How to measure a roof from the ground
You don't need to climb up. Measure the length and width the roof covers seen from above: the outside of the house plus the overhang at the eaves and gable ends. Then find the pitch: how many inches the roof rises for every 12 inches across.
A steeper roof has more surface over the same footprint. For a pitch of 6 in 12, every 12 in across is 13.42 in along the slope, so the roof is 1.118 times the footprint. IKO's estimating guide works roofs out this way.
Source IKO · Estimating How Much Roofing Is Required ↗
How to calculate roofing squares
Here it is with your numbers:
- Work out the footprint, length × width: 50 × 30 = 1,500 sq ft.
- Work out the pitch factor: √(1 + (6 ÷ 12)²) = 1.118034.
- Multiply the footprint by the factor: 1,500 × 1.118034 = 1,677.051 sq ft of roof.
- Divide by 100 sq ft per square: 1,677.051 ÷ 100 = 16.7705 squares.
- Add 10% for waste: 16.7705 × 1.1 = 18.4476 squares, rounded to 18.45.
- Multiply by 3 bundles per square and round up: 18.45 × 3 = 55.35, so 56 bundles.
It means buying 56 bundles of shingles for 18.45 squares, 10% of it for waste.
GAF says contractors usually add 10 – 15% for waste (cutting at hips, valleys and edges), and IKO's estimating example uses 10%, so the waste box starts at 10%. Bundles per square starts at 3, the figure for GAF's Timberline HDZ.
Source GAF · How many bundles of shingles are in a square? ↗GAF · Timberline HDZ shingle specifications ↗
Roof pitch factors
Multiply the footprint by the factor to get the roof's area. Factors are √(1 + (rise ÷ 12)²) rounded to 3 decimals. IKO's printed table gives 1.157 for 7 in 12 and 1.356 for 11 in 12; worked out exactly they round to 1.158 and 1.357.
| Pitch | Angle | Factor |
|---|---|---|
| 1 in 12 | 4.8° | 1.003 |
| 2 in 12 | 9.5° | 1.014 |
| 3 in 12 | 14.0° | 1.031 |
| 4 in 12 | 18.4° | 1.054 |
| 5 in 12 | 22.6° | 1.083 |
| 6 in 12 | 26.6° | 1.118 |
| 7 in 12 | 30.3° | 1.158 |
| 8 in 12 | 33.7° | 1.202 |
| 9 in 12 | 36.9° | 1.250 |
| 10 in 12 | 39.8° | 1.302 |
| 11 in 12 | 42.5° | 1.357 |
| 12 in 12 | 45.0° | 1.414 |
FAQs
How many bundles of shingles are in a square?
For most standard asphalt shingles, 3 bundles make a square (100 sq ft of roof). GAF's Timberline HDZ comes 3 bundles to a square; its heavier Woodland shingles come 4. Check the wrapper of the shingles you buy.
Source GAF · How many bundles of shingles are in a square? ↗
What is a square in roofing?
A roofing square is 100 sq ft of roof, 10 ft × 10 ft. Roofers price shingles, underlayment and labor by the square, so a 2,400 sq ft roof is 24 squares.
How many square feet are in a bundle of shingles?
At 3 bundles to a square, one bundle covers about 33.33 sq ft (100 ÷ 3). At 4 bundles to a square it's 25 sq ft. The exact coverage is printed on each bundle.
How many bundles of shingles do I need for 1,000 square feet?
1,000 sq ft of roof is 10 squares, so 30 bundles at 3 bundles a square. With IKO's 10% for waste it's 11 squares, so 33 bundles.
How do you calculate roof pitch?
Pitch is how many inches the roof rises for every 12 inches it runs across, written like "6 in 12" or 6/12. Measure 12 in level along a rafter or the roof, then measure straight up to the roof. IKO also shows how to read it from the ground with a carpenter's folding rule.
How do you calculate the square footage of a roof?
Measure the area the roof covers from above (the footprint, overhangs included) and multiply by the pitch factor. IKO's example: 1,302 sq ft under a 9 in 12 roof × 1.250 = 1,627.5 sq ft of roof. Roofs with more than one pitch are worked one section at a time.
Sources
- IKO: Estimating How Much Roofing Is Required
- GAF: Timberline HDZ shingle specifications
- GAF: How many bundles of shingles are in a square?