Floor, Wall & Roof Framing Flashcards

What is the unit rise and unit run used for in roof framing calculations?

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.

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?

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.

What is the difference between a common rafter and a jack rafter?

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.

What is the purpose of the birdsmouth cut on a rafter?

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.

According to the NBC, what is the minimum thickness of subfloor plywood required for floor joists spaced at 400 mm O.C.?

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.

When framing a hip roof, the hip rafter runs at what angle to the common rafters in plan view?

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.