Wood and mass timber structures

Wood is the dominant structural material for houses and low-rise residential buildings across Canada and the United States, and mass timber is now being used for mid-rise and even tall buildings. Wood is light, renewable, easy to cut and fasten with simple tools, and familiar to a large workforce. It is also a natural material whose strength depends on species, grade, moisture, load duration and the direction of the grain. This lesson explains how wood structures work, how light-frame and mass timber buildings resist gravity and lateral loads, and the construction issues, especially moisture, connections and temporary stability, that constructors must manage.

Wood as a structural material

Wood is made of long cellulose fibers running along the trunk, so it is much stronger along the grain than across it. Tension and compression parallel to the grain are strong; compression perpendicular to the grain, such as a joist bearing on a plate, is weak, and crushing at bearings is a common issue. Knots, slope of grain and splits reduce strength, which is why lumber is graded.

Wood strength also depends on how long the load is applied. Wood can carry a higher load for a short time, such as a wind gust, than it can carry permanently. Design codes apply load duration factors for this. Wood also creeps: a beam under permanent load will continue to deflect slowly over months and years.

In the United States, wood design follows the National Design Specification (NDS) for Wood Construction (the 2024 edition is current), with grading rules from agencies such as WWPA and SPIB and species groups such as Douglas Fir-Larch, Southern Pine and SPF. Dimension lumber is sold by nominal size, such as a 2 × 4 (actual 1½ × 3½ in) and a 2 × 10 (actual 1½ × 9¼ in).

Moisture: the critical construction issue

Wood shrinks and swells as its moisture content changes. Freshly sawn wood may be above 30 percent moisture content; kiln-dried lumber is typically at or below 19 percent when delivered; in a heated building wood settles to around 8 to 12 percent. Shrinkage is small along the grain but large across it, roughly 1 percent of the dimension for every 4 to 5 percentage points of moisture change across the grain.

Construction consequences of moisture include:

• Shrinkage in floor joists and plates accumulates over several stories of a platform-frame building, causing differential movement against elevator shafts, masonry veneer, stairs, plumbing stacks and other non-shrinking elements.
• Wet wood enclosed in walls or under finishes can grow mold and decay, and wet CLT and glulam can check, stain and swell.
• Strength and stiffness are lower when wood is wet.

Moisture control means buying properly dried material, storing it off the ground and covered, protecting assemblies during construction, measuring moisture content before enclosing and finishing, and detailing for shrinkage.

Light-frame wood construction

Light-frame (platform-frame) construction uses closely spaced dimension lumber studs, joists and rafters, sheathed with plywood or OSB. Each story is framed on the platform of the floor below. Gravity loads travel from sheathing to joists, to beams and walls, down stud walls to foundations. Loads must line up: a point load from a beam above needs a built-up post or stud pack below, continuing all the way to the foundation, with solid blocking through floors.

Lateral loads in light-frame buildings are resisted by diaphragms and shear walls. Floor and roof sheathing acts as a horizontal diaphragm, a deep, thin beam that collects wind or seismic load and delivers it to the shear walls. Shear walls are walls with structural sheathing nailed to the studs at a specified pattern, with hold-downs at the ends to resist overturning.

For constructors, shear walls and diaphragms are invisible structure: a sheathed wall looks the same whether it is a shear wall or not. Nail size, spacing, edge distance, blocking and hold-downs are what make it work. Over-driven nails, missed edge nailing, cutting sheathing for services or omitting hold-downs can greatly reduce capacity. Shear wall schedules on the drawings must be followed and inspected before cover.

In the United States, the International Residential Code allows prescriptive design of one- and two-family dwellings, while larger buildings follow the IBC and NDS. The 2021 and 2024 IBC added construction types IV-A, IV-B and IV-C, allowing mass timber buildings up to 18 stories with protection requirements.

Mass timber

Mass timber uses large engineered panels and members: CLT floors and walls, glulam beams and columns, and nail- or dowel-laminated panels. Pieces are prefabricated with CNC machining, so erection is fast and quiet, with fewer trades on site. Mass timber buildings typically use glulam post-and-beam framing with CLT floors and a concrete or CLT core for lateral resistance.

Construction issues specific to mass timber include:

• Moisture protection during erection. Panels must be kept dry, with temporary roofs or membranes, tape at panel joints and fast enclosure. Standing water on CLT floors causes swelling and staining and must be removed promptly.
• Precision. Panels and connections are made to tight tolerances, so anchor bolts, embeds and steel connections must be accurately placed, and the concrete core must be within tolerance.
• Connections. Proprietary screws, steel plates, dowels and hangers must be installed as specified, with the right screw length, angle and spacing.
• Temporary stability. Walls and columns need bracing until floors are connected; CLT wall panels act as large sails in wind.
• Fire during construction. Exposed timber is combustible before encapsulation, sprinklers and fire separations are complete, so fire safety planning is critical.

Connections and fasteners

Wood structures depend on connections more than most materials do, because wood cannot be welded and its strength perpendicular to the grain is low. Nails, screws, bolts, timber rivets, steel hangers, straps and hold-downs are all structural parts with specified sizes and patterns. Substituting a shorter nail, a different hanger or a smaller screw changes capacity. Edge and end distances matter because wood splits along the grain if fasteners are too close to the edge.

Engineered connectors (joist hangers, hurricane ties and hold-downs) carry published loads only when every hole is filled with the specified fastener. Partial nailing is a common deficiency found at framing inspection.

Temporary stability of wood framing

Light-frame walls are stood up as panels and are unstable until they are connected to the floor or roof above. Temporary diagonal braces, usually lumber nailed to the studs and staked or nailed to the floor, keep walls plumb and resist wind until sheathing, floor framing and roof diaphragms are complete. Roof trusses are a particular hazard: a line of unbraced trusses can domino over, and truss manufacturers and industry guides such as the Building Component Safety Information (BCSI) guide in the United States and the Truss Plate Institute of Canada guidance describe temporary and permanent bracing patterns that must be installed as trusses are set.

In the United States, OSHA requires that structures be braced against collapse during construction, and the truss design drawings and BCSI guidance specify temporary and permanent bracing, including continuous lateral restraint of compression webs.

Engineered wood products have their own handling rules. I-joists must not have their flanges cut or notched, and holes in webs must follow the manufacturer's hole charts for size and location. LVL and glulam beams must not be notched at the bottom (tension) face. Mass timber panels arrive with lifting points designed by the supplier, and panels should be lifted only with the specified hardware.