass.Both the radiant and convective energyheats the glass, but the perimeter ofthe glass is covered and protected by asash. This causes a differential heatingand stressing of the glass, which causesit to crack.

Convective Energy:

70% is de ected away10% is transmitted20% is absorbed

Radiant Energy:

20% is re ected60% is transmitted20% is absorbed

Energy transmission, convential glass

30


Large and small windows


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Large and small windowsEven if the glass does fracture, the hotgasses (convective energy) from the

re and the re itself cannot enter

the building if the glass stays in place.Only the radiant energy heat can getthrough. Eventually, even with the glassin place, combustible materials behindthe window may ignite. (See Low Eglass).

Small windows, less than 2 feet wide ortall on a side, will keep fractured glassin place. The size of glass held in placeby the sash is relatively small with littleweight.Large windows (more than 2 feetwide or tall on a side) cannot keep thefractured glass in place. The size andweight of glass in relationship to thelength of sash is too great.

Thermopane or doubleglazed windowsBecause of current energy codes, mostglass today is double glazed or Thermo-pane. Double-glazed windows last

about twice as long as a single pane, orabout 10 minutes.The same processes of convective andradiant energy affect the front paneof glass. As long as the front pane isin place, the second pane is partiallyprotected. When the front pane failsand falls away, the process continues on

the second pane until it fails and fallsaway.As shown earlier in the re behaviorsection, the duration of a re at a site isdependent on the slope and the fuels. Itcan be as short as 5 minutes in the caseof a grass re.If the duration of the re is any longerthan 10 minutes due to signi cant fuel

supply around the house or preheating,additional protection will be necessaryto prevent glass failure and re enteringthe house.

2 ft.

Small and large windows

5 f

t .

Thermopane window

31


Exterior window covers, Metal shutters are better. They will


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,shutters and screensOnly an additional 10 to 20 minutes ofprotection is necessary for a window

to survive most res. Exterior windowcovers, such as in-place shutters, canadd this time. Shutters originatedin New England as protection fromstorms when the wind would break theglass. They are now readily available inthe Southeast for hurricane protection.

Wood shutters are the most commonand economical, but they will ignitewithin ve minutes. However, as shownin the re behavior section, if thewildland re duration is short enough,an additional ve minutes of protectionmay be all that is needed. Also, eventhough re departments may use foamto protect structures, it will not stick toglass. Therefore, shutters may still beadvisable.

protect the window long enough tolast through the re event and will notignite.

The disadvantage of shutters is thatthey are not completely passive, that is,they require intervention on the part ofthe homeowner or the re departmentto work.

Permanently placed exterior metalscreens eliminate the deploymentproblem. Exterior screens are notgoing to protect the window as muchas a solid cover, but as mentionedbefore, only ve to 10 minutes ofadditional protection may be needed.Screens also provide a surface to whichfoam can adhere. These screens cannotbe used with outward acting windows,like casement or awning windows, butthey can be used with horizontal slidingand double hung windows.

Solidshutter orwindowcover

Exteriormetal

screen

32


Tempered glass


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Tempered glass is both resistant to highimpact and high heat. Most of us arefamiliar with it. Building codes require

that tempered glass be used in patiodoors and all areas subject to humanimpact. It is also the glass used in frontof replaces. Tempered glass will stayin place and intact throughout thewildland re event.

A problem with tempered glass is cost.Windows with tempered glass typicallycost 50 percent more than regularglass. There are strategies around this,and costs are coming down.Patio door replacement units are, asthey infer, used to replace glass in patiodoors. These units are mass producedand stocked by virtually every glassbusiness. As a result they are veryeconomical. In fact they are lessexpensive than conventional glass. Theycome in six sizes, as shown at right, andtypically can be used as a picture unit,or combined to make a window wall orsolar structure.Using patio door replacement units

provides a lot of tempered glass at avery economical price.A few brands of windows are marketedas replacement windows in existingmid-rise urban buildings where theuse of tempered glass is required. Asa result, the additional cost for thesebrands of tempered glass is only 25percent more than standard glass. Yourlocal window supplier can suggestappropriate manufacturers.

2-4

6 - 4

Patio door replacement unit sizes

2-10 3-10

7 - 8

33


Low E glass transfer to combustible materialsb hi d h l h d d


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Low E stands for low emissitivity. Thisis an ultra thin, several microns thick,metallic coating on glass that appears

white or re ective to infrared andultraviolet light. It is used in windowsfor energy ef ciency because it holdsmore heat in during the winter andkeep more heat out during the summer.It also protects fabrics from fading andwood from yellowing.This glazing option is widely used inwindows today and only costs about10 percent more than standard doubleglazed units.The advantage of this glass in a wildland

re is that it stops the radiant energy

behind the glass such as drapes, woodfurniture and walls.The combination of Low E and

tempered glass features for windowsprovides the best possible solution forwindows in a wildland re. The glass willstay intact throughout the re eventand will transfer less radiant energy tocombustibles in the structure.It should be noted that the use oftempered and Low E glass is a recom -

mendation based on observations inthe eld. Actual laboratory studiesin a wildland re setting need to beconducted to give these types of glassspeci c quantitative values.

