Building Performance Series No.

Building Performance Bulletin

For more information and links to helpful and related
information sources, visit the Wood Durability web
site at www.durable-wood.com. A downloadable
PDF version of this publication is also available from
the site.

This publication was produced in collaboration with

Forintek Canada Corporation, Canadas Wood
Products Research Institute.

2004 Copyright
Canadian Wood Council
Ottawa, Ontario, Canada
25M-03-04

Design and production by:

Accurate Design & Communication Inc.
Printed by:
Versant Inc.

Introduction and Background

Wood, a long-lasting, economical, and renewable
resource, is the building material of choice in North
American housing. This is largely due to the proven
performance of properly designed and built wood
frame buildings that have provided strong and
lasting housing for a multitude of people.

Figure 1:
Dimensional Changes in Wood

Although wood can withstand much abuse, it

needs to be stored and handled properly to perform
according to expectations. Managing moisture in
structural wood products is essential in order to
control swelling and shrinkage and prevent problems
associated with mold or decay.

Wood and Moisture

Wood is a semi-porous material. It absorbs moisture
and swells when conditions are wet or humid and
releases moisture during dry periods. The amount of
water that the wood contains at any time is called
Moisture Content, (MC). This value is expressed as
a ratio of the weight of water in the wood to the
weight of the oven-dry wood. For example, if a
board has 25% MC, one fifth of the boards weight
is water.
When wood reaches a balance with the surrounding
atmosphere and does not absorb or release moisture,
it is said to have reached its Equilibrium Moisture
Content.
Wood changes dimensionally with changes in moisture content. Lumber shrinkage is much larger across
the grain (radial or tangential shrinkage), and is
negligible parallel (longitudinal) to the grain, as
shown in Figure 1.
The moisture content of freshly sawn lumber is much
higher than the equilibrium moisture content wood
will reach under normal service conditions. This is
why lumber is generally dried before being shipped
to users. Drying lumber increases its structural
performance, greatly reduces the chances of fungal
growth such as mold, stain, or decay, and reduces
swelling and shrinkage, which could lead to warping
or other problems.

Rule of thumb for every 4% change in MC,

there is approximately 1% change in the
radial/tangential direction.

Figure 2:
Grade Stamp
Grading agency

000

S-P-F

Species group

Assigned grade

S-DRY

Moisture content
Mill designation

The S-Dry label on a North American grade-stamp

for construction lumber indicates that the lumber
was surfaced at a moisture content of 19% or less.

Managing Moisture and Wood

Panel products such as plywood or OSB (Oriented

Strand Board) are at a low moisture content
at the time of manufacturing (see Table 1).
Engineered wood products such as I-joists also
tend to have moisture content lower than S-Dry
lumber, which keeps them very dimensionally
stable as long as they are dry.
Table 1: Typical Moisture Content of Wood
Products at Time of Manufacture

Product

Moisture Content

Lumber S-Dry

19% or less

Lumber S-Green

Usually over 19%

Panel products (OSB, plywood)

4-8%

I-Joists

4-16%

Wood that is Not Kept Dry

Figure 3: Potential Sources of Moisture

Wood that is in contact with water over a long

period of time can become discolored for various
reasons and may grow fungi, including stains,
molds and decay.

Discoloration Is it due to Moisture?

If wood stays wet, it can take on a darker appearance due to the water. This dark appearance will
disappear when the wood dries again, but if it stays
wet too long, it may become permanently discolored.
Discoloration of the wood may be due to moisture
issues or other causes.
Potential causes of discoloration in wood include:

Staining fungi (usually bluestain that goes

deep into the wood)

Weathering (gives wood a grey colour)

Decay fungi (the wood may be weaker)

Mold (may appear in various colours - grey,

red, brown, black, white yellow)

Dirt (will rub off when dry)

Iron stain (caused by steel straps, machinery from

transportation or manufacturing)

Yeasts (black and slimy to the touch)

Miscellaneous chemical stains (caused by

naturally occurring wood chemicals)

Of these, only decay will affect the structural performance of the wood; other forms of discoloration
will not weaken the wood. In the case of staining
fungi, the staining may have occurred in the tree
before the wood was sawn into lumber, although
storing wet lumber over a long period of time may
also cause staining of the wood.
Decay fungi are different from molds and staining
fungi. Molds and stains feed off the free water and
the sugars stored inside the wood but dont affect
the wood strength. On the other hand, decay fungi
will gradually eat the wood if it remains wet for too
long. Although conditions for mold and stain growth
are also favorable to decay, the presence of mold
or stain does not indicate that there is decay. Still,
excess water, as well as moderate to warm temperatures that sustain mold growth, could ultimately
lead to decay, if the wood is not quickly dried.

