Roof Area Calculator
The area of a roof is not the area of the house underneath it. A roof slopes, so its surface is always larger than the ground it covers, and the steeper it is the larger it gets. This works the real figure out from two things you can establish without going anywhere near the roof.
Roof area from footprint and pitch
Turn a building footprint and a roof pitch into the actual roof area, in square feet and in roofing squares.
This calculator needs JavaScript to work. The measurements below are the ones you would need to do the sum by hand.
Roofing squares to order Roof area plus your waste allowance. One square is 100 sq ft.
- Footprint area
- Pitch multiplier How much bigger the sloped surface is than the ground it covers.
- Roof angle
- Actual roof area
- Roofing squares, before waste
- Area including waste
This is the standard footprint-and-pitch method, and it is what a roofer does before climbing anything. It assumes one pitch across the whole roof and a shape that is close to rectangular. Dormers, turrets, a second wing at a different pitch, or a complicated set of valleys will all push the real figure higher, which is what the waste allowance is there to absorb. For an order you are actually placing, have the roof measured.
How the calculation works
Take the footprint of the building, which is its length times its width measured to the outside of the eaves, and multiply it by a number that depends on the pitch. That number is the pitch multiplier, and it is the hypotenuse of the rise-and-run triangle divided by its run.
A 6 in 12 roof rises 6 inches for every 12 inches it travels horizontally. The sloping surface across that 12 inches is the square root of (6 × 6 + 12 × 12), which is 13.42 inches. So the roof is 13.42 divided by 12, or 1.118 times the ground it covers. A flat roof has a multiplier of exactly 1. A 12 in 12 roof, at 45 degrees, has one of 1.414.
A worked example
A house measuring 40 ft by 28 ft has a footprint of 1,120 sq ft. At 6 in 12 the roof surface is 1,120 × 1.118 = 1,252 sq ft, which is 12.5 roofing squares. Add a 10 per cent waste allowance and you would be ordering for about 13.8 squares.
Change nothing but the pitch and the answer moves a long way. The same house at 4 in 12 needs 11.8 squares before waste; at 12 in 12 it needs 15.8. That single number is why a steep roof costs more in materials before anyone has priced the extra labour.
What a roofing square is
One hundred square feet. It is the unit roofing materials are sold in and estimates are written in, and it exists because it is roughly what one person covers in a straightforward day. Bundles, underlayment rolls and labour rates are all quoted against it, which is why the calculator gives squares alongside square feet.
Measuring the footprint
Measure along the ground, around the outside walls, and include the overhang, because the roof covers it and it has to be shingled. Take the measurements at ground level rather than up a ladder.
If the house is L-shaped or T-shaped, break it into rectangles, work each one out separately and add the results together. If you already know the footprint from a plan, a survey or a property listing, switch the first control to the known-area option and skip the tape measure entirely. Check that the figure really is the ground area the roof covers, though, and not the total floor area of a two-storey house.
Where this method is wrong
It assumes one pitch across the whole roof and a shape close to a rectangle. It will understate a roof with dormers, a turret, a wing at a different pitch, or a porch roof tied into the main structure. It knows nothing about chimneys or skylights, which subtract a little area while adding flashing work worth far more than the area they save.
The waste allowance absorbs some of that, which is the point of it. Around 10 per cent suits a simple gable. Fifteen per cent or more is normal once there are hips and valleys, because every course running into them has to be cut to the angle and the offcuts cannot be used anywhere else.
Before you order anything
Have the roof measured properly. A contractor will do it plane by plane, or from aerial imagery, and their figure is the one an order should be based on. Use this to check that the number you have been given sits in the right region, and to understand what is driving it.