What this calculator works out
This calculator converts a rise and a horizontal run into a roof pitch in degrees, and expresses the same slope as a ratio.
Pitch matters because roof coverings have minimum pitches below which they leak. Choosing a covering before knowing the pitch is the wrong way round, and it is a common source of expensive corrections.
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Before you measure
- Rise is the vertical height from the top of the wall plate to the ridge. Run is the horizontal distance from the wall to the point directly below the ridge — half the span on a symmetrical roof, not the full span.
- Both figures must use the same unit. Millimetres are used here.
- Measure inside the loft where you can, since fascia and soffit detail makes external measurement unreliable.
- On an existing roof, a pitch gauge or a phone inclinometer laid on a rafter gives a direct reading.
Plain clay and concrete tiles generally need 35° or more. Interlocking concrete tiles can go to around 17.5°, and some profiled metal and single-ply membranes work close to flat. Slate depends on size and lap. Going below a covering's stated minimum lets wind-driven rain track back under the laps.
How this calculator works
Basic trigonometry, expressed two ways:
Pitch in degrees = arctan(rise ÷ run) × 180 ÷ π
Ratio = run ÷ rise, quoted as 1 in that figureThe ratio form is the older convention and still appears on drawings and in guidance — a 1 in 2 roof rises one unit for every two horizontal. Degrees are more common on modern specifications.
Note that rafter length is longer than the run: it is the hypotenuse, found by Pythagoras from the rise and run.
Worked example: 1 m rise over a 2 m run
Using the default figures — 1,000 mm rise over a 2,000 mm run:
- Pitch: 26.6°
- Ratio: 1 in 2
26.6° sits below the minimum for plain tiles and above the minimum for most interlocking concrete tiles, which places it squarely in the range where the choice of covering is constrained rather than free. The rafter length for this roof would be 2,236 mm — the square root of 1,000² plus 2,000² — which is 236 mm more than the run, and worth knowing before ordering timber.
Common mistakes
- Using the full span as the run. On a symmetrical roof the run is half the span.
- Choosing the covering before checking the pitch. Minimum pitches are not negotiable.
- Mixing units between rise and run. Both must be the same.
- Assuming the rafter length equals the run. It is the hypotenuse and is always longer.
- Measuring from the fascia. Overhang and soffit detail distort the figure.
Frequently asked questions
What is a low pitch roof?
Broadly anything under about 20°, and under 10° is usually treated as flat for detailing purposes. Low pitches restrict the coverings available and demand more careful attention to underlay, laps and fixings, because water moves more slowly and wind can drive it uphill.
How do I find the pitch of an existing roof?
Inside the loft, hold a spirit level horizontally against a rafter and measure the vertical drop over a known horizontal distance — that gives rise and run directly. A digital angle finder or a phone inclinometer app placed flat on a rafter is quicker. Externally, a photograph taken square-on and measured on screen gives a reasonable approximation.
Does changing the pitch need permission?
Almost always. Altering a roof's pitch changes the height and shape of the building and generally requires planning permission, and it certainly engages building regulations for structure, insulation and fire. It is not a DIY alteration.
What pitch do I need for a shed or garden building?
Enough for the covering you intend to use. Felt shingles usually want 20° or more; roll felt and EPDM membranes work at much lower pitches. The other consideration is snow and water shedding — very shallow shed roofs pond and the covering fails early.
Is what I enter stored?
No. Measurements are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — Approved Document guidance on roofs, structure and weather resistance
- Health and Safety Executive — working at height and roof work safety
Sources are checked at publication and can change — how I choose and check references.