Building Performance Series No. 6

Figure 4: Moisture Content vs. Humidity and the Risk of Mold Growth

25
Danger
Zone

Moisture Content (%)

20
Safe Zone
15
Intermediate
Zone
EMC at 21C (70F)

10

0
10

20

30

40

50

60

70

80

90

100

Humidity (%)

Molds are everywhere; therefore, they have the

potential to thrive where there is moisture. They
have received much public attention after having
been identified as a contributor to poor indoor air
quality, along with dust, mites and other factors.
Molds can grow on many surfaces, including wallboard, paint, shower curtains, ceramic tile and grout,
as well as wood. Although there are still many
unknown factors regarding molds and their effects
on health, one thing is for sure: molds indicate a
problem with moisture management in the building.
Water can come from various sources and the way
to solve the problem is to remove these sources.
Ensuring that materials are dry during construction
is a good practice to reduce the chances of problems
down the road.

Wall construction practices have evolved. Air

tightness is emphasized and many new materials
are available for wall construction. Though there are
typically provisions included in the wall design for
the building envelope to dry, it is good practice to
ensure that building materials are dry (ie. 19% MC
or less) at the time of enclosing the wall. Refer to the
Canadian Mortgage and Housing Corporation Best
Practice Guides for a discussion on the proper
designing of wood frame envelopes.
Most building codes require that wood be dried to
19% MC or less. This moisture content is sufficiently
low to virtually eliminate the chances of mold
growth on wood products as well as minimize
dimensional changes in the members.

Potential causes of water entry in the building include:

Plumbing failure

Poor thermal performance

Poor detailing of the envelope (leaks)

Poor ventilation

Managing Moisture and Wood

Moisture and Construction

Wood used in Buildings
In normal applications, wood will undergo changes in
moisture content throughout its life in relation to seasonal changes in relative humidity and temperature.
For example, in more humid regions, wood will have

an average moisture content value that is higher than

in drier regions. In fact, wood is a good moisture sink
and will help equilibrate the humidity fluctuations in
the building. Figure 4 illustrates how wood comes into
equilibrium with relative humidity and temperature in
a building envelope. The Figure also indicates that

The risk of mold increases with higher moisture

content and relative humidity, particularly when
these conditions are sustained for longer time
periods;

Mold can start growing where humidity levels

exceed 80%; although growth is slow, it can
speed up when humidity is higher.

Furthermore, it is worth noting that

Wood moisture content level typically has to be

high (30% and above) for an extended period, in
order for wood to decay;

Most fungi grow fastest in the 60-80F (15-25C)

range; around freezing, they grow very slowly or
not at all.

The following Table lists some examples of expected

long term average MC values for lumber inside the
building envelope in various North American regions.
Table 2: Average Yearly MC for Wood in Various
North American Regions

Region

Average MC Range of Wood (%)

Coastal

8-12

Inland

6-10

Drier climates

5-9

All these values are below 19% moisture content,

which means that wood will slowly keep drying in
service. The time for wood to reach equilibrium with
the surroundings is not critical in such cases, as the
moisture content levels are below those that will support fungal growth. However, changes in moisture
content result in dimensional changes that could lead
to serviceability problems if not considered during
design and construction. Simple construction details,
such as ensuring that materials are compatible and
leaving gaps between sheathing, will accomodate
shrinkage and swelling.
Engineered wood products are manufactured using
dried wood in a controlled environment. As a result,
these products are dimensionally stable. Severe
wetting could compromise quality and dimensional
stability. Panel products and engineered structural
wood products should be protected from moisture.

Wood Outside Buildings

Some outdoor applications such as decks and porches
are more conducive to decay. In such cases, the
wood should be protected by treatment (preservative
treated wood). Otherwise, a naturally durable species,
such as cedar, should be considered. Regardless of
the protection provided, it is good practice to keep
wood away from potential water sources. Should
that not be possible, the structure should be detailed
to ensure that moisture does not accumulate and that
wood will re-dry after wetting.

Figure 5: Design to Accommodate Shrinkage and

Swelling in Building Products

Building Performance Series No. 6

Figure 6:

Potential Sources of Moisture include rain, snow,

ground contact and moisture traps at the interface
between the house and the structure (Note: If wood is
separated from ground by concrete, moisture can still
accumulate unless detailed to drain away from
the wood).

