Wood in Buildings:

Steps to Durability and Longevity

How to Use Wood to Its Full Potential

reThink Wood sponsors this learning unit provided by Hanley Wood, a registered

provider with the American Institute of Architects (AIA) Continuing Education Systems

(CES).

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This program is registered with the AIA; as such, it is not an approval or endorsement by

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Questions related to specific materials and services should be directed to reThink Wood

after you complete this learning unit.

AIA Best Practices

AIA Provider Number: K029

AIA Course number: BNSL1212

AIA Credit: 1 HSW/SD hour

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© 2012, reThink Wood, www.rethinkwood.com

Specifying a building material considers structure, functionality and the

aesthetics of the built project and environment. Architects specify wood for

many reasons, including cost, availability, ease of construction, thermal

performance, aesthetics and design versatility. Research and new product

development have only added to the versatility of building with wood. New

products enable increased dimensional stability, higher strength-to-weight

ratios, and greater long-span capabilities.

Keeping wood free from decay and pests is a function of following good

design, construction and maintenance practices—all areas in which

architects can influence the optimum use of wood to create buildings that

benefit owners, occupants and the environment. Wood products generally

have the lowest environmental impacts of the major construction materials

used in the United States.

Course Description

Discuss wood’s advantages in durability and longevity.

Describe strategies for moisture control.

Explain how to maintain wood buildings and maximize

performance.

Explain issues of quality control for wood construction

and the architect’s role in promoting such control.

Learning Objectives

Section 1

DURABILITY AND LONGEVITY OF WOOD

Learning Objectives

Demand Durability

Photo: Stephanie Tracey

6-story mid-rise in the Pacific Northwest

To meet expectations for building

durability, an architect should

consider quality control such as

detailing and maintenance

techniques, as well as how the

material is handled and installed.

Why Specify Wood?

Reasons to Specify: Cost

Availability

Ease of construction

Thermal performance

Aesthetics

Design versatility

Can be used in: Single- and multi-story residential

Schools

Offices

Industrial facilities

Recreational centers

Arenas

Americana at Brand, Glendale, CA. Architect and photo credit: Togawa Smith Martin, Inc.

El Dorado High School, El Dorado, AR. Architect: CADM Architecture, Inc. Photo credit: WI Bell, courtesy WoodWorks.

Design Flexibility

Light weight

Workability

Adaptable in field

Well suited to additions and

retrofits

Can be dismantled and

materials used elsewhere

Maximum Performance

The performance of all building materials relies

on proper detail to prevent bulk water intrusion

and moisture entrapment.

Tamarack Ski Lodge Heavenly Lake Tahoe Ski Resort, South Lake Tahoe, CA. Architect: Collaborative Design Studio. Photo credit: Carrie Compton.

New Product

Development

Richmond Oval roof structure

Architect: Cannon Design.

Engineer: Fast+Epp Structural Engineers.

Photo credit: www.naturallywood.com

New Products: Structural Composite Lumber (SCL)

Glued Laminated Timber (glulam)

Pre-fabricated paneling

Cross-laminated Timber (CLT)

Benefits: Increased dimensional stability

Higher strength-to-weight

ratios

Greater long-span capabilities

Taller wood buildings

New Product

Development

Stadhaus, London. Architect and photo credit: Waugh Thistleton Sent email

Limnologen, Sweden. Architect: Ola Malm Photo credit: Midroc

Forte Building, Melbourne. Designer and photo credit: Lend Lease Send email through Lend Lease site.

Stadthaus, London

Architect: Waugh Thistleton

CLT Limnologen, Sweden

Architect: Ola Malm

Forte Building, Melbourne

Developer: Lend Lease

Exceeding Expectations

Architects can achieve superior

results when they go beyond merely

specifying wood to advocating for

the right design detailing,

construction, and maintenance

techniques that enable wood

structures to deliver decades, even

centuries, of reliable service.

Marselle Condominium, Seattle, WA. PB Architects. Photo credit: Matt Todd.

Branson Convention Center, Branson, MO. Architect: tvsdesign Atlanta, GA. Photo by Brian Gassel/tvsdesign.

Longevity

Wood

63% of wood buildings 50+ years old

Majority of wood buildings were 75+ years old

Concrete

Half of concrete buildings were 26-50 years old

1/3 of concrete buildings lasted 50+ years

Steel

80% of steel buildings were <50 years old

½ of steel buildings were less than <25 years old

Study of buildings demolished between 2000 and 2003 in Minneapolis/St. Paul

found no significant relationship between the structural system and building life.

Wood buildings had the longest life span.

Service Life

Potential threats to a long service life, such as

moisture and insects, can be controlled with pressure-

treated or naturally durable wood species.

