12.1 Carpentry & Framing

Framing Systems: Platform vs Balloon

Platform framing is the dominant method in modern residential construction. Each floor is built as a separate platform: floor joists and subfloor at level one, walls erected on top, then the next floor platform, and so on. Firestops are naturally created at each floor line because the platform separates stud cavities.

Balloon framing, the older method, runs wall studs continuously from the sill plate to the roof, with floor joists resting on a ribbon board let into the studs. Firestops must be added at each floor line to prevent fire traveling up the continuous stud cavities. Balloon framing is rarely used today but appears in historic homes and some two-story great-room walls.

Quick Answer: Platform framing builds each floor as a separate deck with firestops built in; balloon framing runs studs full height and requires added firestops at floor lines.

Floor Framing

A wood floor system consists of the sill plate (anchored to the foundation with anchor bolts), the girder or beam carrying interior joists, floor joists spanning between foundation walls and girders, and the subfloor (typically 3/4-inch OSB (Oriented Strand Board) or plywood). Joists are sized from span tables — the 2018 IRC (International Residential Code) provides prescriptive tables for residential construction by species, grade, spacing, and live load, while commercial wood framing follows 2018 IBC (International Building Code) Chapter 23 and the ANSI/AWC National Design Specification (NDS) for engineered conditions. Typical residential spacing is 16 inches on center (o.c.), with 24-inch o.c. used for wider-spacing engineered products. Joists must be cross-bridged or solid-blocked at intervals to prevent rotation.

Wall Framing

Walls are built from studs (typically 2x4 or 2x6 at 16 inches o.c.), a single bottom plate (sole plate), and a double top plate. The double top plate overlaps corners and intersections to tie walls together and provide continuous load transfer. Headers span door and window rough openings and transfer loads to trimmer studs (jack studs) and king studs. Cripples carry loads above and below openings to the plates.

Roof Framing: Rafters vs Trusses

Rafters are site-cut framing members spanning from the ridge board to the wall plate, creating the roof slope. A ridge board (not a beam) sits at the peak. Collar ties connect opposing rafters in the upper third to resist wind uplift. Ceiling joists span the walls and tie the bottom of the rafter triangle together, resisting outward thrust.

Trusses are engineered, factory-built triangular assemblies using 2x4 or 2x6 chords and webs joined by metal connector plates (gussets). They span the full width of the building, eliminating interior bearing walls. Trusses are lighter, faster to install, and typically cheaper than stick-framed roofs but restrict attic storage and future modification.

Quick Answer: Rafters are cut on site and rely on ceiling joists to resist outward thrust; trusses are engineered assemblies spanning wall-to-wall with no interior bearing needed.

Engineered Wood Products

• LVL (Laminated Veneer Lumber) — multiple wood veneer layers glued with grains parallel; stronger than sawn lumber, used for beams, headers, and rim boards.
• I-joists — OSB web with solid lumber flanges; straighter and lighter than dimensional joists, span longer distances, with pre-punched knockouts for utilities.
• OSB — wood strands oriented and bonded with resin; the standard subfloor and wall sheathing material, replacing plywood in most residential construction.
• Plywood — alternating wood veneer plies; still used where moisture resistance is critical.

Fasteners and Connectors

Common framing nails include 16d commons (3-1/2 inches) for wall plates and framing, 8d (2-1/2 inches) for sheathing, and 10d for floor joist connections. Joist hangers and other metal connectors (straps, hold-downs, tie plates) are rated by the manufacturer and installed with specified nails; substitutions invalidate the load rating. Anchor bolts (minimum 1/2-inch diameter per the 2018 IRC, with 5/8-inch common in practice) embed a minimum of 7 inches into concrete, spaced at 6-foot maximum intervals with a bolt placed within 12 inches of each plate end.

Spacing and Span Basics

Span depends on species, grade, spacing, and live or dead load. The 2018 IRC publishes prescriptive span tables eliminating engineered calculations for standard residential conditions, while 2018 IBC Chapter 23 governs commercial wood design with engineered calculations per the NDS. Reducing spacing from 24 to 16 inches o.c. increases the allowable span; the exact percentage varies by species, grade, and load condition. Always verify the code edition adopted by the local jurisdiction.

Finish Carpentry Overview

Finish carpentry includes casing (trim around doors and windows), baseboard (wall-to-floor trim), chair rail, crown molding, and interior door hanging. Work is done after drywall, with nail holes filled and joints caulked. Coping inside corners on crown and base produces tighter joints than mitering, especially where walls are not square.

On the B-1/B-2 commercial general building trade exam, Carpentry carries 12 of 100 items.