Project guide
Roofing & exterior walls: material quantities for the building shell
Re-roofing a shed, cladding a garden room, or pricing a contractor’s quote all start with the same question: how much material covers this shell? These calculators give quantity estimates only; design, fixing, and safety stay with the plans and the installer.
Which calculator to use when
What the project involves
The outside of a building is ordered in layers, just like the inside. On walls that usually means insulation, sheathing or boarding, then siding or cladding over the top; on roofs it means decking, underlayment, and the final covering such as asphalt shingles. Each layer is bought by area, but roof area is larger than the footprint because of the pitch, and wall cladding is measured on the net wall area after doors and windows.
Roof and wall work at height carries real fall and structural risk, and fixing schedules, ventilation, and weatherproofing details vary by product and local code. Use these tools to check a quote, plan a budget, or order material for work a qualified installer will carry out, not as a substitute for design or safety planning.
Order of work
- Measure the building footprint and identify the roof pitch from plans or a pitch gauge, without climbing onto the roof.
- Estimate shingle squares and bundles with the Roof Shingles calculator, adding waste for hips, valleys, and starter courses.
- Get net wall area for each elevation with the Wall Area calculator, subtracting doors and windows.
- Size sheathing and insulation for walls or roof from those areas, using the panel and roll sizes you can buy locally.
- Estimate siding or cladding panels from the net wall area and the effective coverage per panel printed on the product.
Common mistakes
- Using the footprint as the roof area; a 6:12 pitch already adds about 12% before any waste.
- Ignoring overhangs and eaves when measuring the footprint for roofing.
- Using the nominal board width for cladding instead of the effective exposed coverage after overlap.
- Forgetting gable triangles when measuring exterior walls for siding.
- Planning roof measurements that require climbing up; take dimensions from the ground, plans, or a professional survey.
Common questions
FAQ
How do I find my roof pitch without going on the roof?
Check the building plans, measure the rise and run on a gable end from a ladder at the eaves, or ask the roofer quoting the job. The Roof Shingles calculator then applies the standard pitch multiplier to your footprint.
What is a roofing square?
A square is 100 square feet (about 9.29 m²) of roof. Asphalt shingles are commonly packed three bundles per square, but check the wrapper: some heavier products use more bundles per square.
Why does the siding calculator ask for panel coverage rather than board size?
Siding and cladding boards overlap or interlock, so the visible coverage per board is smaller than its nominal size. Use the effective coverage printed by the manufacturer to avoid running short.
Can these calculators tell me if my roof can take the load?
No. They estimate material quantity only. Structural capacity, fixing, ventilation, and safe access belong to a qualified roofer or engineer working to local code.