Western red cedar's unique properties make it ideal for weather-resistant applications. Photo credit: KK Law

Pressure Treated Wood

EPA approved preservatives

AWPA standards

WWPI CheckMark program identifies code

compliant products and Best Management

Practices for treated wood in sensitive

environments

“The longer the wood lasts the lower the costs of repair or replacement.”

Dallin Brooks, WWPI

Section 2

MOISTURE CONTROL

Branson Convention Center, Branson, MO. Architect: tvsdesign Atlanta, GA. Photo credit: Brian Gassel.

Natural Moisture Levels

Wood and water are typically

compatible

Wood can absorb and

release moisture

If buildings are properly

constructed, wood performs

well in all types of climates

90% of North American

homes are built with wood

“All materials have challenges when it comes to moisture; however,

when moisture is managed properly, wood exceeds expectations.”

Cheryl Ciecko ALA, AIA, LEED AP

Wood shrinks or swells as its moisture content changes, but only when water is taken up or given off from the cell walls. Photo credit: Canadian Wood Council and Canada Wood

Moisture content (MC) measures

how much water is in a piece of

wood relative to the wood itself.

MC <19% = Dry Wood

MC > 28% = Fiber Saturation

Fiber saturation is the point at which

cell walls are holding as much water

as they can.

Additional water will go to the cell

cavity where decay and fungi can

utilize it.

Moisture Content

North America rain exposure zones. Courtesy, American Wood Council

The fiber saturation point is also the limit for

wood shrinkage

Wood only shrinks and swells when it changes

moisture content below 28%

Expansion and shrinkage occurs in the

dimension perpendicular to the growth rings:

‒ Plates

‒ Band joists

Wood Shrinkage

Photo credit: KK Law

Longitudinal shrinkage is less significant, i.e. wall studs

Natural movement of wood is not problematic with proper

design and construction but can become critical for wood

buildings 6 stories and higher

Every 4% change in MC = 1% change in horizontal

members

Engineered wood is drier and has lower shrinkage

coefficients than lumber, but are more susceptible to water

absorption

Shrinkage

MC of wood will stabilize in an environment

with consistent temperature and relative

humidity:

‒ Interior wood at 8-14% MC

‒ Exterior wood at 12-18% MC

This allows wood to perform its inherent

humidity control function, releasing moisture

in dry conditions and absorbing moisture

when the surrounding air becomes humid

Humidity Control

Start with dry wood and prevent

moisture intrusion during

construction and in building

service

Shrinkage will have occurred prior

to purchase – MC drops from 28%

to 19%

Look for S-DRY or KD stamp

Dry Lumber

Moisture loads must be accounted for and balanced in

building envelope design

Function of: ‒ Climate

‒ Surroundings

‒ Type of building

Managing Moisture Controls: ‒ Swelling

‒ Shrinkage

‒ Pests

‒ Decay

Moisture Loading

Photo credit: Stephanie Tracey

Exterior sources of moisture: Rain

Wind-driven rain

Snow

Irrigation systems

Water vapor from outdoor air

Sources of Water

Studies have found that a 4-member family can

generate 10 gallons of water vapor per day.

Interior sources of moisture: Building occupants

Poor building envelope detailing

Air leaks

Plumbing failures

Poor ventilation

Poor thermal design

Primary objective of addressing moisture loads: ‒ Keep water from entering building envelope

‒ Balance relative humidity of indoor air with building

Moisture control achieved by following the 4 Ds: ‒ Deflection

‒ Drainage

‒ Drying

‒ Durability

4 Ds of Moisture Control

Deflection

Rain deflection prevents rainwater from penetrating a wall and

roof skin and entering the building envelope

Use these design features:

‒ Pitched roofs

‒ Overhangs

‒ Flashing

‒ Rainscreen

Drainage

Drainage allows water that penetrates

the cladding, roof shingles or other

surfaces to flow along a water-resistant

plane to exit building envelope.

Rainscreen. www.buildinggreen.com 12/13/12

Drying

In properly designed building

envelope assemblies, water will

evaporate and the resulting

vapor will go through the

assembly’s outer layers,

providing vapor permeability has

been designed into the building

envelope assemblies.

Mechanism by which building envelope assemblies remove accumulated

moisture by venting and vapor diffusion.

Photo credit: KK Law

Exterior wall assemblies

must be designed to allow

sufficient drying to either the

exterior or the interior, and

the permeability of cladding,

moisture barrier, vapor

barrier and interior finish

materials will greatly affect

the wall’s overall drying

potential.

Drying

Durability

“Wood is not only our most valuable

renewable resource, it is also prized as

a versatile structural material. Its use

in construction affects the environment

in ways that are not always obvious,

such as reducing the effects of climate

change by storing carbon.”

