Roof Pitch & Rafter Length Calculator
Calculate your roof pitch angle, rafter length, and total roof area from span and rise measurements. Includes felt, batten, and tile quantity estimates.
Roof Pitch and Construction Guide
Understanding Roof Pitch
Roof pitch is expressed as the angle in degrees, as a ratio (rise:run), or as a fraction (4/12 pitch means 4 inches rise per 12 inches run — common in US notation). Converting between formats: pitch ratio = rise ÷ (span/2). Angle = arctan(rise/run) = arctan(pitch ratio). Common UK pitches: 22.5° (4:12) — the minimum for most plain tiles and many slates. 30° — typical for many modern homes. 35°-45° — steeper Victorian and Edwardian properties. 15° — minimum for some interlocking concrete tiles (c
Rafter Length Calculation
Rafter length = √(rise² + (span/2)²), then add eaves overhang. For a roof with 8m span and 2m rise: run = 8/2 = 4m. Rafter length = √(2² + 4²) = √(4+16) = √20 = 4.47m. Add 300mm overhang: total rafter = 4.77m. This is the 'theoretical' rafter length — add 75mm to each end for cutting tolerance. Roof area = rafter length × roof length × 2 (for both slopes). For hip roofs, the calculation is more complex — the hip rafters are longer than common rafters.
Minimum Pitch Requirements
Different roofing materials have minimum pitch requirements: most plain clay/concrete tiles: minimum 35° (some approved to 22.5° with special underlay). Interlocking concrete tiles (Marley, Redland): 17.5°-22.5° depending on type and exposure. Natural slate: 25°-30° minimum (steeper for exposed sites). Metal sheet: can be as low as 2.5° for standing seam. Single-lap interlocking tiles in sheltered conditions: as low as 15°. The minimum pitch specified by manufacturers is not optional — going bel
Roofing Materials Quantities
Roofing felt (underlay): typically 1m wide rolls, laid horizontally with 100mm laps — total felt area = roof area × 1.15 (15% for laps). Battens: typically 25×50mm treated softwood. Batten gauge (spacing) depends on tile and pitch — check tile manufacturer specification. Total batten length = (roof area / gauge) × 1.1. Flashings: measure all junctions with walls, chimneys, pipes. Lead flashing is traditional (£15-25/m), code grade 4 for step and cover flashings. Mortar: pointing ridge tiles and
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