For many years, the design values of Canadian dimension lumber were determined by testing small clear samples. Although this approach had worked well in the past, there were some indications that it did not always provide an accurate reflection of how a full-sized member would behave in service.

Beginning in the 1970s, new data was gathered on full-size graded lumber, known as in-grade testing. In the early 1980s, the Canadian lumber industry conducted a major research program through the Canadian Wood Council Lumber Properties Program for bending, tension and compression parallel to grain strength properties of 38 mm thick (nominal 2 in) dimension lumber of all commercially important Canadian species groups. The Lumber Properties Program was conducted as a cooperative project with the US industry with the goal of verifying lumber grading correlation from mill to mill, from region to region, and between Canada and the United States.

The in-grade testing program involved testing thousands of pieces of dimension lumber to destruction in order to determine their in-service characteristics. It was agreed that this testing program should simulate, as closely as possible, the structural end use conditions to which the lumber would be subjected to.

After the test samples were conditioned to approximately 15 percent moisture content, they were tested under short- and long-term loading in accordance with ASTM D4761. Lumber samples in three sizes; 38 x 89 mm, 38 x 184 mm and 38 x 235 mm (2 x 4 in, 2 x 8 in, and 2 x 10 in), were selected across the Canadian growing regions for the three largest-volume commercial species groups; Spruce-Pine-Fir (S-P-F), Douglas Fir-Larch (D.Fir-L) and Hem-Fir. Select Structural, No.1, No.2, No.3, as well as light framing grades, were sampled in flexure. Select Structural, No.1 and No.2 grades were evaluated in tension and compression parallel to grain. Several lesser-volume species were also evaluated at lower sampling intensities.

The in-grade testing resulted in new relationships between species, sizes and grades. The dimension lumber database of results was examined to establish trends in bending, tension and compression parallel to grain property relationships as affected by member size and grade. These studies provided a basis for extending the results to the full range of dimension lumber grades and member sizes described in CSA O86. In Canada, both the CSA O86 and the National Building Code of Canada (NBC) have adopted the results from the Lumber Properties Program. The data has also been used to update the design values in the United States.

The scientific data resulting from the Lumber Properties Program demonstrated:

• close correlation in the strength properties of visually graded No.1 and No.2 dimension lumber;
• good correlation in the application of grading rules from mill to mill and from region to region; and
• a decrease in relative strength as size increases (i.e. size effect) – for example the unit bending strength for a 38 × 89 mm (2 x 4 in) member is greater than for a 38 × 114 mm (2 x 6 in) member.

Following the testing program, the consensus-based ASTM D1990 standard was developed and published. Data for bending, tension parallel to grain, compression parallel to grain, and modulus of elasticity continue to be analyzed in accordance with this Standard.

Unlike visually graded lumber where the anticipated strength properties are determined from assessing a piece on the basis of visual appearance and presence of defects such as knots, wane or slope of grain, the strength characteristics of machine stress-rated (MSR) lumber are determined by applying forces to a member and actually measuring the stiffness of a particular piece. As lumber is fed continuously into the mechanical evaluating equipment, stiffness is measured and recorded by a small computer, and strength is assessed by correlation methods. MSR grading can be accomplished at speeds up to 365 m (1000 ft) per minute, including the affixing of an MSR grade mark. MSR lumber is also visually checked for properties other than stiffness which might affect the suitability of a given piece. Given that the stiffness of each piece is measured individually and strength is measured on select pieces through a quality control program, MSR lumber can be assigned higher specified design strengths than visually graded dimension lumber.