Carol Clausen, US Forest Service

Forest Products Laboratory

Richmond Olympic Oval, Vancouver, BC. Architect: Cannon Design Photo credit: KK Law

Understanding the conditions

under which wood used in

buildings breaks down is a first

step in interrupting the process of

decay and preventing wood

deterioration.

Fungi Control

Overhangs protect end-grain of beams from moisture. Photo credit: Canadian Wood Council and Canada Wood

Decay fungi deteriorates wood

Staining fungi gives wood a blue stain deep into interior of tree

Mold & Staining fungi feed off wood’s free water and sugars but

don’t impair strength

Mold spores: ‒ Thrive in moisture & humid air

‒ Contribute to poor air quality

‒ Signal deficiency in a building’s moisture management program

MC <19% eliminates chance of mold growth

MC > 30% increases risk of mold

Most fungi grow fastest in 60-800 F range

Fungi Control

Section 3

MAXIMIZING PERFORMANCE

Campus Services Building, Western Washington University, Bellingham. Zervas Group Architects. Photo credit: Nic Lehoux

Materials Handling

During Construction

Materials handling during construction results in long-term

performance of wood and the building envelope.

“While an architect’s role typically does not extend beyond

mere specification of wood to materials handling and

construction practices, it would be ideal if architects did

provide recommendations on how wood should be treated

during construction.”

Paul Morris, Research Leader - Durability

and Building Enclosure, FPInnovations

During Construction:

Time delivery of wood close to

installation date

Provide clean, well-drained material

storage area

Inspect wood wrappers – replace with

heavy tarp if faulty

Store 6-8” off ground away from

ponding water

Place tarp or gravel pad on ground if

soil is wet

Storage area free of vegetation

Weather Protection

Weather Protection

If lumber has been significantly wetted during construction,

schedules should allow for drying of framing and sheathing

materials to19% or below.

After Installation:

Protect structure from rain and other sources of bulk water

Install roof covering and moisture barriers

Ventilate materials in building envelope before installing insulation

Avoid premature application of interior membranes (gypsum)

Allow wood framing to acclimate and lower its moisture content

Insects can cause significant property

damage to wood, fixtures and fittings,

furniture, cardboard boxes and books:

‒ Termites

‒ Carpenter ants

‒ Powder post beetles

Subterranean termites cause most damage

Total eradication is unfeasible so must

contain existing insect populations and limit

risk to buildings

Combat termites with 6-S Strategy

Termite Control

S-6 Suppression

Reducing termite populations in a particular area and

preventing spread to new areas.

Methods:

Locating and destroying

termite colonies

Burning infested wood

Heat treating reclaimed

lumber

S-6 Site Management

Careful site preparation and cleanup reduces

potential for termite infestation.

Methods:

Remove tree stumps and

buried wood from site

Remove construction debris

that contains cellulose

Drain water away from

building

Remove wooden concrete

formwork

S-6 Soil Barriers

There should be no contact between the building

woodwork and the soil or fill material.

Methods:

Exterior woodwork 6” above ground

Pressure treat any wood in contact with soil

Establish a chemical barrier between the soil and wood

S-6 Soil Barriers

Because of environmental concerns, chemical barriers in

many instances have been replaced by physical barriers.

Physical Barriers:

Precisely sized sand or crushed material beneath slab foundations

Termite mesh wraps foundation

Termiticide-impregnated membranes

Termiticidal bait systems

Slab & Foundation Details

Slabs and foundation walls should be designed to inhibit the entry of termite

and facilitate inspection of shelter tubes.

Methods:

Cap CMUs or double walls with concrete

or masonry

Keep exterior slab edges and foundation

walls free of cladding for height of

150mm from finished soil level

Terminate exterior insulation and

drainage batts 150mm below cladding

Cladding height should allow for

landscaping

Raise untreated wood from ground Height above ground of non-treated wood elements. 1Based on the U.S. model building codes and American Forest & Paper Association recommendations.

For many decades, preservative treated

framing has been successfully used to deter

termites

Wood products are also treated with borate, a

water soluble chemical that is benign to

humans, but kills insects that feed on it

Structural Protection

.

While suitable for framing and sheathing, borate

treated wood should not be used for outdoor

applications such as decks and porches since

the chemicals are water-soluble

A good quality coating, such as a three-coat film-

forming finish, prevents the borate from moving

out of the wood

Consult the American Wood Protection

Association (AWPA) standards for borate and

other preservative treatments under specific

conditions and types of termites

Structural Protection

Pressurized

‒ Combination of pressure and vacuum achieve a deep,

thorough chemical penetration

‒ Odorless water-borne option is paintable and stainable

Non-pressurized

‒ Brush, spray or dip wood in preservative

‒ Most building codes only allow field treatment for

open side of a pressure treated member that

has been cut

Termite resistant species:

‒ Heartwood of redwood

‒ Resinous heartwood of southern pine

‒ Heartwood of yellow cedar

‒ Cypress

‒ Western red cedar

Preservative Treatments

Surveillance

and

Remediation

The type of action taken against termite colonies will vary widely

depending upon the type of termite, the location and the condition

of the building.

