In the building of a house, most of the material used is wood. Some types of wood are used for structural purposes, such as holding up the floor or roof, while others serve as architectural decoration to enhance the beauty of a home. The size, type and quality of wood should be carefully selected to best meet the particular end use, whether functional or aesthetic.
Most people are familiar with the terms hardwood and softwood. These terms are not directly related to the actual physical hardness or softness, or strength of the wood, but rather, refer to the specific species of trees. Hardwood comes from deciduous trees that lose leaves annually at the end of the growing season. Hardwoods include birch, oak, maple, ash, walnut and cherry. Softwood comes from coniferous (evergreen) trees that retain needle-like leaves all year around. Pine, fir, spruce and cedar are some examples of softwoods.
In Canada, all structural lumber used in houses must be graded to the NLGA “Standard Grading Rules for Canadian Lumber.” Graded lumber bears a stamp, showing species, assigned grade, moisture content at time of surfacing, and the responsible grader at the mill of origin. Lumber grades are determined by the number, character and location of imperfections affecting strength, durability and use.
Common imperfections are knots, checks, pitch pockets, shakes and stains. The best lumber grades are free, or practically free, of imperfections. The lower lumber grades contain more imperfections, and should never be used for structural construction, e.g. floor joists, built-up beams, wall studs, roof rafters or trusses.
Dimensioned lumber is defined as surfaced softwood from 2″ through 4″ thick, designed for use as framing members. When lumber sizes are given in imperial dimensions, the actual size is smaller than the designated, or nominal, size. For example, 2″ X 4″ lumber is actually 1 5/8″ X 3 1/2″ because of surfacing operations. In the National Building Code, all lumber sizes are given in metric dimensions. A 2″ X 4″ stud is called a 38 X 89 mm, a 2″ X 6″ stud is a 38 X 140 mm, a 2″ X 8″ stud is a 38 X 184 mm, and so on. Although the metric sizes are precise actual dimensions, they are certainly harder to remember!
Wood from freshly cut trees is called green, or unseasoned, lumber and it may contain up to 90 percent of its weight in water. Seasoned lumber is green lumber that has been dried in the air or a kiln. Unseasoned lumber is surfaced at a larger size than dry lumber to compensate for shrinkage due to moisture loss. When wet, unseasoned wood is used in new, ventilated houses, structural deformities result as the wood dries out, especially over the first year. The visible symptoms of wood shrinkage are plaster cracks, nail pops, gyprock bulges and truss uplift.
Kiln drying reduces moisture content to 14 percent and kills fungi. However, kiln drying is not a permanent control. Dry lumber reabsorbs moisture to maintain an equilibrium with the surrounding air humidity, whether outdoors or indoors. If the house is poorly ventilated, creating a high humidity level, the moisture content of lumber increases, causing warping, loosening of knots, checks, honeycombing, stains, mold, rot and even collapse in the worst cases. Kiln-dried wood, combined with an air exchanger, is the safest and most maintenance-free type of house construction.
Wood permanently exposed to high levels of outdoor moisture should be pressure-treated with preservative to prevent damage from rot and insects – the two main causes of wood damage. Wood in contact with, or closer than 150 mm to the ground should be pressure-treated, including exterior wall sill plates. Ends of wood structural members bearing on masonry or concrete also require treatment. Regular wood, with surface applied preservative should not be embedded in the ground as it is not sufficiently impregnated. Pressure-treated wood can never be exposed indoors because of chemical outgassing.
The most important thing to remember about using wood for structural purposes, is that the material has physical limitations. The National Building Code contains maximum span tables for the various species and grades of wood. All wood members should be sized and spaced according the loading requirements to ensure safe and sound construction.
WOOD ROT
Wood rot and decay develop when moisture is present. Rot is caused by fungi that feed on wood. Fungi, often seen as mushrooms, are a lower plant form without chlorophyll. They produce microscopic spores spread by wind and water. The spores infect wood that has died and develop into a fungus under certain environmental conditions.
In order to survive, fungi require oxygen, favorable temperatures and a wood moisture content of at least 20 percent. This is why the National Building Code stipulates that lumber used in new houses have a maximum moisture content of 19 percent.
Fungi are the main cause of damage to structural members. Under high moisture conditions, decay begins to develop within six months to two years. Eventually, affected wood subject to a structural load will fail and collapse.
Rot cannot attack dry or pressure-treated wood. However, even kiln-dried wood will reabsorb moisture in a humid house. An air exchanger is the best method for maintaining a dry and rot free house.
