How it works

A useful distinction

Pitch percentage is not extra area

A 6-in-12 pitch has a 50% rise-to-run ratio, but its slope multiplier is about 1.118. The modeled roof surface is therefore about 11.8% above horizontal area before allowance, not 50% larger.

Distinguishing roof footprint from sloped area

A roof's horizontal footprint is different from the surface area on a slope. Choose a footprint-and-pitch mode when that is the information available, or the measured-area mode when the roof surface is already known. Check how overhangs and package coverage are entered. The estimate models the chosen geometry and does not assess structural condition or installation requirements.

Pitched roof area: formula and assumptions

Roof area = plan area×√[1+(pitch/12)²]. One roofing square = 100 ft².

Use dimensions including overhangs. Complex roof geometry, valleys, dormers, and material courses require a measured takeoff.

Footprint, roof angle & overhang: formula and assumptions

Surface area = horizontal projected area × sqrt(1+(rise/12)²), or projected area/cos(angle).

Rectangular overhangs add twice the overhang to length and width. Known projected area must already include overhangs. Use separate sections for different slopes.

Measured roof area & packages: formula and assumptions

Packages = ceil(order area / installed coverage per package).

Measured area is roof surface, not horizontal footprint. Exact decimal package counting avoids rounding a one-third-square package to 33.33 ft².

Worked example

Using the Pitched roof area example:

Inputs

Building length (ft): 40
Building width (ft): 30
Pitch rise per 12 run (inches): 6
Waste allowance (%): 10

Method

Roof area = plan area×√[1+(pitch/12)²]. One roofing square = 100 ft².

Result

Example result

Roof surface: 1,341.64 ft²

With allowance: 1,475.8 ft²

Roofing squares: 14.76

These are illustrative inputs. Change the values to explore a different scenario.

Formula references

References explain the underlying methods; they do not imply endorsement or individual professional review of Finflo.

Common questions

Should known horizontal area already include overhangs?

Yes. In rectangle mode the entered overhang is added on both sides; known-area mode expects those overhangs already included.

Is package coverage based on horizontal footprint?

No. Package mode starts from measured roof surface area, adds the allowance and rounds up using installed coverage per package.

Prepared by Finflo. See our methodology and editorial policy.

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