403A Floor, Wall & Roof Framing 2 — Questions and Answers
Question 1: What is the unit rise and unit run used for in roof framing calculations?
- To express roof slope as rise per 300 mm of run for layout calculations (Correct answer)
- To calculate the total span of the roof
- To determine rafter spacing in the framing plan
- To measure the thickness of roofing materials
Correct answer: To express roof slope as rise per 300 mm of run for layout calculations
Unit rise and unit run define the slope of a roof. In Canada, rise is expressed in millimetres per 300 mm of horizontal run (e.g., 150/300 = a 6/12 pitch equivalent). This ratio is used to calculate rafter length using the rafter tables on a framing square.
Question 2: A common rafter has a total run of 3 600 mm and a unit rise of 200 mm per 300 mm of run. What is the total rise of the roof?
- 2 400 mm (Correct answer)
- 1 800 mm
- 3 000 mm
- 2 700 mm
Correct answer: 2 400 mm
Total rise = (total run ÷ unit run) × unit rise = (3600 ÷ 300) × 200 = 12 × 200 = 2400 mm. The roof rises 2400 mm over a 3600 mm horizontal run.
Question 3: What is the difference between a common rafter and a jack rafter?
- A common rafter spans from ridge to wall plate; a jack rafter spans from ridge or hip/valley to a hip, valley, or plate (Correct answer)
- A common rafter is doubled; a jack rafter is single
- A common rafter is used for flat roofs; a jack rafter for sloped roofs
- A common rafter has no birdsmouth cut; a jack rafter does
Correct answer: A common rafter spans from ridge to wall plate; a jack rafter spans from ridge or hip/valley to a hip, valley, or plate
A common rafter runs from the ridge board perpendicular down to the top wall plate. A jack rafter is shorter — it runs from the ridge to a hip rafter, or from a valley rafter to the plate, at the same slope as a common rafter.
Question 4: What is the purpose of the birdsmouth cut on a rafter?
- To create a flat bearing surface on the top plate and resist horizontal thrust (Correct answer)
- To reduce the rafter weight at mid-span
- To allow ventilation at the eave
- To accommodate the ridge board thickness
Correct answer: To create a flat bearing surface on the top plate and resist horizontal thrust
The birdsmouth (seat cut) is a notch cut into the underside of a rafter where it rests on the top plate. It creates a flat horizontal bearing surface (seat cut) and a vertical plumb cut (cheek cut) that seats squarely on the plate, preventing the rafter from sliding.
Question 5: According to the NBC, what is the minimum thickness of subfloor plywood required for floor joists spaced at 400 mm O.C.?
- 15.5 mm (Correct answer)
- 12.5 mm
- 18.5 mm
- 9.5 mm
Correct answer: 15.5 mm
NBC Table 9.23.16 requires a minimum 15.5 mm (5/8") plywood subfloor for joist spacing of 400 mm O.C. This ensures adequate stiffness and load distribution between joists.
Question 6: When framing a hip roof, the hip rafter runs at what angle to the common rafters in plan view?
- 45° (Correct answer)
- 30°
- 60°
- 90°
Correct answer: 45°
In plan view (looking down from above), hip rafters bisect the corners of the building at 45° to the common rafters for a regular (equal-pitch) hip roof. This 45° angle in plan means the hip rafter's actual length is found using 1.414 (√2) times the common rafter unit run.
403A Floor, Wall & Roof Framing 2
What is the unit rise and unit run used for in roof framing calculations?