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About this tool
True sloped roof area from footprint, eaves overhang and pitch, plus rafter length, ridge, gutter run and sheet count.
This calculator converts a building footprint into the true sloped roof area using the slope factor sqrt(run² + rise²) / run, which is sqrt(144 + rise²) / 12 when pitch is written the trade way as rise per 12 of run. It adds the eaves and rake overhang before applying the factor, then returns common rafter length, ridge, hip rafter, gutter run and the sheet or tile count with a wastage allowance. Built for anyone ordering roofing material who has measured the plan but not the slope.
Open Roof Area Calculator on AltFTool — it loads instantly in your browser.
Enter the values you already know.
Fine-tune the options to match your scenario.
Read the result and use it in your planning or reporting.
Applies the exact geometric factor, so a 6-in-12 roof correctly comes out 11.8% larger than its footprint.
Extends the plan on all four sides first, which is where most manual take-offs lose square feet.
Returns ridge, hip and eaves lengths from the same geometry, ready for the timber and gutter order.
Multiply the plan area, including the overhang on all sides, by the slope factor sqrt(144 + rise²) / 12. A 6-in-12 roof has a slope factor of 1.118, so a 43 x 33 ft plan of 1,419 sqft becomes about 1,586 sqft of actual roof surface.
About 3% at 3-in-12, 12% at 6-in-12, 25% at 9-in-12 and 41% at 12-in-12, which is 45 degrees. The increase depends only on the pitch, not on the size or shape of the building, because both slopes cover the same plan area.
Profiled metal sheeting is normally laid at 2-in-12 (about 9.5 degrees) or steeper, and manufacturers usually require longer end laps and sealed side laps below 4-in-12. Anything shallower should be treated as a flat roof with a membrane rather than lapped sheets, especially in heavy monsoon rainfall.
Allow around 10% for a simple rectangular gable, 12 to 15% for a hip roof because every hip creates an angled cut, and more again where there are dormers or valleys. Ridge caps, barge boards and flashings are measured separately by running length and are not covered by the area wastage.
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