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Roofing · true area
What is the true area of a pitched roof?
Plan measurements turned into real roof area. Every roofing quote starts here, and everyone gets it wrong by forgetting that a 35° roof is 22% bigger than the footprint under it.
1
Measure it
Every box has its own unit next to it.
Change the unit beside any box and the number converts. The (i) beside each question says what the box wants and where the figure usually comes from. Anything marked per cent is a percentage of the quantity above it.
2
Check the rates
Rates and assumptions used
These are the figures the calculator works to. Change any of them and the order list updates — your merchant’s pack sizes and your own waste allowances beat the defaults every time.
How this one works
Rafter length = run ÷ cos(pitch), where run is half the span on a gable roof
Rise = run × tan(pitch)
Gable roof area = 2 × length × rafter length
Hipped roof area = plan area ÷ cos(pitch) — the hips do not change the total
A worked example
With the defaults the page opens on:
How long is the roof on plan? 10 metres
How wide is the span? 8 metres
What is the pitch? 35 °
Gable, hipped or mono-pitch? Gable — two slopes
How far does the eaves overhang? 0.4 metres
And the verge? 0.3 metres
Spare for waste 7.5 per cent
Geometry
Rafter run: 4.4 m — half the span
Rafter length: 5.37 m— run ÷ cos 35°
Ridge rise: 3.08 m — run × tan 35°
Roof multiplier: 1.221 × — how much bigger than the footprint
Areas
Plan area with overhangs: 93.28 m²
True roof area: 113.87 m²
Waste allowance: 7.5 %
Order area: 122.41 m²
Edges
Ridge length: 10.6 m
Eaves length: 21.2 m
Verge / barge: 21.49 m
Hip length, each: 0 m — not a hipped roof
Change any figure above and every line recalculates; the small print under each row is the sum that produced it.
Questions people ask
How do I work out roof area from the pitch?
Rafter length = run ÷ cos(pitch), where run is half the span on a gable roof; then 2 × length × rafter length. A 35° roof is 22% bigger than the footprint under it, which is the bit everyone forgets.
Is a hipped roof bigger than a gable roof?
No - for the same footprint and pitch the total area is the same (plan area ÷ cos pitch). The hips change the cutting and the waste, not the area, which is why the waste hint suggests 12-15% on a hip.
What does it do with overhangs?
It adds the eaves overhang to the span and the verge overhang to the length before working the geometry, because that is the roof you actually cover.
Can it do a pitch of 0°?
No, and it flags it: 0° or 80°+ will not give a sensible answer.
Read before you order. These work out quantities, not structural design — spans, lintels, rebar and foundation depths need an engineer and Building Control sign-off. Waste allowances are defaults, not promises: awkward cuts, long runs and bad weather all eat more than 5%. Where a merchant sells whole packs, the figure is rounded up to whole packs.
