How many squares and bundles of shingles do I need?
Short answer: roof area = footprint × pitch multiplier. Squares = area ÷ 100. Bundles = squares × (1 + waste) × bundles per square, rounded up. A 40 × 30 ft footprint at 6 in 12 is 1,342 sq ft, or 13.42 squares. With 10% waste and 3 bundles per square, buy 45 bundles.
1. Your roof
Measure the house from the ground, including the roof overhangs. Use the outside edge of the eaves and rakes.
The 10% default comes from Roofr, a roofing software company, which gives 10-15% as a rough range. It is not an industry standard. Add more for hips, valleys and dormers. GAF lists 3 bundles per square for Timberline HDZ. Check the wrapper on your shingles.
2. What you need
- Roof area
- 1,342 sq ft
- Squares
- 13.42
- Bundles to buy
- 45
1,200 sq ft footprint × 1.118
1 square = 100 sq ft, before waste
44.3 exact, with 10% waste, rounded up
How this is calculated
- Pitch multiplier = √(1 + (rise ÷ 12)²). For 6 in 12 it is 1.118.
- Roof area = footprint × pitch multiplier. This holds for gable and hip roofs with one pitch on every plane.
- Squares = roof area ÷ 100.
- Bundles = squares × (1 + waste %) × bundles per square, rounded up.
- Starter strips, hip and ridge caps, underlayment and flashing are not included. Buy them separately.
The formula
- Footprint = length × width of the house, measured to the outside of the overhangs.
- Pitch multiplier = √(1 + (rise ÷ 12)²). This is plain geometry: the slope is longer than its level run.
- Roof area = footprint × pitch multiplier. It works for gable and hip roofs when every plane has the same pitch.
- Squares = roof area ÷ 100. Roofers price and order by the square.
- Bundles = squares × (1 + waste %) × bundles per square. Round up.
Pitch is the inches of rise for every 12 inches of level run. A 6 in 12 roof climbs 6 inches per foot. You can often find it on house plans, or a roofer's report.
Pitch multiplier table
The last three columns use a 1,200 sq ft footprint, 10% waste and 3 bundles per square.
| Pitch | Angle | Multiplier | Roof area | Squares | Bundles |
|---|---|---|---|---|---|
| 2 in 12 | 9.5° | 1.014 | 1,217 sq ft | 12.17 | 41 |
| 3 in 12 | 14.0° | 1.031 | 1,237 sq ft | 12.37 | 41 |
| 4 in 12 | 18.4° | 1.054 | 1,265 sq ft | 12.65 | 42 |
| 5 in 12 | 22.6° | 1.083 | 1,300 sq ft | 13.00 | 43 |
| 6 in 12 | 26.6° | 1.118 | 1,342 sq ft | 13.42 | 45 |
| 7 in 12 | 30.3° | 1.158 | 1,389 sq ft | 13.89 | 46 |
| 8 in 12 | 33.7° | 1.202 | 1,442 sq ft | 14.42 | 48 |
| 9 in 12 | 36.9° | 1.250 | 1,500 sq ft | 15.00 | 50 |
| 10 in 12 | 39.8° | 1.302 | 1,562 sq ft | 15.62 | 52 |
| 11 in 12 | 42.5° | 1.357 | 1,628 sq ft | 16.28 | 54 |
| 12 in 12 | 45.0° | 1.414 | 1,697 sq ft | 16.97 | 57 |
Worked example: a 40 × 30 ft house at 6 in 12
The footprint is 40 × 30 = 1,200 sq ft. The 6 in 12 multiplier is √(1 + 0.5²) = 1.118. Roof area is 1,200 × 1.118 = 1,342 sq ft, or 13.42 squares.
| Waste | Exact bundles | Buy |
|---|---|---|
| 0% | 40.2 | 41 |
| 5% | 42.3 | 43 |
| 10% | 44.3 | 45 |
| 15% | 46.3 | 47 |
Waste and bundles per square
- Waste. Roofr, a roofing software company, says a solid waste estimate is around 10% to 15%. That is a rule of thumb, not a standard. The calculator starts at 10%, which is on the cautious side for a simple roof.
- Complexity adds waste. A Fine Homebuilding roofer figures about 1% on a simple roof. He adds 5% or more for complex roofs with open valleys.
- Bundles per square. Fine Homebuilding says three bundles to a square is most common. GAF lists 3 for its Timberline HDZ shingles.
- Heavier shingles need more. Some heavy three-tab and laminated shingles take four or five bundles per square. Check the wrapper.
When to get a professional measurement
This method assumes one pitch on every roof plane. Dormers, valleys, mixed pitches, turrets and porch roofs break that rule. For those roofs, ask a roofer or a satellite measurement report for the real area. Starter strips, ridge cap, underlayment and flashing are separate purchases.
Stay off the roof to measure
Roofs are dangerous. OSHA says falls are the leading cause of death in construction. Do not walk on steep, wet or icy roofs. Measure the footprint from the ground with a tape, or take it from your house plans. If you must use a ladder, set it on stable, level ground and keep three points of contact while you climb.
Common questions
How many square feet is a roofing square?
100 sq ft. A 1,500 sq ft roof is 15 squares before waste.
How many bundles of shingles are in a square?
Usually three. Some heavier shingles need four or five, so read the bundle wrapper for your product.
Can I use the house's square footage?
Not directly. Living area leaves out the overhangs and garage, and it may count a second floor. Measure the footprint under the roof instead.
How do I find my roof pitch without climbing up?
Check the house plans or a past roofing estimate. A roofer can also measure it for you. Do not climb up just to check it.
How much waste should I add?
There is no fixed rule. A Fine Homebuilding roofer uses about 1% on a simple roof, while Roofr, a roofing software company, suggests 10% to 15%. Starting at 10% keeps you from running short. Add more when the roof has many hips, valleys or dormers.
Sources
- GAF: Timberline HDZ shingles product page (3 bundles per square)
- Fine Homebuilding: Don't fall short on shingles (three bundles per square most common, waste by complexity)
- Roofr (roofing software company): How to calculate roof waste factor (10% to 15%)
- OSHA: Protecting Roofing Workers (falls, ladder safety, three points of contact)