Dimension lumber is solid sawn wood that is less than 89 mm (3.5 in) in thickness. Lumber can be referred to by its nominal size in inches, which means the actual size rounded up to the nearest inch or by its actual size in millimeters. For instance, 38 × 89 mm (1-1/2 × 3-1/2 in) material is referred to nominally as 2 × 4 lumber. Air-dried or kiln dried lumber (S-Dry), having a moisture content of 19 percent or less, is readily available in the 38 mm (1.5 in) thickness. Dimension lumber thicknesses of 64 and 89 mm (2-1/2 and 3-1/2 in) are generally available as surfaced green (S-Grn) only, i.e., moisture content is greater than 19 percent.

The maximum length of dimension lumber that can be obtained is about 7 m (23 ft), but varies throughout Canada.

The predominant use of dimension lumber in building construction is in framing of roofs, floors, shearwalls, diaphragms, and load bearing walls. Lumber can be used directly as framing materials or may be used to manufacture engineered structural products, such as light frame trusses or prefabricated wood I-joists. Special grade dimension lumber called lamstock (laminating stock) is manufactured exclusively for glulam.

Quality assurance of Canadian lumber is achieved via a complex system of product standards, engineering design standards and building codes, involving grading oversight, technical support and a regulatory framework.

Checking and splitting

Checking and splitting Checking occurs when lumber is rapidly dried. The surface dries quickly, while the core remains at a higher moisture content for some time. As a result, the surface attempts to shrink but is restrained by the core. This restraint causes tensile stresses at the surface, which if large enough, can pull the fibres apart, thereby creating a check. Splits are through checks that generally occur at the end of wood members. When a wood member dries, moisture is lost very rapidly from the end of the member. At midlength, however, the wood is still at a higher moisture content. This difference in moisture content creates tensile stresses at the end of the member. When the stresses exceed the strength of the wood, a split is formed. Large dimension solid sawn timbers are susceptible to checking and splitting since they are always dressed green (S-Grn). Furthermore, due to their large size, the core dries slowly and the tensile stresses at the surface and at the ends can be large. Minor checks confined to the surface areas of a wood member very rarely have any effect on the strength of the member. Deep checks could be significant if they occur at a point of high shear stress. Checks in columns are not of structural importance, unless the check develops into a through split that will increase the slenderness ratio of the column. The specified shear strengths of dimension lumber and timbers have been developed to consider the maximum amount of checking or splitting permitted by the applicable grading rule. The possibility and severity of splitting and checking can be reduced by controlling the rate at which drying occurs. This may be done by keeping wood out of direct sunlight and away from any artificial heat sources. Furthermore, the ends may be coated with an end sealer to retard moisture loss. Other actions which will minimize dimension change and the possibility of checking or splitting are:

• specifying wood products that are as close as possible in moisture content to the expected equilibrium moisture content of the end use
• ensuring dry wood products are protected by proper storage and handling

Fingerjoined lumber

Fingerjoined products are manufactured by taking shorter pieces of kiln-dried lumber, machining a ‘finger’ profile in each end of the short-length pieces, adding an appropriate structural adhesive, and end-gluing the pieces together to make a longer length piece of lumber. The length of a fingerjoined lumber is not limited by the length of the log. In fact, the manufacturing process can result in the production of joists and rafters in lengths of 12 m (40 ft) or more. The process of fingerjoining is also used within the manufacturing process for several other engineered wood products, including glued-laminated timber and wood I-joists. The specific term “fingerjoined lumber” applies to dimension lumber that contains finger joints.

Fingerjoining derives greater value from the forest resource by using short length pieces of lower grade lumber as input for the manufacture of a value-added engineered wood product. The fingerjoining process utilizes short off cut pieces of lumber and results in more efficient use of the harvested wood fibre. Fingerjoined lumber can be manufactured from any commercial species or species group. The most commonly used species group from which fingerjoined lumber is produced is Spruce-Pine-Fir (S-P-F).

Design advantages of fingerjoined lumber

Fingerjoined lumber is an engineered wood product that is desirable for several reasons:

• straightness
• dimensional stability
• interchangeability with non-fingerjointed lumber
• highly efficient use of wood fibre

The design and performance advantages of this engineered wood product are its straightness and dimensional stability. The straightness and dimensional stability of fingerjoined lumber is a result of short length pieces of lumber, consisting of relatively straight grain and fewer natural defects, being combined with one another to form a longer length piece of lumber. The grain pattern along fingerjoined lumber becomes non-uniform and random by attaching many short pieces together. This results in fingerjoined lumber being less prone to warping than solid sawn lumber. The fingerjoining process also results in the reduction or removal of strength reducing defects, producing a structural wood product with less variable engineering properties than solid sawn dimensional lumber. The most common use of finger-joined lumber is as studs in shearwalls and vertical load bearing walls.

The most important factor for studs is straightness. Fingerjoined studs will stay straighter than solid sawn dimensional lumber studs when subjected to changes in temperature and humidity. This feature results in significant benefits to the builder and homeowner including a superior building, the elimination of nail pops in drywall and other problems related to dimensional changes. This also makes fingerjoined lumber an ideal candidate for interior nonload bearing partitions.

