Do I use the footprint or the sloped area?

The sloped area, always. A 30 degree pitch adds about fifteen percent to the footprint and a 45 degree pitch about forty, and ordering against the footprint is the most common way to run short.

Construction & HomestructuralUnited States only

Roofing Material Calculator

Roofing material from roof area. The number to be careful about is the area itself: a pitched roof is larger than the building footprint by the pitch factor, and using the footprint is the classic underorder.

Also called: roof sheets calculator, shingle bundles.

Roofing units

7.1

7.1 sheets or bundles for 20 m² over 1 coat(s), including 10% wastage.

Area after deductions (m²)

Area across all coats (m²)

Before wastage

6.45

Area one unit covers (m²)

3.1

How this is calculated

Method and background

How this is calculated

Area divided by unit coverage, plus wastage. Enter the true sloped area rather than the plan area: a 30 degree pitch makes the roof about fifteen percent larger than the footprint, and a 45 degree pitch about forty percent. Wastage runs higher than for flat work because hips, valleys and ridges all need cutting.

quantity = area * coats / coverage per unit, plus wastage

A: Net area (m2)
c: Coats (count)
r: Coverage per unit (m2)
s: Wastage (decimal)

Worked examples

Each of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.

a sloped roof area

Length (m): 12
Width or height (m): 8
Or enter the area directly (m²): 0
Openings to deduct (m²): 0
Area one unit covers (m²): 3.1
Coats or layers: 1
Wastage: 12%

Roofing units34.68

96 / 3.1, worked by hand

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a larger sheet needs fewer

Length (m): 12
Width or height (m): 8
Or enter the area directly (m²): 0
Openings to deduct (m²): 0
Area one unit covers (m²): 6
Coats or layers: 1
Wastage: 12%

Roofing units17.92

boundary

Open this example

Method and limits

What it assumes

The area entered is the sloped roof area, not the building footprint.

What it deliberately does not model

Ridge, hip and valley pieces are separate items and are not counted.
Underlay, battens and fixings scale with area but are not included.

Formula version 1.0.0 · definition 1.0.0 · United States · Report a problem with this calculator

Frequently asked questions

Do I use the footprint or the sloped area?: The sloped area, always. A 30 degree pitch adds about fifteen percent to the footprint and a 45 degree pitch about forty, and ordering against the footprint is the most common way to run short.