Why Wood Remains the Dominant Structural Material for Low-Rise Buildings
Despite competition from light-gauge steel and concrete, wood framing accounts for over 90% of residential construction and a significant share of low-rise commercial construction in the United States. It is fast to erect, widely available, easily modified in the field, thermally efficient, and — with proper fire protection — meets IBC fire rating requirements for buildings up to 6 stories (Type III-A) or higher with mass timber (Type IV). Understanding wood structural systems is essential for architects, structural engineers, building officials, and contractors working on any project under approximately 85 feet in height.
Platform Framing: The Standard Wood Construction Method
Platform framing — where each floor is framed as a separate platform before the walls above are erected — is the standard method for wood light-frame construction in North America. It replaced balloon framing (where studs run continuously from foundation to roof) during the mid-20th century because it is safer to erect, produces a natural fire stop at each floor level, and is more dimensionally stable as lumber dries.
In platform framing, floors consist of dimensional lumber joists (typically 2×10 or 2×12) or engineered I-joists spanning between bearing walls or beams. Floor sheathing (typically 3/4" APA-rated tongue-and-groove plywood or OSB) is fastened to the joists to form the platform. Walls are then erected on top of the platform — typically 2×4 or 2×6 studs at 16" or 24" on center — with a double top plate and single bottom plate. Blocking at the floor level between wall studs provides the required fire stops at each story.
Roof systems in platform framing are typically prefabricated trusses (engineered by the truss manufacturer under the supervision of a licensed engineer) or conventional stick-framed rafters with a ridge board, collar ties, and ceiling joists. Trusses have largely replaced stick framing in production housing because they are faster to install, require no interior bearing walls for the roof span, and transfer loads to the exterior walls only.
Engineered Lumber Products
Engineered lumber products (ELPs) are manufactured from wood fiber — either veneers, strands, or fibers — bonded with adhesives under heat and pressure. They offer higher and more consistent structural properties than dimensional lumber, longer spans, and resistance to warping, twisting, and shrinkage. They are now standard for floor beams, headers, and long-span applications in wood construction.
Laminated Veneer Lumber (LVL): Made from thin wood veneers (typically 1/10" to 1/8" thick) with grain running parallel in all plies, then laminated under pressure. LVL has consistent bending values (Fb = 2,600 psi for most products, vs. ~1,000–1,500 psi for dimensional lumber) and is available in depths from 3-1/2" to 24" and lengths up to 60 feet. It is the go-to product for floor beams, headers, and ridge beams. Design per NDS Special Design Provisions for Wind and Seismic and the manufacturer's published design values.
Parallel Strand Lumber (PSL): Made from long wood strands (typically 8" or longer) oriented parallel to the length. PSL has very high compressive strength perpendicular to grain, making it well-suited for columns and posts as well as beams. PSL is stiffer and stronger than LVL for most applications. Trus Joist (Weyerhaeuser) Parallam is the primary brand.
Glued Laminated Timber (Glulam): Made from dimension lumber laminations glued face-to-face, with grain parallel in all laminations. Glulam is available in very large sizes (up to 8-3/4" wide by 80" deep) for long-span applications — gymnasium roofs, large open-plan commercial spaces, and architectural exposed applications. Glulam can be curved or tapered for architectural effect. Design per AITC 117 (Design Standard for Structural Glued Laminated Timber) and the manufacturer's certified design values. Combination symbols (e.g., 24F-V4) describe the lamination grades and species.
Cross-Laminated Timber (CLT): Panels of dimension lumber laminated with alternating grain directions, similar in concept to plywood but much thicker — typically 3 to 12 layers totaling 3-1/2" to 12" in thickness. CLT behaves as a two-way structural panel and can span as a floor, wall, or roof system without additional framing. CLT is the primary structural material for mass timber construction — the building type that allows wood buildings up to 18 stories under IBC 2021 Type IV-C construction. Design per AWC CLT Structural Design Manual and PRG 320 (Standard for Performance-Rated Cross-Laminated Timber).
Wood I-Joists: Flanges of LVL or solid sawn lumber with oriented strand board (OSB) webs, forming an I-shape. I-joists are lightweight, very stiff (high depth-to-weight ratio), available up to 36" deep and 60 feet long, and consistent in dimension (no crown or warp). They are the standard product for floor and roof framing in engineered-design buildings. Install web stiffeners at bearing points and blocking panels per manufacturer requirements — these are frequently omitted in the field and are a leading cause of I-joist failures.
APA-Rated Structural Panels: Plywood and OSB
APA (formerly the American Plywood Association, now the Engineered Wood Association) rates structural panels — both plywood and oriented strand board (OSB) — for specific structural applications. The APA stamp on a panel certifies its performance for the rated use.
The key APA ratings for structural use are:
APA Rated Sheathing: The standard panel for wall, roof, and subfloor sheathing. Rated by Span Rating (e.g., 32/16 — first number is max roof rafter spacing, second is max floor joist spacing) and Exposure Durability Classification (Exposure 1 for most applications, Exterior for fully exposed conditions).
APA Rated Sturd-I-Floor: Single-layer floor system, combines subfloor and underlayment in one panel. 24 o.c. or 48 o.c. span rating. Tongue-and-groove edges prevent differential deflection (squeaking) between panels.
APA Structural I Rated Sheathing: Higher-grade panels for shear walls and diaphragms requiring maximum shear values. Required by some shear wall table footnotes in the AWC Special Design Provisions for Wind and Seismic (SDPWS).
Wood Shear Walls and Diaphragms
In wood-frame construction, lateral loads (wind and seismic) are resisted by a system of horizontal diaphragms (floors and roof) that distribute lateral forces to vertical shear walls, which transfer the forces to the foundation. Understanding this system is critical for structural design of any wood building.
Shear walls consist of wood structural panel sheathing fastened to a wood stud framing with specified nailing. The shear capacity of the wall depends on panel thickness, nail size, nail spacing at panel edges, and framing member size. AWC SDPWS Table 4.3A provides design values for wood structural panel shear walls — for example, 7/16" OSB with 8d nails at 6" edge spacing on Douglas Fir-Larch framing provides 490 lb/ft allowable unit shear (ASD). Doubling the nails (3" edge spacing) roughly doubles the capacity.
Hold-downs at shear wall end posts resist overturning forces. Simpson Strong-Tie HD and HDU series hold-downs are the standard product. The hold-down must be sized for the tension demand at the wall chord, which depends on wall height, length, and total lateral force. Anchor bolts connect the wall base plate to the foundation to resist sliding.
IBC Chapter 23: Prescriptive Wood Construction Requirements
IBC Chapter 23 contains prescriptive requirements for wood construction in buildings that qualify for conventional light-frame construction (Section 2308) — typically 3 stories or less with limited story height, floor area, and load conditions. Buildings outside those limits require engineered design per the NDS (National Design Specification for Wood Construction) and AWC SDPWS.
Key IBC Chapter 23 provisions include: minimum lumber grades and species, stud wall height limitations (typically 10' maximum for 2×4 studs at 16" o.c.), bearing wall requirements, braced wall panel requirements for lateral resistance (prescriptive alternative to engineered shear walls), floor and roof framing span tables, header sizing tables, and fire blocking requirements. Most residential construction uses these prescriptive tables rather than full engineered design.