Ventilation and insulation
How to Measure Roof Pitch and Why the Angle Decides Materials
Roof pitch is calculated from rise over span using two measurements taken inside the roof void, and the resulting angle determines which coverings are legitimate for it. Download the checklist for the roof pitch and book a survey for anything it puts out of reach. This page covers: Roof pitch · Rise · Span.
Pitch is the angle of the slope from horizontal, in degrees. It is derived from the rise divided by the run, and both measurements are taken inside the roof void — never from a roof surface.
The run is half the span, and the pitch is the angle the rise makes across it. Roofs at 15 degrees or less are treated as flat whatever is on them.
Three things the arithmetic will not tell you
- Skip it if you can An angle finder or a phone inclinometer held against a rafter gives the pitch directly, which avoids the calculation entirely.
- Measure both slopes The two sides of a roof are not always the same pitch, and one measurement is not the roof.
- Check the addition A rear addition frequently sits at a lower pitch than the main roof, which is why it sometimes cannot take the same covering.
An approximate figure is enough to shortlist coverings. It is not enough to specify one, and the measurement is confirmed at survey.
How do you measure roof pitch?
Enter the roof void, measure the vertical rise from wall plate level to the underside of the ridge, measure the horizontal span between the wall plates, halve the span to get the run, then derive the angle from rise over run.
Is there a way without the maths?
Yes. Hold an angle finder or a smartphone inclinometer flat against the underside of a rafter and read the angle directly. It gives the same answer in seconds and avoids measuring rise and span at all.
Half the span, used with the rise to derive the angle; the angle is derived from rise divided by run.
What are rise, span and run?
Rise is the vertical height from wall plate level to the ridge underside. Span is the horizontal distance between the two wall plates. Run is half the span, and it pairs with rise to give the angle.
Measuring the angle directly on a rafter avoids the arithmetic entirely. The two sides of a roof are not always the same pitch. Additions frequently have a different and shallower pitch.
Do both sides of a roof have the same pitch?
Not always. Roofs altered over time, and additions built later, frequently carry different pitches on different slopes. Measure each slope you intend to cover rather than assuming one figure applies to the whole roof.
An approximate figure is enough to shortlist coverings, not to specify one. Measurement is taken inside the void, never from a roof surface.
- Rise divided by run gives the pitch angle.
Why does the angle decide the covering?
Every covering has a minimum pitch below which it stops shedding water reliably. The measured angle determines which coverings can legitimately be specified, so it is established before any material discussion begins.
- An inclinometer against a rafter gives the angle directly without arithmetic.
- The two slopes of one roof are not always the same pitch.
- An approximate figure shortlists coverings, but does not and specify one.
An approximate figure is enough to start.
What the measurement is good enough for
An angle finder or phone inclinometer held against a rafter gives the pitch directly, which almost every guide omits in favour of a calculation. The two sides of a roof frequently differ, particularly where an addition or later alteration exists — one measurement is not the roof.
Getting the pitch confirmed
Additions commonly sit at a lower pitch than the main roof, which is why they sometimes cannot take the same covering. The measurement is taken in the void; climbing onto a roof to measure it is both unnecessary and dangerous.
Pitch, and 6 more
- pitch
- rise
- span
- run
- wall plate
- inclinometer
- pitch gauge
Pitch is the angle between the slope and the horizontal, fixed by the rise over the run — the rise being the height from wall plate to ridge, the run half the span. The slope is longer than the run, so the area to be covered is always larger than the footprint of the building. At forty degrees the slope area is roughly thirty per cent greater than the plan area, which is the commonest reason a quotation lands above what a homeowner expected.
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