Roof Square Calc

Measure a roof from the ground

You need the footprint and the pitch. You do not need to walk on the roof.

Free to use. No registration. No uploads.

A wall measurement is the starting point, not the finished roof area. First add the eaves, then account for slope. These diagrams show a simple rectangular example; they are not construction drawings.

Open roof area calculator

Four ground-level steps

  1. Measure the outside walls

    From level ground, measure the outside wall length and width. Divide an L-shaped building into non-overlapping rectangles.

  2. Allow for the roof edges

    Determine horizontal overhang depth on each edge from an accessible position or a reliable plan. Do not climb a roof.

  3. Find the pitch

    Obtain roof pitch from plans or a qualified roofer. Use horizontal run, not rafter length, if calculating pitch.

  4. Estimate, then check

    Enter each rectangle in the roof area tool, with its pitch and overhang. Check the result against roof details before estimating an order.

Plan view: 40 by 30 foot walls with a one foot overhang on each edge make a 42 by 32 foot roof footprint

Top view

Outside walls
40 × 30 ft
Horizontal overhang
1 ft on every edge
Roof footprint
42 × 32 ft

Add the overhang to both sides of each wall dimension.

Roof section: a horizontal run of 12 and a rise of 6 form a 6/12 pitch. Sloped rafter length is not the horizontal run.

Side view

Horizontal run
12 units
Vertical rise
6 units
Roof pitch
6 / 12

Measure across, not along the slope. A 6/12 pitch rises 6 units for every 12 horizontal units.

Work through the ground measurement

For 40 × 30 ft walls with a 1 ft horizontal overhang, the footprint is (40 + 2) × (30 + 2) = 1,344 sq ft. At 6/12, multiply by 1.118033989 to estimate 1,502.64 sq ft of roof surface before waste.

For unequal overhangs, add left plus right to the width and front plus back to the length, then use those roof-edge measurements with the tool’s overhang field set to zero. Do not use a sloped eave-to-ridge distance as the plan width.

Estimates only. Ground measurements can miss hidden roof features; have the roof geometry and material order checked by a qualified roofer.

Before you measure

Can I measure the walls instead of the roof edge?

Yes. Add the horizontal overhang depth on both sides of each dimension. If you already have roof-edge dimensions, use zero overhang.

What if the overhang is different on each edge?

Add the actual opposing overhangs to each wall dimension yourself, enter those roof-edge dimensions, and set overhang to zero in the tool.

How do I handle an L-shaped house?

Split the plan into non-overlapping rectangles. Do not let the shared corner or any intersecting gable projection count twice. More complex geometry needs a professional takeoff.

Will a satellite image be accurate enough?

This tool does not process images. Image scale, eave visibility and roof geometry can introduce errors. Prefer known dimensions or a professional measurement.

Do I need to get on the roof?

No. Stay on accessible level ground. Never climb, lean out or use a ladder to supply calculator inputs. A qualified roofer can supply inaccessible measurements.

What details are missing from a ground estimate?

Dormers, intersecting roof planes, small additions, openings, material overlaps and trim can change the takeoff. Confirm these details before purchasing.