Roofs and Carpentry
What the domain tests
Roofs and Carpentry tests roof types, pitch, trusses, rafters, purlins, battens, coverings, ventilation, gutters, timber joints and timber defects.
Core rules
- Pitch is rise over run and may be given in degrees or as a ratio. Minimum pitch depends on the covering.
- Rafter length for a simple pitched roof is found from the square root of rise squared plus run squared.
- A truss uses top chords, a bottom chord and webs; triangulation gives strength and stiffness.
- Purlins support rafters; battens carry tiles or sheets. Spacing follows the covering manufacturer and design.
- Ventilate the roof space, use flashings at junctions and give gutters a fall.
- Timber must be seasoned, graded and protected from damp; common joints include halving, mortise and tenon, and housing.
Worked example
A roof rises 2.5 m over a half-span of 5 m. Pitch tangent = 2.5/5 = 0.5, so the pitch is 26.6 degrees. Rafter length = square root of 25 + 6.25 = 5.59 m. Add a 0.4 m overhang, giving 5.99 m. Reasoning: pitch comes from the tangent; rafter length comes from Pythagoras.
Traps
- Confusing full span with half-span.
- Treating purlins as battens.
- Cutting truss members without design approval.
- Omitting roof-space ventilation.
- Using unseasoned or untreated timber in exposed positions.