For further information, refer to the following resources:

Canadian Lumber Properties (Canadian Wood Council)

ASTM D1990 Standard Practice for Establishing Allowable Properties for Visually-Graded Dimension Lumber from In-Grade Tests of Full-Size Specimens

ASTM D4761 Standard Test Methods for Mechanical Properties of Lumber and Wood-Based Structural Materials

National Lumber Grades Authority (NLGA)

The durability of wood is often a function of water, but that doesn’t mean wood can never get wet. Quite the contrary, wood and water usually live happily together. Wood is a hygroscopic material, which means it naturally takes on and gives off water to balance out with its surrounding environment. Wood can safely absorb large quantities of water before reaching moisture content levels that will be inviting for decay fungi.

Moisture content (MC) is a measure of how much water is in a piece of wood relative to the wood itself. MC is expressed as a percentage and is calculated by dividing the weight of the water in the wood by the weight of that wood if it were oven dry. For example, 200% MC means a piece of wood has twice as much of its weight due to water than to wood. Two important MC numbers to remember are 19% and 28%. We tend to call a piece of wood dry if it is at 19% or less moisture content. Fiber saturation averages around 28%.

Fiber saturation is an important benchmark for both shrinkage and for decay. The fibers of wood (the cells that run the length of the tree) are shaped like tapered drinking straws. When fibers absorb water, it first is held in the cell walls themselves. When the cell walls are full, any additional water absorbed by the wood will now go to fill up the cavities of these tubular cells. Fiber saturation is the level of moisture content where the cell walls are holding as much water as they can. Water held in the cell walls is called bound water, while water in the cell cavities is called free water. As the name implies, the free water is relatively accessible, and an accessible source of water is one necessity for decay fungi to start growing. Therefore, decay can generally only get started if the moisture content of the wood is above fiber saturation. The fiber saturation point is also the limit for wood shrinkage. Wood shrinks or swells as its moisture content changes, but only when water is taken up or given off from the cell walls. Any change in water content in the cell cavity will have no effect on the dimension of the wood. Therefore, wood only shrinks and swells when it changes moisture content below the point of fiber saturation.

Like other hygroscopic materials, wood placed in an environment with stable temperature and relative humidity will eventually reach a moisture content that yields no vapor pressure difference between the wood and the surrounding air. In other words, its moisture content will stabilize at a point called the equilibrium moisture content (EMC). Wood used indoors will eventually stabilize at 8-14% moisture content; outdoors at 12-18%. Hygroscopicity isn’t necessarily a bad thing – this allows wood to function as a natural humidity controller in our homes. When the indoor air is very dry, wood will release moisture. When the indoor air is too humid, wood will absorb moisture.

Wood shrinks/swells when it loses/gains moisture below its fiber saturation point. This natural behaviour of wood is responsible for some of the problems sometimes encountered when wood dries. For example, special cracks called checks can result from stresses induced in a piece of wood that is drying. As the piece dries, it develops a moisture gradient across its section (dry on the outside, wet on the inside). The dry outer shell wants to shrink as it dries below fiber saturation, however, the wetter core constrains the shell. This can cause checks to form on the surface. The shell is now set in its dimension, although the core is still drying and will in turn want to shrink. But the fixed shell constrains the core and checks can thus form in the core. Another problem associated with drying is warp. A piece of wood can deviate from its expected shape as it dries due to the fact that wood shrinks different amounts in different directions. It shrinks the most in the direction tangential to the rings, about half as much in the direction perpendicular to the rings, and hardly at all along the length of the tree. Where in the log a piece was cut will be a factor in how it changes shape as it shrinks. One advantage of usingdry lumber is that most of the shrinkage has been achieved prior to purchase. Dry lumber is lumber with a moisture content no greater than 19%; wood does most of its shrinking as it drops from 28-19%. Dry lumber will have already shown its drying defects, if any. It will also lead to less surprises in a finished building, as the product will stay more or less at the dimension it was upon installation. Dry lumber will be stamped with the letters S-DRY (for surfaced dry) or KD (for kiln dry).

