Calculate horizontal projected roof area and actual sloped surface area for simple gables, sheds, single planes, known projections, or multiple independently sloped sections.
A roof plane covers a horizontal projected area. Multiplying that projection by the rise-to-horizontal-run slope factor gives actual sloped surface area.
All dimensions and areas are normalized before section totals, allowances, conversions, and optional area-based cost are calculated.
effective length × effective projected widthrise ÷ horizontal run√(1 + (rise ÷ horizontal run)²)projected area × slope factorbuilding length × building width × slope factoractual area × (1 + extra percentage ÷ 100)actual square feet ÷ 100A 40 × 30 ft centered gable at 6:12 has a 1,200 ft² projection. With a 1.118034 slope factor, actual area is about 1,341.64 ft², or 13.42 roofing squares.
A flat 30 × 20 ft projection has 600 ft² of actual area because its slope factor is 1.
A known 1,000 ft² horizontal projection at 4:12 becomes approximately 1,054.09 ft² of sloped area.
Adding 1-ft gable and eave overhangs to a 40 × 30 ft gable produces effective projected dimensions of 42 × 32 ft and a 1,344 ft² projection. At 6:12, actual sloped area is about 1,502.64 ft².
In multiple-section mode, a 1,000 ft² projection at 6:12 plus a 200 ft² projection at 3:12 totals 1,200 ft² projected and about 1,324.19 ft² of actual roof area before any project-level extra allowance.
Projected dimensions and run are plan-view measurements, not distances along the roof slope.
Eave overhang extends the projected width/run; gable or rake overhang extends the roof length at its ends.
Roofing-square output is an area equivalent only. Use Roofing Calculator for bundles and material quantities.
Split main roofs, porches, garages, and dormers into simple planes with their own rise-per-12 slope and quantity.
This calculator expresses slope as rise per 12 units of horizontal run. It divides rise by run and uses √(1 + slope²) to convert a horizontal projection to its simple sloped equivalent.
Projected area is the roof shadow in plan view. Actual area grows with slope according to the shared slope factor.
Each gable side projects across half the building width. Two times length × half-width simplifies to length × full width, so multiplying the full footprint by two would double-count the roof.
A shed has one plane. The calculator uses effective projected length and run once, then applies slope factor once. Single-plane mode expects projected dimensions already including overhangs.
For gables, eave overhang is added to both projected width edges and gable/rake overhang to both length ends. Shed mode uses the same fields as per-edge run and end overhangs.
Plane sections use length × projected width; known-area sections accept a horizontal projection. Each receives its own rise-per-12 slope and quantity before canonical areas are summed without intermediate rounding.
One roofing square equals 100 square feet of area. Extra allowance is shown separately and applied once after actual area is calculated.
This geometry calculator does not estimate bundles, sheathing, underlayment, fasteners, flashing, labor, structural loads, framing, drainage, product suitability, or code compliance.
Calculate horizontal projected area and multiply it by the rise-to-horizontal-run slope factor.
Greater rise relative to horizontal run increases slope factor and therefore actual surface area for the same projection.
It is the horizontal plan-view area covered by the roof.
It is the sloped plane area after slope factor is applied to the horizontal projection.
No. The full footprint already combines the horizontal projection of both equal roof planes.
Multiply projected length by horizontal run, then multiply once by slope factor.
Add eave overhang to both projected width edges and gable/rake overhang to both length ends.
Divide actual square feet by 100. This is an area equivalent, not a purchase quantity.
Yes. Add plane or known-projection sections with independent rise-per-12 values and quantities.
No. Use the Roofing Calculator for purchasing estimates.
No. Product suitability, drainage, structural design, and code requirements must be verified separately.
NRCA defines roof slope using vertical rise relative to horizontal run and recognizes ratio and degree expressions.