Common Mistakes When Replacing a Shingle Roof (Squares, Pitch)
Roofing is ordered in squares of 100 sq ft, and almost every mistake is a measuring one. A 40 × 30 ft house with a 6/12 pitch and 12 in overhangs is 15.0 squares, or 16.5 with 10% waste, which takes 50 bundles of 3 per square. The roofing calculator does the geometry.
Part of the project planners: Planning a roof replacement and Fall jobs for the yard and the outside of the house.
Mistake 1: using the footprint, not the roof area
A sloped roof is bigger than the floor under it. The same house by pitch:
| Pitch | Squares | Bundles (3 per square) |
|---|---|---|
| 4/12 | 14.2 | 47 |
| 6/12 | 15.0 | 50 |
| 8/12 | 16.2 | 54 |
| 10/12 | 17.5 | 58 |
| 12/12 | 19.0 | 63 |
Measure the pitch with the roof pitch calculator if you do not know it.
Mistake 2: forgetting the overhang
Eaves and rakes carry shingles too. By overhang:
| Overhang | Squares | Bundles |
|---|---|---|
| 0 in | 13.4 | 45 |
| 6 in | 14.2 | 47 |
| 12 in | 15.0 | 50 |
| 18 in | 15.9 | 53 |
Mistake 3: too little waste
Starter strips, ridge caps, cuts at valleys and hips and a few damaged bundles all take material. By waste allowance:
| Waste | Squares ordered | Bundles |
|---|---|---|
| 10% | 16.5 | 50 |
| 15% | 17.3 | 52 |
| 20% | 18.0 | 55 |
Mistake 4: skipping the underlayment and flashing
- Fit drip edge, underlayment and flashing, since they keep water out where shingles alone cannot.
- In cold climates add ice and water membrane at the eaves: see the ice and water shield calculator.
- Replace flashing at chimneys and walls instead of reusing worn pieces.
Mistake 5: nailing and safety
- Use the right nails in the right place, driven flush, not over-driven or crooked.
- Stagger the cutouts so that the joints do not line up course on course.
- Use a harness and anchors, and stay off the roof when it is wet, windy or frosty.
For underlayment, nails, vents and the rest on one list, use the roof replacement materials calculator. Estimates only: confirm bundles per square for your shingle.
How many bundles? See bundles of shingles for 1,000 square feet.
Frequently asked questions
How many bundles of shingles per square?
Usually three bundles make one square (100 sq ft), some products use four. The example roof of 15.0 squares takes 50 bundles of 3 per square including waste.
Does the pitch change how much I need?
Yes: a steeper roof has more surface than its footprint. The same house at 12/12 is 19.0 squares, against 15.0 at 6/12.
How much waste should I allow?
Ten percent for a simple gable, and 15 to 20 percent for a roof with hips, valleys and dormers: 50 bundles at 10%, 55 at 20%.
Is roof work a DIY job?
It can be, but falls from roofs are among the most serious home injuries. Use a harness and proper roof anchors, and do not work alone or in wet weather. Many people hire this one out.
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.