Another way to avoid shrinkage and warp is to use composite wood products, also called engineered wood products. These are the products that are assembled from smaller pieces of wood glued together – for example, plywood, OSB, finger-jointed studs and I-joists. Composite products have a mix of log orientations within a single piece, so one part constrains the movement of another. For example, plywood achieves this crossbanding form of self-constraint. In other products, movements are limited to very small areas and tend to average out in the whole piece, as with finger-jointed studs.

Non-Pressure Treated Wood

For most treated wood, preservatives are applied in special facilities using pressure. However, sometimes this isn’t possible, or the need for treated wood was not apparent until after construction or building occupancy. In those cases, preservatives can be applied using methods that do not involve pressure vessels.

Some of these treatments can only be done by licensed applicators. When using wood preservatives, as with all pesticides, the label requirements of the Pest Management Regulatory Agency (in Canada) or the EPA (in the USA) must be followed.

Five categories of non-pressure treatments

Treatment during Engineered Wood Product Manufacture

Some engineered wood panel products, such as plywood and laminated veneer lumber (LVL) are able to be treated after manufacture with preservative solutions, whereas thin strand based products (OSB, OSL) and small particulate and fibre-based panels (particleboard, MDF) are not. The preservatives must be added to the wood elements before they are bonded together, either as a spray on, mist or powder.

Products such as OSB are manufactured from small, thin strands of wood. Powdered preservatives can be mixed in with the strands and resins during the blending process just prior to mat forming and pressing. Zinc borate is commonly used in this application. By adding preservatives to the manufacturing process it’s possible to obtain uniform treatment throughout the thickness of the product. 

In North America, plywood is normally protected against decay and termites by pressure treatment processes. However, in other parts of the world insecticides are often formulated with adhesives to protect plywood against termites.

Surface pre-treatment

This is anticipatory preservative treatment applied by dip, spray or brush application to all of the accessible surfaces of some wood products during the construction process. The intent is to provide a shell of protection to vulnerable wood products, components or systems in their finished form. One example would be spraying house framing with borates for resistance to drywood termites and wood boring beetles in some cases. Such treatments may also be applied to lumber, plywood and OSB to provide additional protection against mould growth.

Sub-surface pre-treatment (Depot treatment)

This is preservative treatment applied at discrete locations, not to the entire piece, during the manufacturing process or during construction. The intent is to pro-actively provide protection only to the parts of the wood product, component or systems that might be exposed to conditions conducive to decay. One example would be placing borate rods into holes drilled in the exposed ends of glulam beams projecting beyond a roof line.

Supplementary treatment

This is preservative treatment applied at discrete locations to treated wood in service to compensate for either incomplete initial penetration of the cross section, or depletion of preservative effectiveness over time. The intent is to boost the protection in previously-treated wood, or to address areas exposed by necessary on-site cutting of treated wood products. One example would be the application of a ready-made bandage to utility poles that have suffered depletion of the original preservative loading. Another example is field-cut material for preserved wood foundations.

Remedial treatment

This is preservative treatment applied to residual sound wood in products, components or systems where decay or insect attack is known to have begun. The intent is to kill existing fungi or insects and/or prevent decay or insects from spreading beyond the existing damage. One example would be roller or spray application of a borate/glycol formulation on sound wood left in place adjacent to decayed framing (which should be cut out and replaced with pressure-treated wood).

Formats of non-pressure treatments

Non-pressure treatments come in three different forms: solids, liquids/pastes, and fumigants. Unlike pressure-treatment preservatives, which rely on pressure for good penetration, these rely on the mobility of the active ingredients to penetrate deep enough in wood to be effective. The active ingredients can move in the wood via capillarity or can diffuse in water and/or air within the wood. This mobility not only allows the active ingredients to move into the wood but can also allow them to move out under certain conditions. This means the conditions within and around the structure must be understood so the loss of preservative and consequent loss of protection can be minimized. Borates, fluorides and copper compounds are particularly suitable for use as solids, liquids and pastes. Methyl isothiocyanate (and its precursors), methyl bromide, and sulfuryl fluoride are the only widely used fumigant treatments. Methyl bromide was phased out, except for very limited uses, in 2005.

