403A Floor, Wall & Roof Framing 1 — Questions and Answers
Question 1: According to the National Building Code of Canada, what is the maximum allowable span for 38×184 mm (2×8) spruce-pine-fir floor joists at 400 mm O.C. spacing in a residential application?
- Approximately 3.6 m (Correct answer)
- Approximately 4.5 m
- Approximately 2.8 m
- Approximately 5.2 m
Correct answer: Approximately 3.6 m
NBC span tables for 38×184 SPF joists at 400 mm O.C. for residential floor loads (1.9 kPa live load + dead load) yield approximately 3.6 m. Always consult current NBC tables as values depend on load, species, and grade.
Question 2: What is the purpose of blocking installed between floor joists at the bearing wall?
- To prevent joist rotation and transfer loads to the bearing wall (Correct answer)
- To provide nailing for subfloor edges
- To create a fire stop between floors
- To support the rim joist
Correct answer: To prevent joist rotation and transfer loads to the bearing wall
Blocking (or solid bridging) at bearing walls prevents joist rotation under load, transfers vertical loads to the bearing wall, and helps distribute point loads. It also acts as a fire stop in some assemblies.
Question 3: When framing a load-bearing wall, what is the minimum number of jack studs (trimmer studs) required on each side of a door opening wider than 1 800 mm?
- Two jack studs on each side (Correct answer)
- One jack stud on each side
- Three jack studs on each side
- No jack studs — king studs carry the load
Correct answer: Two jack studs on each side
For openings wider than 1800 mm, the NBC and standard practice require two jack studs on each side to adequately support the increased header load. Narrower openings typically use one jack stud per side.
Question 4: What is the purpose of a let-in brace in wood frame wall construction?
- To provide racking resistance (lateral load resistance) in the wall (Correct answer)
- To support the top plate during framing
- To carry vertical loads from roof to foundation
- To create a nailing surface for exterior sheathing
Correct answer: To provide racking resistance (lateral load resistance) in the wall
A let-in brace is a piece of 19×89 mm (1×4) lumber notched into studs at a 45° angle from top corner to bottom corner. It resists racking forces (lateral loads from wind and seismic) that would otherwise push the wall out of plumb.
Question 5: What does 'crown up' mean when installing floor joists?
- Install the joist with the natural bow facing upward so floor loads will straighten it (Correct answer)
- Install the joist with the crown (label) side facing up
- Use the joist with the largest cross-section facing upward
- Orient the joist so the growth rings face up
Correct answer: Install the joist with the natural bow facing upward so floor loads will straighten it
Lumber often has a slight natural bow or crown along its length. Installing joists crown up means the convex side faces up. Floor loads compress the crown down toward straight, preventing a visible hump. A crown installed downward will sag further.
Question 6: When doubling a top plate in wall framing, how must the lap joints be positioned?
- Joints in the upper plate must be offset a minimum of 1200 mm from joints in the lower plate (Correct answer)
- Joints must align directly over each other for maximum strength
- Joints must occur over a stud location only
- Joints may be anywhere as long as nailing is adequate
Correct answer: Joints in the upper plate must be offset a minimum of 1200 mm from joints in the lower plate
The NBC requires top plate lap joints to be offset by at least 1200 mm (or spanning at least two stud spaces) between the lower and upper plates. This ties the wall together and prevents a continuous weak plane.
403A Floor, Wall & Roof Framing 1
According to the National Building Code of Canada, what is the maximum allowable span for 38×184 mm (2×8) spruce-pine-fir floor joists at 400 mm O.C. spacing in a residential application?