What a roof measurement has to give you

Whatever the method, a shingle order and a roofing estimate need the same handful of numbers:

  • The area on the slope, in squares. A square is 100 square feet of roof; the guide to roofing squares counts them.
  • The pitch, how many inches the roof rises for every 12 inches it runs.
  • The edges, in feet: eaves and rakes for drip edge and starter, the ridge and hips for cap, valleys for valley metal or a woven valley. The guide to the parts of a roof shows where each one is.

There are four ways to get them. They trade time on a ladder for money or accuracy.

1. From the ground, with a tape

This is the free way, and for a simple gable or hip roof it’s close enough to order from.

  1. Measure the house’s length and width along the outside walls.
  2. Add the overhang. Measure how far the eave sticks out past the wall, usually 12 to 24 inches, and add it on both ends of each side. A 50 by 30 foot house with 12 inch overhangs has a roof edge of 52 by 32 feet.
  3. Multiply for the footprint: 52 × 32 = 1,664 square feet. That’s the flat area the roof covers, not the roof.
  4. Find the pitch (next section).
  5. Multiply the footprint by the pitch’s roof factor. At 6/12 the factor is 1.118, so 1,664 × 1.118 = 1,860 square feet on the slope.
  6. Divide by 100 for squares: 18.6 squares.

The roofing calculator does steps 3 to 6 and adds the waste, bundles, drip edge, starter and ridge cap.

Roof factors by pitch

The factor is how much bigger the sloped roof is than the flat footprint under it.

Pitch Angle Multiply the footprint by
3/12 14.0° 1.031
4/12 18.4° 1.054
5/12 22.6° 1.083
6/12 26.6° 1.118
7/12 30.3° 1.158
8/12 33.7° 1.202
9/12 36.9° 1.250
10/12 39.8° 1.302
12/12 45.0° 1.414

Finding the pitch

The pitch is the rise in inches for every 12 inches of level run. Three ways to find it without guessing:

  • In the attic. Hold a level against the underside of a rafter, level it, and mark 12 inches along the level from where it touches. Measure straight up from that mark to the bottom of the rafter. A 6 inch measurement is a 6/12 pitch.
  • At a gable end. From a ladder, hold the level against the rake board the same way. The rise over 12 inches is the pitch.
  • On the roof. Set one end of a level on the shingles, level it, and measure from the 12 inch mark straight down to the roof.

The roof estimate guide shows how the measurements become a priced estimate. The roof pitch calculator turns a rise and run, or an angle from a phone’s level, into the pitch and its factor. The guide to finding a roof’s pitch has each way step by step, and the angle and percent for every pitch.

Pitch also decides what can go on the roof. Asphalt shingles need at least 2/12, and from 2/12 up to 4/12 the code calls for two layers of underlayment (IRC R905.2.2).

2. On the roof, with a tape

For a cut-up roof (dormers, several pitches, valleys everywhere), measuring each plane on the roof is more exact than the ground method. Measure each plane’s width along the eave and its length from eave to ridge along the slope, multiply, and add the planes up. Measure along the slope and no pitch factor is needed, because the tape is already on the slope.

It’s also the slowest way and the one with the most risk. Walk a roof only with fall protection, and not on a steep or wet one.

3. A roof report from the address

Aerial roof reports such as EagleView measure every facet, pitch and edge from imagery, without anyone on the ladder. They’re common in insurance work, and they’re the fastest way to an exact number on a complicated roof. A report costs money per address; for a plain gable, the ground method gets the same answer for free.

4. From a phone

Some apps measure the roof from the address on the phone and put the numbers straight into the estimate. In BluMatrix, an EagleView report can be ordered from the address, and the satellite roof sketch gives the roof’s outline and size; either way the squares, pitch and edges flow into the estimate instead of being typed in.

Turning the measurement into an order

  • Waste. Add about 10% for a plain gable and about 15% for a hip roof, more for roofs cut up by valleys and dormers. The cut pieces at hips, valleys and rakes never cover roof.
  • Bundles. Most 3-tab and architectural asphalt shingles come three bundles to a square (GAF), so 18.6 squares with 10% waste is 20.5 squares, 62 bundles.
  • Edges. Drip edge goes on the eaves and rakes (IRC R905.2.8.5), sold in 10 foot pieces. Starter strip runs the eaves; ridge cap runs the ridge and hips.

Common questions

What is a square in roofing?

100 square feet of roof, measured on the slope. A 1,860 square foot roof is 18.6 squares.

How big is the roof on a 1,400 square foot house?

It depends on the footprint, the overhang and the pitch, not the living area. A one-story 40 by 35 house (1,400 square feet) with 12 inch overhangs has a 42 by 37 footprint, 1,554 square feet; at 6/12 that’s about 1,737 square feet of roof, 17.4 squares. A two-story house with the same living area has about half the footprint, so about half the roof.

What does a 10/12 pitch mean?

The roof rises 10 inches for every 12 inches it runs: an angle of about 39.8°, with a roof factor of 1.302.

Can I measure a roof from the ground?

Yes, for a simple roof. Length and width from the outside walls, the overhang, and the pitch from a gable end are all the math needs. For a roof with many planes, a report from the address is more exact.

Is the roof area the same as the house’s square footage?

No. The roof covers the overhangs too, and the slope makes it bigger than the flat footprint. A 6/12 roof over a 1,500 square foot footprint with 12 inch overhangs is about 1,860 square feet.