The roof is bigger than the building
This is the whole tool, and it is the most expensive misunderstanding in roofing.
A roof is tilted, so it has further to travel than the footprint it covers. The multiplier is the slope factor: one divided by the cosine of the pitch.
- 15°: 3.5% bigger than the plan
- 30°: 15% bigger
- 35°: 22% bigger
- 45°: 41% bigger
A 10m × 8m footprint at 35° is 80 m² on plan and 97.7 m² on the slope. Order tiles to 80 and you are seventeen and a half square metres short, before waste, and you discover it with three courses to go on a Friday.
So the tool shows plan area and slope area side by side, permanently. Hiding the plan figure and just giving the answer would conceal the exact thing somebody came here to understand.
Every plane gets its own pitch
Real roofs are not one pitch. A main roof at 35°, a rear lean-to at 15°, a porch at 30°. Averaging them, or running the whole footprint at the main pitch, gets every part of it wrong at once.
So each plane is entered separately with its own pitch and its own slope factor, and each row shows its own plan-to-slope conversion as you type. Add as many as the roof has.
Tile counts, and who the authority is
Roughly 60 plain tiles a square metre, 10 interlocking concrete, 16 pantiles, 21 slates at 500 × 250. Those are typical at a common gauge and they are genuinely only typical.
The real figure depends on the gauge the tiles are set to, which depends on the pitch and the required headlap, and a shallower pitch needs a bigger lap, which means a tighter gauge, which means more tiles per square metre. It moves in the direction people do not expect.
The manufacturer's fixing instructions are the authority and there is a box to put their number in. A calculator that pretends to know your tile better than its maker does is not being helpful.
Waste depends on how cut up it is
Five per cent on a plain rectangular roof with two verges and a ridge. That is genuinely enough.
A cut-up roof is a different job. Every hip and valley produces a raking cut down its whole length, and the offcuts are the wrong shape for anywhere else. Dormers, abutments and chimneys all add more. A heavily cut roof can run to ten per cent or beyond, and the tool lets you say so rather than assuming.
One thing about sheet
Profiled sheet is sold by area, so the slope figure is the order. But watch the side lap: the coverage width of a sheet is always less than its actual width, sometimes by a full profile. Ordering on sheet width rather than cover width leaves you short across the roof, in the same way ordering on plan leaves you short up it.
Plan into slope, and nothing beyond that
It does not size anything or decide anything. It converts plan dimensions into slope dimensions and counts what covers them.
Next to it: roof pitch for rafter and hip lengths off the same geometry, timber quantities for the carcass, and soakaway sizing, which needs the plan area rather than the slope area: the one place on the site where the footprint is the right number.
Common questions
How do I work out roof area from plan dimensions?
Multiply the plan area by the slope factor, which is one divided by the cosine of the pitch. A 10m by 8m footprint at 35 degrees is 80 square metres on plan and 97.7 on the slope. The tool shows both side by side, because the gap between them is the whole point.
Why is the roof bigger than the building?
Because it is tilted. The steeper the pitch the further the roof has to travel to cover the same footprint. At 30 degrees the roof is 15 per cent bigger than the plan, at 35 degrees 22 per cent, at 45 degrees 41 per cent. Order tiles off the plan and you are short by exactly that, and you find out on the last day.
How many roof tiles do I need, and how many per square metre?
Roughly 60 for plain tiles, 10 for interlocking concrete, 16 for pantiles and 21 for 500 by 250 slates. Those are typical at a common gauge: the actual figure moves with the pitch and the lap, and the manufacturer’s fixing instructions are the authority. There is a box to put their figure in.
Can I mix pitches on one job?
Yes, and you should. Each roof plane carries its own pitch, so a main roof at 35 degrees and a rear lean-to at 15 go in the same calculation with the right slope factor applied to each. Adding them at one pitch is a quiet way to get both wrong.
How much waste should I allow on tiles?
Five per cent on a plain rectangular roof. More on a cut-up one: hips, valleys, dormers and abutments all produce cuts you cannot use elsewhere, and a heavily cut roof can run to ten or more. The tool defaults to five and lets you push it.
Does it work for profiled sheet?
Yes, and sheet is sold by area rather than by unit so the slope area is the order. One thing to watch that no calculator can do for you: the coverage width of a sheet is always less than its actual width, because of the side lap. Order on cover width, not sheet width.