403A Floor, Wall & Roof Framing 4 — Questions and Answers
Question 1: What is the purpose of a strongback in floor joist framing?
- A continuous horizontal member nailed to the bottom of joists mid-span to reduce deflection and vibration (Correct answer)
- A doubled joist used at stairwell openings
- A steel strap connecting joists to the foundation
- A bridging method using cross-bracing
Correct answer: A continuous horizontal member nailed to the bottom of joists mid-span to reduce deflection and vibration
A strongback is a 38×140 mm (or larger) board nailed vertically to the bottom edge of joists at mid-span, running perpendicular to the joists. It ties multiple joists together, distributes point loads across several joists, and reduces bounce and vibration.
Question 2: When framing a valley on a roof where two roofs of equal pitch intersect, what is the mathematical relationship between the valley rafter length and the common rafter run?
- The valley rafter run equals the common rafter run multiplied by 1.414 (√2) (Correct answer)
- The valley rafter is the same length as the common rafter
- The valley rafter run equals the common rafter run divided by 1.414
- The valley rafter run is always 300 mm longer than the common rafter
Correct answer: The valley rafter run equals the common rafter run multiplied by 1.414 (√2)
Like hip rafters, valley rafters run at 45° in plan view. For a regular equal-pitch roof, the valley rafter's horizontal run equals the common rafter run times √2 (1.414). The actual rafter length is then calculated using the slope.
Question 3: What nailing pattern is required when attaching plywood sheathing to wall studs for shear wall construction?
- Closer nailing at panel edges (e.g., 75 mm O.C.) and field nailing at 300 mm O.C. (Correct answer)
- Nailing only at panel corners
- Uniform 300 mm O.C. nailing throughout the panel
- Nailing pattern is not critical — any schedule is acceptable
Correct answer: Closer nailing at panel edges (e.g., 75 mm O.C.) and field nailing at 300 mm O.C.
Shear walls resist lateral loads from wind and seismic forces. Edge nailing must be denser (typically 75 or 150 mm O.C. per the engineer's schedule) because panel edges carry the most shear. Field nailing is at wider spacing (300 mm O.C.).
Question 4: What is the purpose of a scab or splice plate when joining two pieces of lumber end-to-end in a beam?
- To restore structural continuity and transfer load across the joint (Correct answer)
- To protect the end grain from moisture
- To mark the location of the joint for inspection
- To allow thermal expansion between members
Correct answer: To restore structural continuity and transfer load across the joint
A scab or splice plate is nailed or bolted over an end-to-end joint to transfer load. Without it, the joint has no tensile or bending capacity. Splices must be located at points of low bending moment (near supports) and designed by an engineer for structural members.
Question 5: In platform frame construction, at what location must firestopping be installed in wall cavities according to the NBC?
- At each floor level and at the top of each stud space adjacent to concealed combustible spaces (Correct answer)
- Only at the ceiling level of the top floor
- Only at the first floor level in two-storey buildings
- Firestopping is not required in platform frame construction
Correct answer: At each floor level and at the top of each stud space adjacent to concealed combustible spaces
The NBC requires firestopping at each floor level in exterior walls and within concealed spaces to prevent fire and smoke from spreading vertically through wall cavities. Platform framing provides inherent firestopping at floor levels due to the rim joist and plates.
Question 6: What is the maximum opening width allowed in a load-bearing wall before an engineer-designed header is required under the NBC Part 9?
- 3 000 mm (Correct answer)
- 2 400 mm
- 1 800 mm
- 3 600 mm
Correct answer: 3 000 mm
NBC Part 9 (Housing and Small Buildings) prescribes header sizes for openings up to 3000 mm wide. Beyond 3000 mm, engineered design is required. Standard prescriptive tables cover common residential window and door sizes.
403A Floor, Wall & Roof Framing 4
What is the purpose of a strongback in floor joist framing?