Energy transmission, Low E glass

Convective Energy:

70% is de ected away10% is transmitted20% is absorbed

Radiant Energy:

70% is re ected

10% is transmitted20% is absorbed

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Glass block It does not contribute to the combus-tion of the house The problem is that


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Glass block is the most re- resistiveglass available. It has the highestavailable rating of 90 minutes. It has

an excellent appearance but provides apoor view. It does not have the Low Eoption.

A good use may be in a situation whereonly day lighting is needed, a view isnot a factor and the orientation of thewindow may be toward a very high rehazard.

Frames and sashesWindows with improved glass technol-ogy will only work as long as the glassremains in place. The glass is held inplace by the frame, so the frame needsto withstand the re.Wood frames will burn. Since theyhave a high surface-to-volume ratiothey will readily ignite and burn freely.They are not a good choice.Vinyl frames seldom ignite, and if theydo, the combustion rate is very slow.

tion of the house. The problem is thatvinyl frames melt and structurally fail,allowing the glass to fall away. They arenot a good choice either.Aluminum clad wood frames delay theignition of the wood frame. They donot completely protect the windowbecause the aluminum conducts theheat to the wood. This delay is enoughin most wildland res.All aluminum frames are even better.

Since there are no combustiblematerials, they remain fully intact duringa re. These frames are now availablewith a thermal break, a plastic spinethat connects the interior frame to theexterior frame.

35


Doors Metal doors, steel andaluminum


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Wood doors

Residential buildings typically use wooddoors with glass inserts. The same reissues related to window glass apply toglass in doors. An unrated wood dooris typically 1 1/2 to 2 inches thick. It canreadily ignite and burn through in only10 minutes, which is much faster thanthe rest of the structure will burn.

Wood doors are available with a classC, 20 minute rating. These doors aretypically used between the garage andthe house. They are a good solution inmoderate re hazard situations. In veryhigh re hazard situations, they maynot resist burning for the re durationand will allow other exterior buildingcomponents to ignite.

aluminumMetal doors are non-combustible andavailable with 20 minute, 45 minute and

one and one half-hour ratings, whichmakes them the most appropriatesolution for very high hazard situa-tions. Glass sizes are restricted in thesedoors. The surfaces are available withembossing to simulate wood grain andraised panel designs.

Just as in energy conservation, a goodre-resistive door requires adequate

weatherstripping so that the sealprevents hot gasses or burning embersfrom entering the building.

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6. SummarySummary


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A major wild re can be an overwhelm -ing event to experience. It can be huge,blotting out the sun and creating itsown winds. It can throw ames andburning embers everywhere. Wild reis a natural part of our environmentthat we can either respect or fear. Ifwe make adjustments and modi ca -

tions to our homes and the sites theyoccupy, then we can live con dentlywith re. Each Interface resident mustunderstand the basic characteristicsof wildland re and how it puts theirproperty and lives at risk. Then theactions they take by building appropri-ate structures and properly caring fortheir Interface environment can signi -cantly reduce the re hazard.

A comparison is often made betweenre and water. Fire, like water, tries tond a way into our homes. It does not

matter how re-resistive some parts ofa structure are if weak points let a rein. An awareness of how each buildingcomponent is affected by re willenable the owner, architect or builderto eliminate those weak points.And nally, each of us needs to under -stand that, when we suppress wildland

res we must enhance our forestmanagement policy to reduce re fuels.

When res do occur they will be moremanageable and less destructive toboth the forest and our buildings.

References and additional informationThe following is a partial list of publications and/or organizations canprovide more information on this topic.

Californias I-ZoneRodney Slaughter, editor.Available from the CFESTES bookstore in the California State FireMarshals Of ce, 1131 S St., Sacramento, California 95814.Ph: 916-445-8200

Brush re Prone Areas: Siting and Design of Residential Buildings

Construction of Buildings in Bush reProne AreasQueensland Department of Local Government and Planning

P.O. Box 187, Brisbane Albert Street Qld 4002, AustraliaPh: 07-3237-1703, Fax: 07-3235-4071

The Urban Wildland Interface CodeThe International Fire Code Institute, International Conference ofBuilding Of cials

5360 Workman Mill Road, Whittier, California 90601-2298Ph: 562-699-0541

NFPA 299 Standard for Protection of Life and Property from Wild reNational Fire Protection Association, (NFPA) 11 Tracy Drive, Avon,Massachusetts 02322Ph. 800-344-3555

www. rewise.org

a web site maintained by NFPA covers much of what is in thispamphlet.

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