Figure 7: Wrapping at the Mill

Areas of potential
accumulation

Ground
Contact

Site Considerations
Most wood construction products, even those
intended for protected interior use, are often left
outside for a length of time, where they are subject
to picking up moisture from rain, snow, or the
ground. Outdoor storage on the building site should
be minimized to limit potential damage caused by
exposure to moisture. This can often be achieved
by careful planning.

Upon Delivery
Do

Dont

Prepare the material storage area.

The site should be clean, and well-drained

Let water infiltrate through torn wrapper

Examine the state of the wrapper upon delivery

Drop the bundles in puddles or allow them to

be dumped on the ground

Leave intact specialty wrapping on the bundles

Store wood products in basements

Fix or replace damaged wrapper with a good

quality tarp

Lift I-joists in ways they are not designed for;

lift with the web in the vertical position, how they
are designed to take structural loads

Store materials off the ground

Leave the materials in an area that incites theft

Managing Moisture and Wood

Construction Sequence
Do

Dont

Have the material delivered as close to the

installation date as possible

Let the materials sit too long. Materials left exposed

risk water absorption and deterioration, as well
as theft

Protect the materials after they have been installed

by closing the structure quickly (walls/roof)

Hurry to close-in the walls, especially if the

wood is green and/or has been wetted

Provide enough time and ventilation for the materials

inside the wall cavities to dry to an acceptable level
before installing the vapour barrier, when elevated
moisture levels have been detected

Storage and Handling Techniques

Figure 8: Drying need of Construction

Vertical wood
members can
dry relatively
quickly after
wetting

Horizontal wood components, however,

need more time to dry. Temporary protection
such as tarps may be required.

Storage Area

Ensure storage area is well-drained and

clear of obstacles, debris, and vegetation
(Vegetation can harbour insects, prevent good
ventilation and keep the air moist).

If the soil is wet or poorly drained, place a polyethylene tarp or a temporary gravel pad on the
ground below the supports for the materials.

Storage Off Ground

Keep packages of lumber, trusses, I-Joists and

panels 6 to 8 inches off the ground (dry wood
in contact with the ground will quickly take
up water).

Store bundles as level as possible.

Building Performance Series No. 6

Wrapping
Figure 9: Well protected bundles

Keep wrappers on the bundles until they are

ready to be used.

Re-cover using a waterproof tarp if the wrapping is damaged. WARNING: Tarps develop
holes easily on lumber corners or other
points of stress.

The tarp should not be tucked under the pile

or wrapped tightly.

Make sure the tarp covering extends beyond

the bundles.

Beware of polyethylene tarps; though they are

efficient at protecting the wood from soil moisture, their use as a wrapping material may keep
the moisture trapped inside the pile.

Figure 10: Storage off-ground

If Wood is Re-Wetted
Kiln dried lumber and panels have some innate
degree of water repellence and are not easily
wetted by a small amount of rain. Wood gets
wet and dries from the outside in. In cases of
wetness, the wood will dry quickly on the surface
and limit mold growth.
Wood products are most commonly stored in
solid piles. With tidy piles of panel products, the
top panel can retain the moisture but most of
the water runs off and does not readily penetrate
into the pile. Piled lumber can be more vulnerable as rainwater or snowmelt percolates in
between the lumber pieces and may wet several
layers. Such water is held between the boards as
a film that wets the surfaces. Wood in this situation could remain wet long enough to develop
surface mold or wood-staining fungi. For longer
term storage, lumber should be stickered.

Managing Moisture and Wood

Handling Green Lumber

A water-based formulation containing a low
level of an antimicrobial chemical is sometimes applied to lumber that will be sold
green. This is to prevent the growth of
staining fungi or molds on the lumber
before it is used. The products used to treat
the wood are registered for commercial use
by the Pest Management Regulatory Agency
in Canada or by the Environment Protection
Agency in the USA. Prior to their use, risk
assessments determine that the products
are safe for use under the conditions given
on the pesticide label.
The chemicals used for treatment are mild,
such as detergent type chemicals or fungicides used on agricultural food products.
Their protection time on green lumber is
therefore limited. The treatment is meant
to give a few months protection against
fungi to cover the time between manufacturing to receiving the wood at the
building site. It is not intended to replace
preservative-treated wood.
Often, by the time green lumber has been
shipped from the sawmill, it is partially airdried. Manufacturers rarely wrap packages
of green lumber. Upon arrival at the building
site, green lumber is best stored indoors.

