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Generator Hut

Roofing Calculator

Roof area, squares and shingle bundles from footprint and pitch.

Your measurements

The footprint at ground level, not along the slope.

Rise in inches per 12 inches of horizontal run.

Covers starter courses, ridge caps, hips and valleys.

Roofing required

14.76squares

14.76 squares — 45 bundles

Footprint area40 ft × 30 ft
1200 sq ft
Slope factorfor a 6/12 pitch
× 1.118
Actual roof area
1341.64 sq ft
With waste allowance+10%
1475.8 sq ft
Roofing squaresone square covers 100 sq ft
14.76
Bundles of shingles3 bundles per square
45

This assumes a simple gable roof over the whole footprint. Dormers, hips, valleys and overhangs all change the figure, and a complex roof can run 15% or more above the simple calculation.

Underlayment, drip edge, starter strip, ridge cap and nails are separate purchases. Ridge cap in particular is sold by the linear foot, not by the square.

Steep roofs are not a DIY job. Above about 6/12 you are working on a surface you cannot stand on unaided, and fall protection stops being optional.

Measure the building's footprint at ground level, pick the roof pitch, and it works out the actual sloped area — then converts that to roofing squares and bundles. The pitch is what makes roofing estimates go wrong: a 12/12 roof has over forty per cent more surface than the footprint it sits on.

Why use this tool?

Pitch handled properly

The slope factor is derived from the rise per 12 inches of run, so the area is the real sloped surface rather than the footprint.

Squares and bundles

Roofing is quoted in squares of 100 square feet and sold in bundles of three per square. Both figures are given.

Ground-level measurement

Measure the footprint from the ground rather than the slope from the roof. Safer, easier and the arithmetic handles the rest.

Honest about its limits

This models a simple gable. Hips, valleys and dormers add material, and the tool says so rather than pretending otherwise.

How this roofing calculator works

The slope factor is the hypotenuse of a right triangle with a 12 inch base and a rise equal to the pitch, divided by 12. A 6/12 pitch gives about 1.118, so the roof surface is nearly twelve per cent larger than the footprint beneath it. A 12/12 gives 1.414 — over forty per cent more.

Footprint area times that factor is the actual roof area. Adding the waste allowance and dividing by 100 gives roofing squares, the unit suppliers quote in, and multiplying by three gives bundles, since standard three-tab and architectural shingles are packed three bundles to the square.

The waste allowance covers starter courses, ridge caps and the cuts at hips and valleys. Ten per cent suits a simple gable; a roof with several planes and valleys can consume fifteen per cent or more, because every valley is a run of angled cuts down both sides.

How to use it

  1. Step 1: Measure the footprint

    Length and width of the building at ground level, including the overhang if the roof extends past the walls. Do not climb up to measure the slope.

  2. Step 2: Determine the pitch

    Rise in inches over 12 inches of horizontal run. A 6/12 is the most common residential pitch; you can measure it inside the loft against a level.

  3. Step 3: Set the waste allowance

    Ten per cent for a simple gable. Fifteen or more if the roof has hips, valleys or dormers.

  4. Step 4: Add the accessories separately

    Underlayment, drip edge, starter strip and ridge cap are all separate purchases, and ridge cap is sold by the linear foot rather than by the square.

Example usage

A 40 by 30 gable at 6/12
1,200 square feet of footprint times a 1.118 slope factor is 1,342, plus 10% is 1,476 — 14.76 squares, or 45 bundles.
The same house at 10/12
The slope factor rises to 1.302, giving 1,562 square feet before waste and 52 bundles. Seven extra bundles purely from the pitch, on an identical footprint.
A flat-roofed extension
At 0/12 the slope factor is 1, so the roof area equals the footprint. Flat roofs are not usually shingled, but the area figure is still the starting point for whatever membrane goes on.

Frequently asked questions

How many bundles of shingles are in a square?

Three, for standard three-tab and most architectural shingles. A square covers 100 square feet of roof. Heavier premium shingles are sometimes four or five bundles per square, so check the packaging.

How do I find my roof pitch?

Pitch is the rise in inches over 12 inches of horizontal run. The safest way is from inside the loft: hold a level horizontally against a rafter, measure 12 inches along it, and measure vertically from that point up to the rafter.

Why is the roof bigger than the house footprint?

Because a sloped surface is longer than its horizontal projection. At 6/12 the roof is about 12% larger than the footprint; at 12/12 it is over 40% larger. Estimating from the footprint alone is the most common roofing mistake.

Does this work for a hip roof?

Approximately. The total area of a hip roof over a given footprint is close to that of a gable at the same pitch, but the cuts along the hips generate substantially more waste — raise the allowance to 15% or more.

Can I re-roof over existing shingles?

Sometimes, but it depends on local building codes, how many layers are already there, and the condition of the deck underneath. It also adds weight the structure has to carry. Worth confirming before ordering material.

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