Finger-joined lumber is also commonly used for flange material in wood I-joists. This application of the product requires the wood fibres and the glued joint to resist long term tension loads when in use. For this reason, fingerjoined lumber used for the manufacture of I-Joists must comply with the requirements of NLGA SPS 1. Wood I-joist manufacturers undertake additional quality assessment procedures during production.

Types of fingerjoined lumber

Canadian fingerjoined lumber is manufactured in conformance with either NLGA Special Products Standards SPS 1, Fingerjoined Structural Lumber, SPS 3, Fingerjoined “Vertical Stud Use Only” Lumber, or SPS 4, Fingerjoined Machine Graded Lumber. In almost all cases, fingerjoined lumber manufactured to the requirements of SPS 1 is interchangeable with solid sawn lumber of the same species, grade and length, and can be used for either horizontal or vertical load bearing applications, such as joists, rafters, columns and wall studs. Fingerjoined lumber manufactured according to SPS 3 can only be used as vertical end-loaded members in compression, e.g., wall studs, where bending and tension loading components do not exceed short term duration and where the moisture content of the wood will not exceed 19% and the temperature will not exceed 50 °C for an extended period of time. SPS 3 lumber is manufactured in section sizes up to 38 x 140 mm (2 x 6), in lengths up to 3.66 m (12 ft). Fingerjoined machine graded lumber manufactured in accordance with SPS 4 can be used for wood I-joist flanges and metal plate connected truss applications. SPS 4 graded fingerjoined lumber designated as “Dry Use Only” shall only be used in applications where the equilibrium moisture content of the lumber is not expected to exceed 19%. Fingerjoined lumber is typically produced from lumber that has no more than 19% moisture content for ease of manufacturing the joint to meet the strict quality control standards. For this reason, fingerjoined lumber is almost always sold as ‘S-Dry’.

There are several different types of adhesives used in the manufacture of fingerjoined lumber. The National Lumber Grades Authority (NLGA) Special Product Standards (SPS) outline what types of adhesives can be used in SPS 1, SPS 3 and SPS 4 fingerjoined lumber as well as the test standards that those adhesives must meet. SPS 1, sometimes referred to as a structural fingerjoint, uses a phenol-resorcinol formaldehyde (PRF) adhesive, similar to what is used in structural panel products or in glued-laminated timber. SPS 3 typically uses a polyvinyl acetate adhesive. Adhesives used in the manufacture of SPS 3 fingerjoined lumber are not suitable for joining wet lumber and therefore only ‘S-Dry’ lumber is utilized in order to ensure a quality joint.

Adhesives used in fingerjoined lumber are designated as either a Heat Resistant Adhesive (HRA) or Non-Heat Resistant Adhesive (Non-HRA). Qualification as an HRA adhesive requires an adhesive to be exposed to elevated temperatures during a standard fire resistance test of a loadbearing fingerjoined stud wall assembly loaded to 100 percent of the wall’s allowable design load. All SPS 1 products must be manufactured using HRA adhesives. SPS 3 products may be manufactured with either HRA or non-HRA adhesives. All SPS 4 products must be manufactured using HRA adhesives.

Structural testing protocols for fingerjoined lumber

The strength of the finger joints is controlled by stipulating the quality of wood which must be present in the area of the joint. For the majority of fingerjoined lumber, the segments between the fingerjoints are visually graded in accordance with the NLGA rules for the lumber grade indicated on the grade stamp. Near the fingerjoints, more restrictive visual limits are generally imposed. The structural properties are confirmed through a comprehensive quality assurance program with independent third party verification. Daily structural tests are certified to verify that the product meets the requirements as set out by the North American lumber grading system. Each piece must be comprised of species from the same species group, and strict tolerances are established for the machining of the fingers; the quality, the mixing, and the curing of the adhesive. Depending on the type of fingerjoined lumber being manufactured, edge and flat bending tests and tension tests are performed on each piece to ensure the joint can meet the engineering design values for the lumber.Fingerjoint lumber test requirements are selected to enable the same specified strength and stiffness as non-finger-joined lumber of the same grade and size to be assigned to the fingerjoined lumber. Test methods (e.g. bending or tension tests) and target test load (e.g. minimum and 5th percentile finger joint strengths) for samples of single fingerjoints are not only linked to the size, grade and species to be joined, but also take into account the average fingerjoint spacing. Fingerjoints used at lower average fingerjoint spacing need to achieve a higher 5th percentile strength level than the same fingerjoints used at higher average fingerjoint spacing. In selecting the tests, only some properties, such as bending strength, are directly tested. Others characteristics are established by correlation to the property monitored, or implied by the specification imposed on the adhesive (e.g. adhesive bondline performance). For further information on the performance of adhesives in fingerjoined lumber in fireresistance-rated wall assemblies, refer to the following document

