11.5 Wood Framing, Trusses & Carpentry

Why Carpentry Matters on the Limited Building Exam

The PSI Limited Building Contractor outline gives 12 of its 80 scored items to carpentry, more than any other area. Its references include Carpentry and Building Construction (Feirer), the BCSI guide to handling, installing, restraining, and bracing metal-plate-connected wood trusses, and the building code. Expect questions on lumber quantities, framing members, code cutting limits, and truss safety.

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Lumber Sizes, Board Feet, and Grade Stamps

Lumber is sold by nominal size but delivered dressed (surfaced) to smaller actual dimensions:

Board feet are figured with nominal dimensions:

$$\text{Board Feet} = \frac{\text{Thickness (in.)} \times \text{Width (in.)} \times \text{Length (ft)}}{12}$$

Example: one 2x10 that is 16 feet long contains 2 × 10 × 16 ÷ 12 = 26.67 board feet. Twenty of them contain about 533 board feet. Estimators add a waste factor, typically 8% to 10% for framing lumber, before pricing.

A grade stamp identifies five things:

• the grading agency
• the mill number
• the grade (for example, No. 2)
• the species (Southern Pine is common in South Carolina)
• the moisture condition at surfacing: S-DRY or KD means 19% moisture content or less, and S-GRN means over 19%

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Treated Wood and Decay Protection

• Preservative-treated lumber is specified by AWPA Use Category:
  • UC3B for above-ground exterior exposure
  • UC4A for general ground contact
  • UC4B for heavy-duty or hard-to-replace ground contact
• IBC Section 2304.12 requires naturally durable or preservative-treated wood in these locations:
  • framing that rests on exterior foundation walls and is less than 8 inches from exposed earth
  • sills and sleepers on a concrete slab in direct contact with the earth
  • other locations exposed to moisture
• Fasteners and connectors in treated wood must resist corrosion. Code-accepted choices are hot-dip galvanized steel, stainless steel, silicon bronze, or copper (IBC Section 2304.10).

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Framing Systems and Fireblocking

• Platform framing: Each floor is built as a platform, and walls are framed one story at a time on top of it. The floor assembly interrupts the stud cavities at each level. It is the dominant system today.
• Balloon framing: Studs run continuously from the sill to the roof. It needs added fireblocking at every floor line and is rarely used now.

Wall framing vocabulary appears on the exam in plan-reading questions:

• Sole (bottom) plate and double top plate
• Studs, usually 16 or 24 inches on center
• At openings: king studs run full height, jack (trimmer) studs support the header, and cripple studs fill the space above the header and below the rough sill

Fireblocking (IBC 718.2): Concealed spaces in stud walls and partitions must be fireblocked vertically at the ceiling and floor levels and horizontally at intervals not exceeding 10 feet. Acceptable materials include:

• 2-inch nominal lumber
• two thicknesses of 1-inch nominal lumber with broken lap joints
• 23/32-inch wood structural panels with joints backed
• 1/2-inch gypsum board
• mineral-wool or glass-fiber batts installed as the code allows

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Cutting, Notching, and Boring Limits

These are the conventional light-frame limits (IRC R602.6 and R502.8, with matching IBC Section 2308 provisions). They apply unless an engineer approves otherwise:

Engineered wood products such as I-joists, LVL, and glulam follow the manufacturer's hole charts. Never cut the flange of an I-joist.

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Structural Sheathing

• An APA span rating such as 32/16 means the panel can span 32 inches between roof supports and 16 inches between floor joists.
• Install panels with the strength axis (long dimension) perpendicular to the supports. Leave about a 1/8-inch gap at panel edges and ends for expansion.
• Typical wall and roof sheathing nailing is 6 inches on center at panel edges and 12 inches in the field. Shear walls and high-wind zones often need closer nailing per the drawings.
• Panel clips (H-clips) support unblocked roof panel edges between rafters or trusses.

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Metal-Plate-Connected Wood Trusses (BCSI)

The truss design drawing shows loads, member sizes, plates, and the permanent individual-member restraint the truss needs. The building designer is responsible for the overall permanent bracing of the roof or floor system.

Handling and storage: Store truss bundles off the ground on blocking and brace them so they cannot tip. Lift at designated points, never by the webs. Use spreader bars and strongbacks for long spans so the trusses do not buckle sideways during the lift.

Installation and temporary restraint:

1. Brace the first truss securely to the ground or end wall, because every other truss is braced back to it.
2. Install top-chord temporary lateral restraint at BCSI spacings. The maximum is 10 feet on center for spans up to 30 feet, 8 feet for 30 to 45 feet, and 6 feet for 45 to 60 feet.
3. Add diagonal bracing in the planes of the top chord, the bottom chord, and the webs, so the lateral restraint cannot move together as a unit.
4. For trusses with clear spans of 60 feet or greater, IBC Section 2303.4.1.3 requires the owner to contract with a registered design professional to design the temporary installation restraint and bracing and the permanent individual-member restraint. BCSI likewise directs users to one for trusses longer than 60 feet.

Never cut, drill, notch, or alter a truss without a written repair detail from the truss designer. Do not stack plywood, drywall, or roofing bundles on unbraced trusses. Place construction loads near supports and within the limits BCSI gives.