FitAndFillShingle quantities are not based on the size of the house; they are based on the size of the roof, and a roof is always bigger than the footprint under it. The pitch is what tells you how much bigger. This guide explains what a pitch like 6/12 means, how to find yours without climbing up, and how it changes the number of bundles you buy.
What "6/12" means
Roof pitch is written as two numbers: how many inches the roof rises for every 12 inches it runs horizontally. A 6/12 roof climbs 6 inches for every 12 inches of horizontal distance. The 12 is always the same; only the first number changes.
Why the pitch changes your shingle count
The footprint is what you see on a plan: length times width. The roof surface is longer because it climbs. Pythagoras gives the factor that converts one to the other:
slope factor = √(1 + (rise ÷ 12)²) roof area = footprint × slope factor For 6/12 the factor is √(1 + 0.5²) = 1.118, so each square foot of footprint becomes about 1.12 square feet of roof.
| Pitch | Angle | Slope factor | Roof area for a 1,000 sq ft footprint |
|---|---|---|---|
| 2/12 | 9.5° | 1.014 | 1,014 sq ft |
| 3/12 | 14° | 1.031 | 1,031 sq ft |
| 4/12 | 18.4° | 1.054 | 1,054 sq ft |
| 5/12 | 22.6° | 1.083 | 1,083 sq ft |
| 6/12 | 26.6° | 1.118 | 1,118 sq ft |
| 7/12 | 30.3° | 1.158 | 1,158 sq ft |
| 8/12 | 33.7° | 1.202 | 1,202 sq ft |
| 9/12 | 36.9° | 1.25 | 1,250 sq ft |
| 10/12 | 39.8° | 1.302 | 1,302 sq ft |
| 12/12 | 45° | 1.414 | 1,414 sq ft |
How to find your pitch
From the attic
Hold a level horizontally with one end touching the underside of a rafter. At 12 inches along the level, measure straight up to the rafter. That distance in inches is the first number of the pitch. If it is 6 inches, the roof is 6/12.
From the end of the house, without climbing
Look at the triangle at the gable end. The rise is the height of the ridge above the line of the eaves, and the run is half the width of the house. Then pitch = rise ÷ run × 12.
Do not climb a steep, wet or damaged roof to measure it. The attic and the ground methods above give you the pitch safely.
Worked example: a 40 × 30 ft roof at 6/12
The footprint is 40 × 30 = 1,200 sq ft, including the overhangs. At 6/12 the roof area is 1,200 × 1.118 = 1,341.6 sq ft, which is 13.42 roofing squares (one square is 100 sq ft of roof).
With 10 % extra for cuts and waste that is 14.76 squares. If the shingles come 3 bundles to the square, you need 45 bundles. Had you used the footprint alone, with no pitch, you would have ordered about 40 bundles, 5 too few.
Common mistakes
- Using the footprint as the roof area. Always apply the slope factor.
- Leaving out the overhangs. The footprint should include the eaves and the rakes, not just the walls.
- Assuming one pitch for the whole roof. Porches, dormers and additions often have their own pitch. Calculate each part separately and add the areas.
- Forgetting the extras. Starter strips, ridge caps, underlayment and flashing are not part of the shingle count.
Frequently asked questions
Is a bigger first number a steeper roof?
Yes. 8/12 is steeper than 4/12. The angle for each common pitch is in the table above.
How many bundles are in a square?
It depends on the shingle, so read the package. The Roofing Shingle Calculator lets you choose 3, 4 or 5.
Can I use this for a flat roof?
A flat roof has a pitch of 0, so the slope factor is 1 and the roof area equals the footprint. Flat roofs usually use different materials, so these shingle numbers may not apply.