Sloped roof area is larger than the horizontal plan area; the angle determines the geometric correction.
Plan area and actual surface
The horizontal projection is the area a roof covers when viewed from above. The sloped surface is where tiles, sheets or membranes are installed. For a uniform slope at angle θ from horizontal, an approximate geometric relation is:
Sloped area = horizontal area / cos(θ)
Worked example
A roof over a 9 × 6 m rectangle projects to 54 m². With a uniform 30° pitch, cos(30°) ≈ 0.866. The combined sloped area is 54 ÷ 0.866 ≈ 62.35 m². If a hypothetical 12% allowance is applied, 62.35 × 1.12 ≈ 69.83 m² of equivalent material. Convert that figure to rolls, tiles or sheets using the manufacturer's useful coverage, not nominal dimensions.
Gable roofs and different slopes
The formula also describes the combined faces of a simple symmetrical gable roof when both pitches match. If slopes or face lengths differ, calculate each face separately and sum them.
Skylights, chimneys and edges
Openings may remove some coverage but introduce flashing, overlaps and special cuts. Do not assume deducting the opening produces an equal saving in purchased material. TechoExacto lets you draw these features and choose whether to deduct an area.
Limits and safety
Valleys, dormers, curved surfaces, varying pitches and overhangs require separate measurement. The estimate does not check structural strength, drainage, fasteners or local building rules.
Try it in TechoExacto
Enter your own measurements and compare the result. Actual quantities depend on your materials.