Light-Frame Wood Construction Is the Most Common Building System in the US
Wood light-frame construction accounts for the vast majority of residential buildings and a significant portion of light commercial construction in the United States. It is fast to build, relatively inexpensive, easily modified, and supported by a well-established supply chain. The ARE PPD division tests wood framing concepts because architects designing residential and light commercial projects must understand structural behavior, member spacing, lateral resistance strategies, and the interplay between framing systems and building envelope performance.
Platform Framing vs. Balloon Framing
The two historic types of wood light-frame construction differ in how continuous the stud is:
Platform framing (the modern standard): Each floor is framed as a separate "platform" - the floor structure (joists and subfloor) is built first, and the walls of the next floor rest on top of the platform. Wall studs run only one story tall. This method provides a natural firestop at each floor level (the rim joist and subfloor block the path for fire to run up the wall cavity), is simpler to build, and is the standard method for all contemporary light-frame construction.
Balloon framing (historic, rarely used today): Wall studs run continuously from foundation to roof plate - potentially two or three stories tall. Floor joists hang from the studs at each level. Balloon framing allows easier installation of vertical elements (flues, plumbing stacks) but lacks inherent firestopping and is practically obsolete because tall studs are difficult to source and erect.
Stud Spacing and Member Sizes
Standard stud spacing in light-frame construction is 16 inches on center for most applications and 24 inches on center for advanced framing (optimum value engineering). Stud sizes are typically 2×4 for shorter walls and less demanding loads, and 2×6 for exterior walls where added insulation depth is desired or for taller walls. Headers over openings must be sized to span the opening and carry loads from above.
Lateral Resistance in Wood Light-Frame
Wood light-frame buildings resist lateral loads through shear walls - wall segments sheathed with structural panel sheathing (plywood or OSB) and nailed to the framing according to specific nail patterns. The shear capacity of a wall is determined by the panel thickness, the nail spacing at panel edges, and the aspect ratio of the wall segment. Hold-downs (metal hardware that anchors the end stud of a shear wall to the foundation or floor below) resist overturning forces. The nailing schedule is specified in the structural drawings and must be followed precisely during construction.
Fire Performance
Wood frame construction has fire performance requirements addressed through: fire-retardant-treated (FRT) lumber or panels in areas where combustible construction is used near property lines; fire-resistive assemblies (gypsum board on both faces of a wall assembly) to achieve 1-hour or 2-hour fire resistance ratings as required by the IBC based on occupancy; and automatic sprinkler systems that are increasingly required in wood frame multi-family construction.
Key Exam Points
- Platform framing: studs one story tall; inherent firestopping at each floor - modern standard.
- Balloon framing: continuous studs; lacks firestopping - historic, not used today.
- Standard stud spacing: 16 inches o.c.; advanced framing uses 24 inches o.c.
- Shear walls: structural sheathing (plywood/OSB) with nail schedule; hold-downs resist overturning.
- 2×6 exterior walls: deeper insulation cavity + same structural capacity.
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