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.
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Roof Pitch and Construction Guide
What's the key thing to understand about 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, but always check the manufacturer's minimum pitch rating for your specific tile before assuming it's suitable at the lower end of this range.
What do I need to know about 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.
What do I need to know about 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 below it risks water ingress, even if the roof looks fine when first installed.
What's the key thing to understand about 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 verges typically needs a 4:1 or 5:1 sand:cement mix, similar to standard bedding mortar used elsewhere on the roof.