Solids

The major advantage of solids in these applications is that they maximize the amount of water-soluble material that can be placed into a drilled hole, due to the high percentage of active ingredients contained in commercially-available rods. The major disadvantage is the requirement for sufficient moisture and the time needed for the rod to dissolve. The earliest and best-known solid preservative system is the fused borate rod, originally developed in the 1970s for supplementary and remedial treatment of railroad ties. These have since been used successfully on utility poles, timbers, millwork (window joinery), and a variety of other wood products. A mixture of borates is fused into glass at extremely high temperatures, poured into a mould and allowed to set. Placed into holes in the wood, the borate dissolves in any water contained in the wood and diffuses throughout the moist region. Mass flow of moisture along the grain may speed up distribution of the borate. Secondary biocides such as copper can be added to borate rods to supplement the efficacy of the borates against decay and insects. While all preservatives should be treated with respect, many users feel more comfortable dealing with borate and copper/borate rods because of their low toxicity and low potential for entry into the body.

Fluorides are also currently available in a rod form. The rod is produced by compressing sodium fluoride and binders together, or by encapsulation in a water-permeable tubing. Fluorides diffuse more rapidly than borates in water and may also move in the vapour phase as hydrofluoric acid.

Zinc borate (ZB) is a powder used to protect strand-based products. It is blended with the resins and stands during the manufacturing processes for OSB and other strand based products becomes well dispersed throughout. Zinc borate has very low water solubility and can protect strand based products from decay and termites.

Liquids, Pastes and Gels

Liquids can be sprayed or brushed on to surfaces, or poured or pumped into drilled holes. Pastes are most often brushed or troweled on, then covered with polyethylene-backed kraft paper creating a “bandage.” Pastes can also be packed into drilled holes or incorporated into ready-to-use bandages for wrapping around poles. Borates and fluorides are commonly used in these formulations because they diffuse very rapidly in wet wood. Copper moves more slowly because it reacts with the wood. For dryer wood, glycols can be added to borate formulations to improve penetration. Over-the-counter wood preservatives available for brush application are based on either copper naphthenate (a green colour), or zinc naphthenate (clear). Both are dissolved in mineral spirits-type solvents. In addition, water-borne borate/glycol formulations can also be purchased over-the-counter as roll-on liquids.

Fumigants

These treatments are typically delivered as liquids or solids; they change to a gas upon exposure to air, and become mobile in the wood as a gas. Some solid and liquid fumigants are packed in permeable capsules or aluminum tubes. Methyl isothiocyanate (MIT), and chemicals that produce this compound as they break down, are used for utility poles and timbers. This compound adsorbs to wood and can provide several years of residual protection. Sulfuryl fluoride and methyl bromide are used for tent fumigation of houses to eradicate drywood termites.

Repairing Cuts in the Treated Shell

Pressure-treated wood in the ground can undergo significant internal decay within just six or seven years if cuts, bolt holes and notches are not brush treated with a field-cut preservative. Common over-the-counter agents for this purpose include copper naphthenate (a green colour), or zinc naphthenate (clear). Both are dissolved in mineral spirits-type solvents. Other brush-on agents include water-borne borate/glycol formulations which can also be purchased at building supply outlets.

Forgetting this critical step will almost certainly shorten the life span of the product and will void any warranties on the product. Although brush-on application of wood preservatives isn’t nearly as effective as pressure-treatment, the field-cut preservatives are usually applied to the end grain, whereby the solution will soak in further than if applied to the side grain.

In FPInnovations’ field tests of these preservatives, copper naphthenate performed best. Zinc naphthenate (2% zinc), which is colourless, was not as effective but may be suitable for above-ground applications where the decay hazard is lower and if the dark green colour of copper naphthenate is undesirable. Note that the dark green of the copper-based product will fade after a few years.