Roof Cover Area by Slope
Computes real roof cover area from projected area and slope in degrees.
—
Roof area from horizontal projection and pitch
A pitched roof always covers more surface than its horizontal projection, since the slope stretches the plane out. The exact relation is A_roof = A_projection / cos(α), with α being the slope angle measured from horizontal. At a 30° pitch the projected area gets multiplied by 1.155; at 45° it's 1.414. If the pitch comes to you as a percentage (rise over run), convert it first with α = arctan(p/100) — a 30% pitch is the same as 16.7°.
The minimum pitch you can use depends on the tile. Ceramic colonial wants 30–40%, Roman/Portuguese around 30%, corrugated fiber-cement gets away with 9% (5–25°), American shingle sits at 15–30%, and standing seam metal can drop to 5%. The Brazilian standards ABNT NBR 15310 (ceramic) and NBR 7196 (fiber-cement) define each manufacturer's minimum. Drop below it and you void the leak warranty while inviting water to creep in by capillarity.
Applications
It feeds material take-off for tiles, underlayment and battens, structural load on the rafters, gutter sizing from the collected area, thermal and acoustic insulation, solar panel surface estimates, and how much waterproofing membrane to buy.
FAQ
Why isn't projected area enough? Order tiles off the floor plan footprint and you'll come up short by 15–40%. What actually needs covering is the real sloped surface, not its shadow on the ground.
Pitch in % or degrees? On Brazilian sites people work in %, but manufacturers often quote degrees. Convert between them with arctan and write both onto the project drawing so nobody guesses.
Does eave overhang count? It does. Take the projection with the overhangs included, usually 40–80 cm past the wall, and only then apply the cosine factor.
Related Tools
Laundry Room Size
Estimates minimum residential laundry area by equipment count.
Paint Calculator (Gallons)
Calculate how many gallons of paint you need for a wall or room, from the area and the coverage per gallon. Avoid buying too much or too little.
Roof Tiles Count by Area
Estimates roof tiles needed per square meter by type.
Number of Sprinklers
Calculate the number of automatic sprinklers needed, N = area ÷ coverage area per head, dividing the total area to protect (m²) by the maximum coverage area of each sprinkler (m²). The result is the minimum number of heads to cover the space, spaced within code limits (coverage per head depends on hazard class and sprinkler type). In practice, always round up and adjust to the piping and beam layout. It is an initial quantity calculation in sprinkler system design. Enter the area to protect and the coverage area per head.
Garden Plants Coverage
Computes number of seedlings to cover a garden by spacing.
Rainfall Runoff Coefficient by Area Calculator
Computes the weighted average rainfall runoff coefficient from the areas and partial coefficients of each surface type.
The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.