How this calculator works
This runs our roofing and attic ventilation calculators. The roof area is the footprint with overhangs times the slope factor for the pitch, then plus your waste allowance, in squares of 100 ft² at three bundles a square. Underlayment covers the whole deck, and the drip edge and starter follow the eaves and rakes.
Worked example: a 40 × 30 ft house with a 6/12 roof and 12 inch overhangs has a roof of 1,503 ft², which is 16.5 squares with 10% waste: 50 bundles of shingles, 2 rolls of underlayment, 15 pieces of drip edge and 4,800 nails, plus 16 ft of ridge vent.
Roofing materials by house size (6/12 pitch, 12 in overhang, 10% waste)
A simple gable roof.
| House footprint | Squares | Bundles | Underlayment rolls | Drip edge |
|---|---|---|---|---|
| 24 × 24 ft | — | 25 | 1 | 11 |
| 28 × 40 ft | — | 47 | 2 | 15 |
| 30 × 40 ft | — | 50 | 2 | 15 |
| 32 × 48 ft | — | 63 | 3 | 18 |
| 36 × 50 ft | — | 73 | 3 | 19 |
Tips before you buy
- Shingle bundles weigh 60 to 80 lb each. Spread them across the rafters, never in one pile.
- Check your local rules on permits, the number of layers allowed, and the shingles you may use.
- Plan the work for a dry spell, with the old roof torn off and the new one underlaid within the same day.
- A steeper roof is bigger: the slope factor is 1.118 at 6/12 and 1.302 at 10/12.
- Replace flashing and pipe boots when you reroof. They age faster than the shingles and are the usual leak points.
Tools & materials for this project
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Frequently asked questions
How many bundles of shingles for a 40x30 house?
About 50 bundles (16.5 squares) for a simple gable roof with a 6/12 pitch, 12 inch overhangs and 10% waste. A steeper 10/12 roof needs 58.
How many squares of roofing is a roof?
A square is 100 square feet of roof. Divide the roof area by 100, then add 10 to 20% for waste. Hip roofs and valleys waste more than a simple gable.
How much underlayment do I need?
The whole roof deck, with about 10% for laps: 2 rolls of 1,000 ft² for a roof of 1,503 ft². Add self-adhesive membrane along the eaves and in valleys where your climate calls for it.
Do I need a permit to replace a roof?
Usually yes. A permit also covers an inspection, and your building department can tell you how many layers are allowed and what shingles you may use.
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Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.