Work out roof pitch in degrees, as a ratio, and as a slope factor for material take-offs — from rise and run, or from a known angle.
Use any unit you like (mm, cm, inches) — just use the same unit for both.
Vertical height from wall plate to ridge.
Horizontal distance, wall to ridge centre (half the span).
Pitch
33.7°
Ratio (rise : run)
1 : 1.5
X-in-12 pitch
8.0 in 12
Slope factor
×1.20
Per 1000mm run, rises
667mm
Roof area estimate: multiply your building's flat plan area by the slope factor above to estimate the actual sloped roof area. Example: a 60m² flat plan area × 1.20 slope factor ≈ 72m² of actual roof covering needed.
For UK roofing companies reviewing their online presence, enquiries or suitable contract opportunities.
These are common UK industry ranges, not a substitute for the manufacturer's data sheet or current Building Regulations and BS 5534 guidance. Always check the exact minimum pitch for the specific product you're specifying.
| Covering | Typical minimum pitch |
|---|---|
| Single-ply / GRP / EPDM flat roofing | 0°–5° (with adequate falls for drainage) |
| Standing seam metal roofing | 1°–10° depending on system |
| Interlocking concrete/clay tiles | ~17.5°–22.5° depending on manufacturer |
| Natural slate | ~20°+, often 25°–30° recommended |
| Plain clay/concrete tiles | ~35°+ |
| Thatch | ~45°+ |
Pitch angle is arctangent(rise ÷ run). The ratio is rise expressed against run; the X-in-12 figure (a common rafter-square notation) is that same ratio multiplied by 12. The slope factor is the hypotenuse divided by the run — multiply your flat plan area by this number to estimate the true sloped roof area for ordering tiles, membrane or felt.
Measuring rise and run: from inside the loft, place a spirit level horizontally against a rafter and measure straight down to the rafter at a fixed horizontal distance (the run) — the vertical drop at that point is the rise. No loft access? A smartphone inclinometer app held flat against an accessible part of the slope reads the angle directly — use the "I know the angle" tab.
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