High Moisture Content Before Installation

In the event that a user would like to dry lumber before
installation, rather than install it green and let it dry in
place, the proper procedure involves stacking the lumber
with stickers (small sticks of dry wood about 3/4 inch
thick by 1/2 inch wide) to provide sufficient air-flow
between the layers of boards.

Place stickers across the pile, no farther than

2 feet apart;

Align stickers vertically with the ones of the row

below so that the weight is transferred directly
to the pile below;

Place first and last stickers as close to the end of the

pile as possible to prevent checking in the lumber

A makeshift roof made up of longer pieces of wood

could be placed on the wood pile to help reduce wetting
and to protect the pieces from the hot sun, which could
cause checking in the uppermost boards. Otherwise, a
sheet of plywood over the pile or a tarp under the top
layer will help shed water and prevent the lower tiers
from getting wet. Weight should also be placed on top
of the pile to prevent the upper pieces of wood from
warping as the wood dries.

Wetting After Installation

For surface-wetted material used in framing, getting the
roof sheathing and roof membrane on quickly and allowing the wood to air dry in place is a common strategy
that works. The lumber will dry out naturally so long as

Figure 11: Piling

10

Building Performance Series No. 6

Clean Up Material Affected by Mold

Recommendations on remedial procedures for materials damaged by water and mold growth are
identified in various publications and articles such as Remedial Procedures for Water Damage in
Buildings by Public Works and Government Services Canada (T. Nathanson, 2002), or New York
Citys Dept, of Health and Mental Hygienes Guidelines on Assessment and Remediation of Fungi
in Indoor Environments. In all cases, the first recommendation is that the source of moisture and
the conditions suitable for mold growth should be eliminated before surface mold is cleaned.
Clean-up measures depend on the extent of material affected by water and/or fungal attack.
In cases involving small amounts of mold, it is generally recommended that molds be removed
from solid or semi-porous materials (such as wood) by cleaning with a water/detergent solution
or wet vacuuming with a HEPA equipped vacuum cleaner, after which the materials should then
be thoroughly dried. Disinfectants or biocides are generally not recommended, since they may
contain respiratory irritants and are not always completely effective in destroying mold organisms.
Good practice also considers the health of workers in cleaning up affected areas. In order
to minimize risk, it is recommended to wear protective equipment such as a N95 particulate
half facemask, rubber gloves and eye protection. Larger areas, however, may need professional
clean up. On-site professional advice is available should there be doubt on the appropriate
course of action.

it is well ventilated and the humidity level of the air

is not high. The latter, of course, depends on the
local weather during the time of construction. In cold
weather, space heaters should be used to help with
drying. The site manager should make sure the framing is dry when the whole building is closed and
sealed, otherwise moisture trapped in the structure
may cause problems, given the tight construction
methods in use today.

A Few Points to Remember

The versatility and long-term performance of wood
products has ensured that traditional and innovative
wood materials will continue to be important to the
North American construction industry. The trends
towards using engineered wood products and the
advances in wall construction places a greater
emphasis on managing moisture in wood.

Managing Moisture and Wood

As a final reminder, here are a few points to remember when handling wood products to ensure that
they remain dry and mold free:

Examine the wrapper on delivery to make sure

that it is fully protecting the wood;

Minimize outdoor on-site storage;

Store wood products in a dry, well drained area;

Keep an intact protective cover on the products

until ready to use;

Allow sufficient time for the materials to dry if

they have been wetted;

Avoid closing in walls that have wet building

materials.

11

References
Remedial Procedures for Water Damage in Buildings.
Tedd Nathanson, Environmental Services, Public Works
and Government Services Canada. 2002
www.pwgsc.gc.ca/rps/aes/docs/pdf/iaq_pub_remedial-e.pdf
Guidelines on Assessment and Remediation of Fungi in Indoor Environments.
New York City Department of Health and Mental Hygiene. 2002
www.ci.nyc.vs/html/doh/html/epi/moldrpt1.html
Wood Frame Envelopes Best Practice Guide, Building Technology.
Canadian Mortgage and Housing Corporation. 1999.

Canadian Wood Council

99 Bank St., Suite 400
Ottawa, ON K1P 6B9
Tel: 1-800-463-5091
Fax: 1-613-747-6264
e-mail: info@cwc.ca