Offline
No internet, no account required.
IRC R802
Validated against the standard.
PDF export
Pro-only. Generate professional reports.
$2.99 Pro
Lifetime unlock. No subscriptions.
How it works
Enter your values
Type in your inputs — the app fills code tables and constants for you, so there's nothing to look up.
Get a code-checked result
Instant answer with a clear PASS / FAIL and the exact IRC R802 reference behind every number.
Save & export
Keep a history of calculations and export a clean PDF report to share or attach to a permit (Pro).
What this calculator does
The footprint of a building is not its roof area — pitch adds surface. This app applies the slope factor to the footprint to get true roof area and converts it to roofing squares for ordering shingles or metal.
The slope factor
Roof area = footprint area × slope factor, where slope factor = √(rise² + run²) ÷ run.
For a 6/12 pitch the factor is √(6² + 12²) ÷ 12 = 1.118; a 12/12 pitch is 1.414. One roofing square = 100 ft².
Worked example
A 40 x 30 ft footprint (1,200 ft²) at 6/12 pitch: 1,200 × 1.118 = 1,342 ft² of roof = about 13.4 squares, plus waste for hips, valleys, and starter.
Related calculators
Cut the rafters with the rafter calculator and figure trim with the trim and molding calculator.
Roof area FAQ
Why multiply by a slope factor?
A pitched roof has more surface than the flat footprint beneath it. The slope factor — the rafter length divided by the run — scales the footprint up to the true sloped area.
What is a roofing square?
One square equals 100 square feet of roof area, the unit shingles and roofing materials are sold and estimated in.
Does it add waste?
Yes. You can add a waste percentage for hips, valleys, ridge, and starter course so the order is complete.
Does it work offline?
Yes, it runs 100% offline with no account. Free with rewarded ads; a one-time $2.99 Pro unlock adds PDF export and saved history.
Get Roof Area Calculator on your phone
Free with rewarded ads · $2.99 lifetime Pro · iOS & Android. Works 100% offline on the job site.