How to measure a roof from the ground: walls, overhang and pitch
You can measure most roofs without climbing on them. Tape the walls at the ground, add the overhang on every side, read the pitch at a gable end, then multiply that footprint by the pitch factor. Walls of 30 × 20 ft with a 12 in overhang make a 32 × 22 ft footprint, and at 6 in 12 that is 704 × 1.118 = 787 sq ft of roof.
The pitch factor works for the whole roof when every slope has the same pitch, a hip roof as well as a gable: the area comes from the footprint and the pitch, not from the shape. A porch or garage roof with a pitch of its own is a section of its own; for a dormer, take its footprint off the main section and add it back at the dormer’s pitch.
Worked example
Example: walls of 30 × 20 ft with a 12 in overhang all around, a gable roof at 6 in 12, 10% extra and 3 bundles to a square.
This example gives 26 shingle bundles.
- Section 1 (gable): 32 × 22 ft footprint with the overhang = 704 sq ft × 1.118 = 787.1 sq ft
- 787.1 × (1 + 10/100) ÷ 33.3333 = 25.97 shingle bundles before rounding.
- Underlayment: 787.1 sq ft ÷ 935.63 sq ft a roll = 0.84, rounded up (GAF FeltBuster, 48 in × 250 ft, after its 3 in side laps and 6 in end laps; double coverage below 4 in 12). The extra for cuts is for the shingles only.
- Starter strip along the eaves and rakes: 113.19 ft ÷ 120 ft a bundle (GAF Pro-Start) = 0.94, rounded up.
- Drip edge: 113.19 ft ÷ 9.83 ft a piece (10-ft pieces lapped 2 in, as the residential code requires) = 11.51, rounded up.
- Ridge cap over the ridges and hips: 32 ft ÷ 25 ft a bundle (GAF Seal-A-Ridge) = 1.28, rounded up.
- Nails: 8.66 squares × 256 a square (64 shingles a square, 4 nails each, the maker’s standard pattern; 6 nails each in high-wind areas), rounded up to the next hundred.
- Vents: 600 sq ft of attic ÷ 150 = 4 sq ft (576 sq in) of net free ventilating area (International Residential Code R806.2).
Other calculators for this job
Every step of a roof project: measure, the list, what it costs and the quotes.
Pitch from a gable end
How far the gable rises from the top of the wall to the peak, for a gable centered on its wall. Find your wall’s width and the rise you counted in siding rows: the table row it falls on gives your pitch. For any other width, divide the rise by half the wall and multiply by 12.
| Pitch | 20 ft wall | 24 ft wall | 28 ft wall | 32 ft wall | Pitch factor |
|---|---|---|---|---|---|
| 3 in 12 | 2 ft 6 in | 3 ft | 3 ft 6 in | 4 ft | 1.031 |
| 4 in 12 | 3 ft 4 in | 4 ft | 4 ft 8 in | 5 ft 4 in | 1.054 |
| 5 in 12 | 4 ft 2 in | 5 ft | 5 ft 10 in | 6 ft 8 in | 1.083 |
| 6 in 12 | 5 ft | 6 ft | 7 ft | 8 ft | 1.118 |
| 7 in 12 | 5 ft 10 in | 7 ft | 8 ft 2 in | 9 ft 4 in | 1.158 |
| 8 in 12 | 6 ft 8 in | 8 ft | 9 ft 4 in | 10 ft 8 in | 1.202 |
| 9 in 12 | 7 ft 6 in | 9 ft | 10 ft 6 in | 12 ft | 1.250 |
| 10 in 12 | 8 ft 4 in | 10 ft | 11 ft 8 in | 13 ft 4 in | 1.302 |
| 12 in 12 | 10 ft | 12 ft | 14 ft | 16 ft | 1.414 |
What changes the order?
Same footprint, different pitch: a steeper roof has more surface to cover. A roof report that already gives the sloped area needs no pitch factor.
| Calculator input | Result from those inputs |
|---|---|
| Pitch rise, in 12: 3 | 24 shingle bundles |
| Pitch rise, in 12: 6 | 26 shingle bundles |
| Pitch rise, in 12: 9 | 30 shingle bundles |
| Pitch rise, in 12: 12 | 33 shingle bundles |
Try your own measurements in the roofing calculator.
Tape the walls at the ground
Walk around the house with a 100 ft tape or a measuring wheel and measure each wall along its base, corner to corner. Sketch the outline as you go and write each measurement on it. An L-shaped house is two rectangles that do not overlap.
Add the overhang
The roof runs past the walls at the eaves and at the gable ends. On a one-story eave, clear of the power line where it reaches the house, hold a long straight board upright against the end of the fascia, the board the gutter hangs on, check it plumb with a level, and measure along the ground from the wall to the board. Measure to the fascia, not the gutter. Add the eave overhang to both sides and the gable-end overhang to both ends, on the outside edges only: 30 × 20 ft walls with 12 in all around become a 32 × 22 ft footprint.
Read the pitch at a gable end
Stand square to a gable end. The wall rises in a triangle from where the roof edges cross the corners of the wall up to the peak, over half the wall’s width. Count the rows of siding in that triangle and measure one row at the ground: 12 rows of 5 in siding is a 5 ft rise. Over half of a 20 ft wall, a 10 ft run, the pitch is 5 ÷ 10 × 12 = 6 in 12, a rise of 6 inches for every 12 across.
No gable to read? Use the attic
In the attic, stepping only on the joists, hold a level with its bubble centered, one end touching the underside of a rafter and the other reaching toward the ridge. Mark 12 inches along the level and measure straight up from the mark to the rafter: that is the rise in 12. From the ground, a phone angle app lined up with the roof edge while you stand square to the gable gives degrees; the roof pitch calculator turns them into inches in 12.
Multiply by the pitch factor
Multiply the footprint by the pitch factor: 1.054 at 4 in 12, 1.118 at 6, 1.202 at 8 and 1.414 at 12. A section with another pitch gets its own line. Then add about 10% for cuts on a simple roof and 15% or more on one with hips, valleys and dormers.
Check it against a quote
An aerial measuring report gives the sloped area, or squares of 100 sq ft, and should come close to your footprint times the pitch factor. A quote’s squares usually add the extra for cuts. A much larger figure is a reason to ask what it counts: more waste, or a section such as a garage or porch that you left out. An area measured on an online map is seen from above, so it is the footprint, not the sloped area.