How this calculation works
Translate a rectangular horizontal footprint into sloping surface area when the whole roof has one common pitch. The overhang expands each side of the footprint before the slope factor is applied. This helps distinguish floor-plan area from the roofing surface that needs covering, but it does not account for valleys, dormers or roof sections with different pitches.
Worked example
This example uses illustrative inputs. Your current result is shown above.
With a 10 m × 8 m footprint, 1 m horizontal overhang on every side, a 30° pitch and 10% waste, the plan area is 12 × 10 = 120 m². Dividing by cos(30°) gives 138.56 m² sloped area; the material allowance is 152.42 m².
Assumptions and limits
- Footprint and overhang use the same length unit; overhang is its horizontal projection on each side, not a sloping rafter length.
- Pitch is degrees and must be below 89°. Complex roofs need separate sections; sheet laps, fastening and structural design are outside this area model.
Questions people ask
Is pitch entered as an angle or a rise/run ratio?
Enter the angle in degrees. A rise/run notation must first be converted using the arctangent of rise divided by run.
Do I double the footprint for a simple gable roof?
No. The full horizontal footprint already includes both sides. Applying the common slope factor accounts for their combined area.