Roofing Calculator

Enter the length, width, overhang and pitch, and get the roof surface in square feet and square metres, the number of roofing squares, and an optional cost.

Waste is included, and underlayment is calculated alongside so the order is complete.

How to use the roofing calculator

Enter the building footprint and overhangs.

Set the roof pitch and waste allowance.

Read roof area, squares and cost.

Why use the Build Tally roofing calculator

Pitch-corrected area

Slope factor applied automatically.

Overhangs included

Eave and rake added before the calculation.

Squares and metres

Both the trade unit and metric area.

Underlayment too

Rolls estimated alongside the field material.

Footprint, overhang and slope

Three things separate the number you measure on the ground from the number a roofer bills for. The footprint is the building outline; the overhang adds to it on every side, so a 1 ft eave on a 40 × 30 ft building adds 148 sq ft before pitch is even considered; and then the slope factor multiplies the whole thing. A 6/12 roof on that footprint with 1 ft eaves is about 1,507 sq ft of surface, against a 1,200 sq ft footprint — 25% more than the ground area.

Roofing squares and how material is ordered

A roofing square is 100 square feet, and nearly everything on a roof is quoted in squares — shingles, underlayment, labour. Underlayment is sold in rolls typically covering four squares for synthetic or two for felt at standard overlap. Drip edge, starter strip, ridge cap and ice-and-water shield at the eaves and valleys are all separate items priced by the linear foot, and they are the parts most often left off a self-made estimate.

Simple and complex roofs

This calculator handles a straightforward gable or hip roof over a rectangular footprint well. Every dormer, valley, chimney and change of plane adds both area and waste, and complex roofs run 15% waste or more against 10% on a plain gable. For an L-shaped building, calculate each rectangle separately and add them, then treat the valley where they meet as extra waste. If the roof has more than a couple of planes, have a roofer measure it before you order.

Read the guide: Roof Pitch Explained: Rise Over Run, Degrees and the Slope Factor

Frequently asked questions

How do I calculate roof area?

Multiply the footprint including overhangs by the slope factor for your pitch. Enter both above and the calculator does the rest.

What is a roofing square?

One hundred square feet of roof surface. Materials and labour are usually quoted per square.

How much bigger is a roof than its footprint?

It depends on pitch. A 4/12 roof is about 5% larger, a 6/12 roof 12% larger, and a 12/12 roof over 41% larger.

Does the calculator include overhangs?

Yes. Enter your eave and rake overhang and they are added to the footprint before the slope factor is applied.

How much waste should I allow?

Ten percent on a simple gable, fifteen percent or more on hip roofs and anything with dormers and valleys.

Can I estimate a complex roof with this?

Break it into rectangular sections, calculate each, and add extra waste for the valleys. For anything intricate, have a roofer measure it.