Home & Construction⏱ 5 min read
How to Calculate Roof Pitch and What the Numbers Mean
Roof pitch affects everything from materials to drainage to attic space. Here's how to measure it, what the different systems mean, and which pitches suit which roofing materials.
Roof pitch is one of the most practically important measurements in home construction and renovation. Get it right and water runs off efficiently; get it wrong and you end up with leaks, trapped debris, and materials that fail faster than they should.
How Roof Pitch Is Measured
Roof pitch describes the steepness of a roof as the ratio of vertical rise to horizontal run. There are three ways it's commonly expressed:
Pitch as a ratio (most common in UK): rise:run
e.g. 1:2 means 1 unit up for every 2 units across
Pitch as degrees: the angle of the roof slope
e.g. 30° from horizontal
Pitch as a fraction (common in US): rise/12 inches of run
e.g. 6/12 means 6 inches up per 12 inches horizontal
Converting Between Systems
Ratio to degrees:
Angle = arctan(rise ÷ run)
1:2 ratio → arctan(0.5) = 26.6°
Degrees to ratio:
Run = 1, Rise = tan(angle)
30° → rise = tan(30°) = 0.577 → ratio 1:1.73
US pitch (x/12) to degrees:
Angle = arctan(x ÷ 12)
6/12 pitch → arctan(0.5) = 26.6°
Common Pitch Categories
CategoryAngle RangeTypical Applications
Flat (low pitch)0°–10°Flat roofs, EPDM / felt coverings — requires special waterproofing
Low pitch10°–20°Some flat-profile tiles, corrugated metal
Medium pitch20°–35°Most concrete/clay tiles, slate, fibre cement
Steep pitch35°–50°Traditional slate, some clay tiles, thatched roofs
Very steep50°+Decorative, turrets — specialist materials needed
Minimum Pitches for Roofing Materials
MaterialMinimum Recommended Pitch
Plain clay/concrete tiles (standard)35° (1:1.4)
Interlocking concrete tiles17.5°
Natural slate25° (shorter lap at 30°+)
Fibre cement slate15°
Corrugated metal sheeting5°–10°
Standing seam metal3°+
EPDM / GRP (flat roofing)1.5° minimum fall
Installing tiles below their minimum pitch voids most manufacturer warranties and causes premature failure — water backs up under the tiles instead of running off cleanly.
How to Measure Existing Roof Pitch
Using a spirit level and tape measure: Hold a 600mm spirit level horizontally against the roof slope with one end touching the surface. Measure vertically from the far end of the level down to the roof. That measurement (in mm) ÷ 600 = the tangent of the pitch angle.
Example: level reads horizontal, vertical drop = 300mm
tan(angle) = 300 ÷ 600 = 0.5
angle = arctan(0.5) = 26.6°
Using a smartphone: A spirit level app can measure angle directly — hold the phone against the roof slope.
Roof Area Calculation
Roof surface area = Plan area × Roof pitch factor
Pitch factor = 1 ÷ cos(pitch angle)
Example: 8m × 6m house, 30° roof pitch
Plan area = 48 m²
Pitch factor = 1 ÷ cos(30°) = 1 ÷ 0.866 = 1.155
Roof area = 48 × 1.155 = 55.4 m²
This matters for calculating roofing material quantities — always base your tile or slate order on roof surface area, not plan area.