Roof area, rafters, ridge height and how many sheets to buy — for gable and lean-to roofs.
Checked by the editorial teamRates and formulas: September 2026
Roof area—
How roof area is calculated
The slope length is the hypotenuse of a right-angled triangle where one side is half the width of the house (for a gable roof) or the full width (for a lean-to roof), and the angle is the roof pitch. The eaves overhang is added to it.
Example. A 10 × 8 m house, 30° pitch, overhangs of 0.5 and 0.3 m. Slope: 4 / 0.866 + 0.5 ≈ 5.12 m. Area: 2 × 5.12 × 10.6 ≈ 108.5 m². Ridge height — 2.31 m.
How many sheets to buy
The number of sheets in a row is the length of the roof slope along the eaves divided by the effective width of a sheet (usually 1.1 m for metal tiles, 1 m for S21 profiled sheeting). Sheets are ordered to the length of the slope to avoid horizontal joints. Allow 5–10% extra for trimming, and up to 20% for complex roofs with valleys.
Rafter size and spacing depend on the regional snow load and the roofing material. The calculator works out quantities, not structural strength.
This information is for reference only and does not constitute financial, tax or legal advice. Prepared by our editorial team, last checked: September 2026. Found an error? Write to us.
Frequently asked questions
How do I calculate the area of a gable roof?
Find the slope length: divide half the width of the house by the cosine of the pitch angle and add the overhang. Multiply by the roof length including overhangs, and by 2.
What roof pitch is best?
For most regions of Russia, 25–45°: snow slides off on its own and wind load is still moderate. For an attic room, 35–45° is usual.
How many rafters do I need?
Divide the length of the house by the rafter spacing (usually 0.6–1 m) and add one pair. For a house 10 m long with 0.6 m spacing, that is 18 pairs.
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