Wood Building Materials: Lumber, Panels and Mass Timber

Wood building materials are sawn lumber plus the products glued up from veneers, strands, particles or fibers. The USDA Wood Handbook's classification of wood composites (Table 11-1) lists 19 of them; add sawn lumber and you have 20:

• Sawn wood - lumber and timbers.
• Veneer based - plywood, laminated veneer lumber (LVL), parallel strand lumber (PSL).
• Laminates - glued laminated timber (glulam), overlaid materials, laminated wood-nonwood composites, multiwood composites.
• Composite materials - low-, medium- and high-density fiberboard, cellulosic fiberboard, hardboard, particleboard, waferboard, flakeboard, oriented strandboard (OSB), laminated strand lumber (LSL), oriented strand lumber (OSL).
• Wood-nonwood composites - wood fiber-polymer composites (the material in most composite decking), inorganic-bonded composites such as cement-bonded board, and cellulose nanocomposites.

The Handbook describes them as products that can be made in a great variety of sizes and geometries, which is why they have replaced sawn lumber in many roles.

Solid lumber is still the frame of most houses. The Handbook defines boards as under 2 in nominal thickness, dimension lumber as 2 in up to 5 in, and timbers as 5 in and thicker. Posts and timbers are 5 by 5 in and larger and resist mostly axial loads; beams and stringers are 5 in or thicker and more than 2 in wider than they are thick. Retail yards stock dimension lumber in 2, 4, 6, 8, 10 and 12 in nominal widths and 8 to 18 ft lengths.

Sizes, grades and the species on the stamp are covered in types of lumber.

• Plywood is built from an odd number of veneer layers with the grain of adjacent layers at right angles. Construction plywood is classed Exposure 1 (not for permanent exposure to weather) or Exterior (repeated wetting and drying or long-term weather exposure). Details in types of plywood.
• OSB is a structural panel of thin strands bonded with waterproof resin, typically phenol-formaldehyde or MDI. Strands are at least three times as long as they are wide and laid in three layers, the faces aligned along the panel. The Handbook says OSB's performance became similar to structural plywood and it replaced softwood plywood in many construction uses. Aspen is the main North American species.
• Particleboard is made from particles, typically in three layers with fine faces for a smooth surface. The Handbook calls it the main material of ready-to-assemble furniture, and it is also used in flooring systems and as underlayment. Because most uses are interior, it is usually bonded with urea-formaldehyde resin.
• MDF is made from fibers, usually with urea-formaldehyde type resins. The Handbook notes moisture breaks down those bonds, so MDF and particleboard are interior products.

• LVL - thin veneers, 1/8 to 1/10 in thick, hot-pressed with phenol-formaldehyde into lengths from 8 to 60 ft or more. It began in the 1940s as high-strength aircraft parts from Sitka spruce veneer, and today its common job is the flanges of composite I-joists.
• PSL - veneer about 1/8 in thick clipped into strands about 3/4 in wide, with strands averaging at least 150 times their thickness in length; sold commonly as Parallam and used for large beams and columns in place of sawn lumber or glulam.
• LSL and OSL - an extension of OSB technology; one type of LSL uses strands about 12 in long.
• I-joists - prefabricated joists; the Handbook says LVL is commonly used for their flanges and lists plywood among their components. Its own research building uses glulam timbers supporting composite I-joists and plywood sheathing.

• Glulam - two or more layers of lumber glued with the grain of all layers parallel to the length. Laminations are at most 2 in thick, and glulam can be curved, with the radius limited to 100 to 125 times the lamination thickness. Douglas fir-larch, Southern pine, hem-fir and SPF are the usual US species (USDA Wood Handbook). NC State Extension notes glulam arrived in the US in the early 1900s.
• CLT (cross-laminated timber) - an odd number of lumber layers, typically three to nine, stacked crosswise and glued into panels up to 60 ft long and 10 ft wide (NC State Extension).
• NLT and DLT - lumber on edge fastened with nails or screws (NLT) or with hardwood dowels and no glue or metal (DLT).
• Mass plywood panels - large panels built from engineered veneer layups instead of lumber.

The 2021 International Building Code allows mass timber buildings up to 18 stories (NC State Extension).

Every product above is still wood, and wood decays when it stays wet. The Handbook says serious decay needs moisture above about 30%, and that decay fungi can even form voids in wood-plastic composites by degrading the wood fibers after water gets in through surface cracks. Where members touch the ground or stay damp, use treated material or naturally durable heartwood; see treated wood for decks and best wood to use outdoors.

For the framing systems these materials go into, see our guide to types of wood. To check what species a beam, post or salvaged board is, WoodLens identifies wood from a photo.