How do I work out roof pitch without going on the roof?

From inside the loft, measure the rise and run along a rafter using a spirit level and tape measure, or use a smartphone inclinometer app resting flat against the rafter or roof slope to read the angle directly in degrees.

What is a low pitch roof?

In the UK, roofs under about 10 to 15 degrees are generally considered low pitch and need coverings rated for low-pitch use, such as single-ply membranes, GRP, EPDM or standing seam metal. Always check the manufacturer's minimum pitch rating before specifying a covering.

How do I convert roof pitch to the X-in-12 format?

Multiply the rise-over-run ratio by 12. For example, a roof with 600mm rise over 1200mm run has a ratio of 0.5, so it is a 6-in-12 pitch. This calculator shows the figure automatically.

Why does roof pitch affect how much material I need?

A sloped roof has more surface area than the flat plan area of the building below it. Multiply the flat plan area by the slope factor shown in this calculator to estimate the actual roof area for tiles, membrane or felt.

Roof Pitch Calculator (Free, UK) — Degrees, Ratio & Slope Factor

Use any unit you like (mm, cm, inches) — just use the same unit for both.

Rise

Vertical height from wall plate to ridge.

Run

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.

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Typical minimum pitch by covering (general guidance)

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°+

How the calculation works

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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