What this tool does
Converts a horizontal footprint and roof pitch into true sloped area, then divides by the coverage of one package to give the number of packages.
How to use it
- Enter the horizontal footprint in square metres.
- Enter the pitch as rise and run — a 4/12 pitch is rise 4, run 12.
- Enter the waste allowance.
- Enter the coverage of one package and run it.
Worked example
A 100 m² footprint at 4/12 pitch with 10% waste and 3 m² packages gives a sloped area of about 105.41 m² and 39 packages.
Calculation method
Sloped roof area
area*sqrt(1+(rise/run)^2)Packages required
ceil(area*sqrt(1+(rise/run)^2)*(1+waste/100)/coverage)Frequently asked questions
- How do I measure the footprint?
- It is the outline area seen from above. Use the plan area from drawings rather than measuring the slope, and include the eaves overhang.
- Does a steeper pitch need more material?
- Yes. For the same footprint, a steeper pitch has more true area — about 1.054× at 4/12 and about 1.202× at 8/12.
What this tool does not cover
It assumes a single pitch across the whole roof. Valleys, skylights, vents, and ridge or hip trim must be computed separately.