Regular inspections are necessary:

‒ Baiting

‒ Chemical fumigation or heat

treatment

Both types of remediation must be done

by licensed contractors

Do not protect against re-infestation

Vigilant monitoring is key to identify food

sources and moisture

Protecting the structure by building with borate-treated wood products is a suppression option in certain situations. Photo credit: Louisianna-Pacific® SmartGUARD™

Wood does not decay merely because it gets wet,

but because fungi and insects consume the wood

fiber as food

Critical to separate untreated wood from the ground

and other moisture sources

Section 2304.11 International Building Code (IBC):

‒ Requires separation from soil level

‒ Addresses decay and termites

‒ Requirements for non-residential construction

applications

‒ Requirements for wood used above ground for

framing, decks, stairs, etc

Code Requirements

Section 4

QUALITY CONTROL

Ballard Library and Neighborhood Center, Seattle, WA. Architect: Bohlin Cywinski Jackson. Photo credit: Structurlam

Buildings That Work

Every building material has its challenges, and despite the

concerns outlined above, wood remains a sound choice in

most situations.

Over the years, experience has shown that the wood buildings

that work over long service lives share several characteristics. Photo credit: Tien Sher Group of Companies

Durability By Design:

Store wood properly, design the building to keep the wood dry and

observe proper maintenance practices

Dry Buildings

Basements

Because they are surrounded by

soil, basement walls are subject to

penetration by tiny amounts of

water through concrete foundation

walls that can cause wood to rot

where exterior foundations don’t

have a solid moisture barrier.

3rd Party Inspection required for all construction

materials covered by building codes

Chapter 17 of IBC, Structural Test and Special

Inspections

‒ Specific construction practices critical to

building’s structural capability

Treated wood products should:

‒ Be consistent with AWPA standards

‒ Carry the quality mark of an accredited

inspection agency of the ALSC

Buildings with

Quality Assurance

Photo credit: Jerry Parks

Untreated, exposed exterior wood should be inspected

5 years after installation and every 2 years after

Sunken paint and surface collapse indicate wood decay

Inspect cladding annually for signs of wear and green

algae stains

Examine through-wall flashing for brown stains

Properly paint or stain exposed wood

Re-treat with termiticides at appropriate intervals

Keep heavy shrubbery away from walls

Point sprinklers away from walls

Keep roof gutters unclogged

Promptly repair plumbing leaks

Regular Maintenance

WOOD:

A Material of Choice

“Wood products generally have the lowest environmental impacts of

the major construction materials used in the United States. In addition,

wood is made from trees, a renewable resource that sequesters

carbon while growing and stores carbon while in-use, thus helping to

mitigate the impacts from climate change.”

Richard Bergman, US Forest Service’s Forest Products Laboratory

Because wood is versatile and long-lasting, it will continue to

be the material of choice for many structural applications.

Keeping wood free from decay and pests is a function of

following good design, construction and maintenance

practices—all areas in which architects can influence the

optimum use of wood to create buildings that benefit owners,

occupants and the environment.

Use these resources for more information about building with wood:

http://designbuildsource.com.au/plans-for-worlds-tallest-timber-skyscraper-revealed-in-

melbourne

http://www.cwc.ca/documents/durability/BP1_MoistureAndWoodFrameBuildings.pdf

http://www.canadawood.cn/english/downloads/pdf/moisture/moisture_english.pdf

http://www.cwc.ca/documents/durability/BP6_ManagingMoistureAndWood.pdf

http://www.cwc.ca/index.php/en/design-with-wood/durability/durability-hazards/about-moisture-

and-wood

http://www.madehow.com/Volume-3/Lumber.html#b

http://www.buildingscience.com/glossary/kilndriedlumber

http://www.canadawood.cn/english/downloads/pdf/moisture/moisture_english.pdf

http://www.fpl.fs.fed.us/documnts/techline/blue-stain.pdf

http://www.fpl.fs.fed.us/documnts/informationalkits/infokit_0011_Mold_A525.pdf

http://www.ncagr.gov/SPCAP/structural/pubs/preconstruction.htm

http://www.awpa.com/standards/organization.asp

http://www.fpl.fs.fed.us/products/publications/several_pubs.php?grouping_id=100&header_id=p

http://www.awc.org/pdf/WCD6.pdf

Resources

Wood in Buildings:

Steps to Durability and Longevity

How to Use Wood to Its Full Potential

PROPERTIES

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