Roof Calculator
Work out the actual roof surface area to cover — from the building footprint, roof pitch, and overhang. Covers single-pitch and gable roofs only; rafters, trusses, and load-bearing design are out of scope.
Building & pitch
Material & buffer
Before you order
- This calculator computes covering area ONLY — not rafters, trusses, purlins, or snow/wind load. Load-bearing roof design needs a qualified structural engineer, not this tool.
- The material you pick only changes the suggested buffer percentage (overlap and trim vary by material), not the area formula itself — the geometry is the same regardless of what you're covering the roof with.
- Eave overhang is a common design choice, not a fixed rule — check your actual project drawings or local practice; 0.3-0.6m is a typical range, but yours may differ.
- Hip roofs (four sloped sides) aren't covered here — their geometry (hip and valley lines) is meaningfully more complex than the two simple roof types this calculator supports.
How this is calculated
A sloped roof plane's real (on-slope) area is its horizontal footprint stretched by 1/cos(angle) — standard trigonometry, exact for a simple rectangular footprint. That gives the same total roof area for a single-pitch and a gable roof of the same footprint and angle, since a gable's two symmetric slopes cover the same horizontal projection as one single-pitch slope. What actually differs between the two types is slope length (useful for ordering sheet material by length): a gable roof's slope runs from eave to ridge over half the building width, a single-pitch roof's slope runs the full width. A buffer is added on top for cutting and overlap.
Common questions
Why isn't a hip roof (four-sided) included?
Because its geometry is meaningfully more complex — hip and valley lines create triangular and trapezoidal roof planes instead of simple rectangles, and getting that area right needs more careful geometry than this tool has been built and verified for. Rather than guess, it's left out for now, same reasoning as excluding I-beam from the metal weight calculator.
Why doesn't this calculate rafters, trusses, or load?
Because that's structural engineering, not a coverage-area calculation — rafter sizing and spacing depend on your local snow/wind load requirements and building codes, and getting it wrong is a genuine safety issue. This tool covers roof surface area only; talk to a structural engineer or your local building authority for the load-bearing design.
Why is the roof area the same for single-pitch and gable, but the slope length is different?
Because roof area only depends on the horizontal footprint being covered and the pitch angle — a gable roof's two slopes together cover exactly the same horizontal area as one single-pitch slope over the same footprint. Slope length is different because a gable roof splits the footprint's width in half between its two slopes (each one runs from the eave to the central ridge), while a single-pitch roof's one slope covers the full width.
Part of Home Improvement Calculators