Fingerjoined lumber grading and grade stamps Fingerjoined lumber must meet the identical requirements found in the grading rules for regular sawn lumber. Grading rules do not consider the presence of finger joints to reduce strength properties. Fingerjoined lumber must also meet special product standards on quality control requirements for strength and durability of the joints. The National Lumber Grades Authority (NLGA) Special Product Standards SPS 1, SPS 3 and SPS 4 in Canada or Western Wood Products Association (WWPA) Glued Products Procedures & Quality Control, C/QC 101.97 are examples of these product standards. All fingerjoined lumber manufactured to the Canadian NLGA Standards carries a grade stamp indicating: • the species or species combination identification • the seasoning designation (S-Dry or S-Green) • the registered symbol of the grading agency • the grade • the mill identification • the type of adhesive used (HRA or Non-HRA) • the NLGA standard number and the designation SPS 1 CERT FGR JNT (certified finger joint), or SPS 3 CERT FGR JNT-VERT STUD USE ONLY (certified finger joint for vertical use only), or SPS 4 CERT FGR JNT (certified finger joint) Additional information on SPS 1 and SPS 3 fingerjoined lumber is provided in Table 1 below.

Grade Stamp Designation	Grade Stamp Facsimile	Product Standards	Comparison to Non-Finger-joined Lumber	Permissible Uses	Adhesives	Grades Allowed	Dimensions and Lengths
VERTICAL STUD USE ONLY – SPS 3 CERT FGR JNT		SPS 3 and C/QCl0l.97	Intended for use as wall studs, limited to normal short-term bending and tension loads	Load-bearing studs, non-load-bearing studs, interior use only	Typically polyvinyl acetate, but any glue meeting standards	Stud, Construction, Standard, No.1, No.2, No.3	2×2, 2×3, 2×4, 2×6, 8′ to 12′
STRUCTURAL FINGERJOINT – SPS 1 CERT FGR JNT		SPS 1 and C/QCl0l.97	Fully interchangeable with lumber of the same grade and species	Load-bearing and non-load-bearing studs, headers, lintels, beams, joists	Phenol-resorcinol or equivalent, dark-colored	Select Structural (SS), No.1, No.2	2×2, 2×3, 2×4, 2×6, 2×8, 2×10, 2×12, 8′ to 40′

Dimension Lumber Sizes

Notes:

• 38mm (2″ nominal) lumber is readily available as S-Dry.
• S-Dry lumber is surfaced at a moisture content of 19 percent or less.
• After drying, S-Green lumber sizes will be approximately the same as S-Dry lumber.
• Tabulated metric sizes are equivalent to Imperial S-Dry sizes rounded to the nearest millimeter.
• S-Dry is the final size for seasoned lumber in place and is the size used in design calculations.

Moisture content

Wood will gain or lose moisture depending on the environmental conditions to which the wood is subjected. Changes in moisture affect wood products in two ways. First, change in moisture content causes dimensional changes (shrinkage and swelling) of the wood. Secondly, when combined with other necessary preconditions, excessive moisture can result in deterioration of wood by decay. Moisture content (MC) is the weight of water contained in the wood compared to the wood’s oven-dry weight. A change in the size of a piece of lumber is related to the amount of water it absorbs or loses. For moisture contents from 0 to about 28 percent, the moisture is held within the walls of the wood cells. At about 28 percent MC the cell walls reach their capacity or fibre saturation point (FSP) and any additional water must be held in the cell cavities.

Moisture content stamps

Lumber stamped ‘S-Grn’ (surfaced green) is lumber which had a moisture content exceeding 19 percent at the time of manufacture (planing or dressing). S-Grn lumber is also called unseasoned lumber or green lumber. Lumber stamped ‘S-Dry’ (surfaced dry) is lumber that had a maximum moisture content of 19 percent or less at the time of manufacture. The moisture content stamp will not indicate whether seasoning resulted from air drying or kiln drying. Some mills apply a voluntary stamp, ‘KD’, indicating that the lumber was kiln dried. Both air dried lumber and kiln dried lumber have the same specified strengths used for engineering design. S-Dry lumber is up to 15 percent more expensive than S-Grn lumber, as a result of increased costs related to packaging and drying.

Moisture content measurement

Measurement of moisture content of wood products can be difficult, particularly if done in variable site conditions. Guidelines should be followed to measure and interpret results to correctly assess whether wood products are dry at installation time. For example, when measuring the moisture content of a piece of wood the following factors affect the individual result:

• type of test (oven dry is most accurate)
• type of meter (dielectric, DC resistance)
• product type
• temperature
• wood species
• variation of wood (wet pockets)
• frequency, location and depth of sampling to correctly represent the entire piece

The following factors should be considered when measuring and assessing the performance of a wood structure, under given end use conditions and moisture changes:

• moisture distribution throughout structure
• location(s) in which moisture will accumulate
• number of storeys
• construction type(s)
• orientation, exposure and shading
• sampling and analysis of individual results

Resources in PDF:

• Checking and splitting
• Fingerjoined Lumber
• Lumber Sizes
• Moisture content
• Surface Smoothness