Roofing Calculator
Calculate the total roof area for gable, hip, or flat roofs. Get roofing material quantities in squares, sheets, and tiles with waste allowance.
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How to use this calculator
Roof area is always larger than the floor footprint because of the pitch. A steeper pitch increases the roof area and material required.
- 1
Enter the building length and width (not including overhang).
- 2
Select the roof type and enter the pitch angle in degrees.
- 3
Enter eave overhang on each side (typically 0.3–0.6 m).
- 4
Enter the roofing sheet or tile area to get a sheet count.
Frequently asked questions
What formula does this roofing calculator use?
Both sides of the footprint are extended by twice the eave overhang, the extended footprint is multiplied by 1 ÷ cos(pitch), and the waste percentage is added. For the default 12 × 8 m building with a 0.5 m overhang and a 25° pitch, 13 × 9 = 117 m², the pitch multiplier is 1.103, and the roof area is 129.10 m², or 142.00 m² with 10% waste.
Is there any difference between the gable and hip options?
Not in this tool. Gable and hip use the same arithmetic, the overhang-extended footprint divided by cos(pitch), so the default building shows 142.00 m² for both. That is consistent for a hip roof whose four faces all share the entered pitch, but it does not model ridge length, hip rafters or different pitches on different faces.
What does the flat option change?
It ignores the pitch entirely and uses the extended footprint, so the default building drops to 117.00 m² before waste and 128.70 m² after it. The pitch multiplier line still displays a number based on the pitch you typed, but it is not applied for a flat roof.
How do I convert rise-over-run into degrees?
Degrees = arctan(rise ÷ run). A 4 in 12 slope is about 18.4° and a 6 in 12 slope is about 26.6°. Enter the angle in degrees, between 0 and just under 90; a value of 90 or more is rejected because the multiplier would be infinite or negative.
What is a roofing square and how many does the tool show?
A roofing square is 100 square feet, and this tool treats it as 9.29 m². It divides the waste-adjusted area by 9.29, so the default building shows 15.3 squares. Some materials, including shingles, are priced per square, so the line lets you compare quotes with your own measurements.
How is the sheet or tile count worked out?
If you enter an area per sheet or tile above zero, the waste-adjusted roof area is divided by it and rounded up. A 10 × 7 m building at 35° with a 0.4 m overhang and 15% waste is 118.26 m², and with 0.66 m² sheets that is 180. Leave the field at 0 and the line is hidden. Use the effective covered area per piece, not its overall size, because overlaps reduce the coverage.
Does it include ridge caps, flashings, valleys, underlay or gutters?
No. It reports surface area and a piece count for the main roof planes. Accessories, penetrations such as chimneys and skylights, dormers and valley sections are not itemised, and the single waste percentage is the only allowance made for them.
Can I use it to decide what roof is safe or allowed?
No. This is a material estimate, not a structural design. Rafter sizes, spans, loads, minimum pitches for each covering and local building rules are for a qualified professional or inspector to determine, and the tool checks none of them.
Roofing calculator — roof area, squares and sheet count
Why roof area exceeds the floor plan
A pitched roof covers more surface than the building beneath it, because the slope stretches each metre of horizontal run into a longer sloping length. The factor that does the stretching is 1 ÷ cos(pitch): about 1.103 at 25°, 1.221 at 35°, and exactly 1 for a flat surface. The calculator first widens the footprint by the overhang on every side, since the covering extends past the walls, and then applies the factor to that wider rectangle. Material is therefore ordered against the sloping area, not the plan area on the drawings.
Worked examples
Four buildings were run through the calculator. The table lists the pitch, overhang and waste used with each, and shows the resulting area, squares and sheets. The last case is a pitch of 0° with no overhang and no waste, which confirms that the output reduces to the plain footprint.
| Building and settings | Area (no waste) | Area (with waste) | Squares | Sheets |
|---|---|---|---|---|
| 12 × 8 m, 25°, gable, 0.5 m overhang, 10% waste | 129.10 m² | 142.00 m² | 15.3 | not requested |
| 12 × 8 m, 25°, flat type, 0.5 m overhang, 10% waste | 117.00 m² | 128.70 m² | 13.9 | not requested |
| 10 × 7 m, 35°, gable, 0.4 m overhang, 15% waste, 0.66 m² sheets | 102.84 m² | 118.26 m² | 12.7 | 180 |
| 10 × 7 m, 0°, gable, no overhang, 0% waste | 70.00 m² | 70.00 m² | 7.5 | not requested |
Comparing the first two rows
The first two rows use the same footprint and differ only in the roof type. Selecting the flat option drops the pitch factor, and the area falls from 129.10 m² to 117.00 m², a difference of 12.10 m² before waste. That gap is the extra surface created by a 25° slope, and it is the quantity a quote based on the plan area alone would miss. The same calculation also shows that the overhang matters: the 12 × 8 m building has a 96.00 m² footprint, but 117.00 m² of covering once 0.5 m is added on each side.
From area to material
The with-waste area is converted two ways. Dividing by 9.29 m² gives roofing squares, the 100 square foot unit used to price shingles and many contracted jobs, and dividing by your sheet or tile coverage gives a piece count, rounded up. The 180 sheets in the third row come from 118.26 ÷ 0.66 = 179.2. Remember to enter the area a piece actually covers once overlaps are accounted for, as quoted by the manufacturer.
Boundaries of the tool
Only a rectangular building with a uniform pitch and overhang is modelled. Complex roofs with dormers, valleys, different pitches or cut-outs should be broken into separate rectangles and added by hand. The output is a surface-area and quantity estimate, not a structural design, and it does not check spans, loads or the minimum pitch a given covering needs. Those depend on the product and on local building rules, so confirm them with a qualified professional or your inspector.
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Results are estimates for informational purposes only and do not constitute professional financial, medical, legal, or technical advice. Read full disclaimer →