403A Floor, Wall & Roof Framing 3 — Questions and Answers
Question 1: What is the maximum allowable height of an unbraced stud wall using 38×89 mm (2×4) studs according to the NBC?
- 3 000 mm (Correct answer)
- 2 400 mm
- 3 600 mm
- 4 200 mm
Correct answer: 3 000 mm
The NBC limits 38×89 mm studs in bearing walls to a maximum unsupported height of 3000 mm (approximately 10 feet). Beyond this, larger members (38×140 mm) are required unless the wall is laterally braced.
Question 2: What is the function of a rim joist (band joist) in platform frame construction?
- To close off the ends of floor joists and provide a nailing surface for wall framing (Correct answer)
- To support the centre beam under the floor
- To carry the staircase opening trimmer
- To distribute roof loads to the foundation
Correct answer: To close off the ends of floor joists and provide a nailing surface for wall framing
The rim joist (band joist) runs perpendicular to the floor joists and is nailed to their ends. It closes off the floor system, ties joist ends together, provides a nailing surface for the bottom plate of the wall above, and helps distribute loads.
Question 3: A wall is framed out of plumb by 12 mm over a height of 2 400 mm. Is this within the NBC allowable tolerance for residential construction?
- No — the NBC limit is typically 6 mm in 3 000 mm (1 in 500) (Correct answer)
- Yes — 12 mm in 2 400 mm is within the 1% tolerance
- Yes — any wall under 20 mm in 3 000 mm is acceptable
- No — walls must be perfectly plumb with zero tolerance
Correct answer: No — the NBC limit is typically 6 mm in 3 000 mm (1 in 500)
The NBC and trade standards generally require framed walls to be plumb within 6 mm over 3000 mm (approximately 1:500 ratio). 12 mm in 2400 mm exceeds this (ratio of ~1:200), so the wall would need correction.
Question 4: What is engineered lumber (ELP/LVL) and what advantage does it offer over sawn lumber for long-span beams?
- Laminated Veneer Lumber — consistent strength, no knots, available in longer lengths (Correct answer)
- Lumber treated with preservatives — resists rot and insects only
- Finger-jointed lumber — lower cost than solid lumber
- Green (unseasoned) lumber — used only in temporary structures
Correct answer: Laminated Veneer Lumber — consistent strength, no knots, available in longer lengths
LVL (Laminated Veneer Lumber) is manufactured by bonding thin wood veneers with structural adhesive. The manufacturing process eliminates knots and random defects, producing consistent, predictable strength in longer lengths than sawn lumber — ideal for headers, beams, and long-span applications.
Question 5: When installing wood I-joists, why must holes for mechanical runs be limited to the web area and not cut through the flanges?
- The flanges carry bending stress — cutting them severely reduces the joist's load capacity (Correct answer)
- The flanges contain the structural adhesive that holds the web
- The web is a fire-rated component and must remain intact
- Holes in flanges cause moisture problems at the rim joist
Correct answer: The flanges carry bending stress — cutting them severely reduces the joist's load capacity
In an I-joist, the flanges carry the tensile and compressive bending stresses while the web resists shear. Cutting the flanges destroys the joist's bending capacity. Manufacturers specify hole size and location limits for web penetrations only.
Question 6: What is the minimum bearing length required for a floor joist on a wood plate according to the NBC?
- 38 mm (Correct answer)
- 25 mm
- 50 mm
- 90 mm
Correct answer: 38 mm
The NBC requires a minimum bearing of 38 mm for floor joists resting on wood plates. This ensures adequate load transfer from the joist to the support below without crushing the wood fibres at the bearing point.
403A Floor, Wall & Roof Framing 3
What is the maximum allowable height of an unbraced stud wall using 38×89 mm (2×4) studs according to the NBC?