Roof Framing and Roof Construction
This section tests roof types, pitch, loads, member names, fixing, and weatherproofing. You should know rafters, purlins, ridge, wall plate, tie beams, bracing, birdsmouth joints, overhangs, and the different requirements of clay tiles, concrete tiles, and metal sheeting.
Core rules
- Pitch controls water shedding and material choice. Tiles generally need a steeper pitch than profiled metal sheets; follow the manufacturer’s minimum.
- Rafter spacing and purlin spacing must suit the covering and the design load. Wind uplift matters in exposed locations.
- Cut a birdsmouth so the rafter seats fully on the wall plate without over-cutting the effective depth.
- Fix rafters to wall plates and ridge with appropriate straps, nails, or connectors; provide diagonal bracing and ceiling ties where required.
- Maintain consistent overhang and fascia line; check the roof square before fixing the covering.
- Lap sheeting and tiles correctly and fix at every recommended point, especially at edges and ridges.
Worked example
A roof has a 4 m span and a 30° pitch. The half-span is 2 m. Rafter length from wall plate to ridge = half-span ÷ cos 30° = 2 ÷ 0.866 = 2.31 m. Add a 0.45 m overhang: 2.76 m. Allow for the birdsmouth and order 2.8 m rafters. The reasoning: use half the span, not the full span, and add the overhang before ordering.
Traps
- Using the full span instead of half-span in rafter calculations.
- Forgetting the overhang when ordering rafters.
- Cutting a birdsmouth too deep and weakening the rafter.
- Fixing roof sheets only at the ridges and ignoring edge fixing.