Roofing calculator
Voir le calcul — avec vos chiffres
This counts field shingles by area. It does not size rafters, judge whether the deck will take the load, or count ridge, hip, valley, starter and flashing — those come off your own measured lines and your supplier's product sheet.
Type the roof footprint and the pitch and you get the true sloped area, the number of roofing squares (100 ft² each) and the whole bundles to buy at your manufacturer's bundles-per-square. The footprint is stretched by the pitch factor — √(rise² + 12²) ÷ 12 — then the waste is added and the total is divided into squares and bundles.
Exemple détaillé
A gable roof over a 40 ft × 30 ft footprint at 6/12. Flat area is 40 × 30 = 1,200 ft². The pitch factor for 6/12 is √(6² + 12²) ÷ 12 = 13.416 ÷ 12 = 1.118, so the true roof is 1,200 × 1.118 = 1,341.6 ft². Add 10% waste: 1,475.8 ft², which is 14.76 squares. At 3 bundles per square that is 44.3, so buy 45 bundles.
The same method in metric: a 12 m × 9 m footprint at 6/12 is 108 m² flat, × 1.118 = 120.7 m² on the slope, and 132.8 m² with 10% waste. That is 1,430 ft², or 14.3 squares — at 3 bundles per square, 42.9, so buy 43 bundles.
Sur le chantier
The factor is not optional. A 6/12 roof (26.6°) is 12% bigger than the footprint, a 9/12 is 25% bigger, a 12/12 is 41% bigger. Order off the flat number and a steep roof leaves you a pallet short on a Saturday. The factor is the same one the roof pitch page calls the rafter multiplier — it stretches every dimension that runs up the slope.
Squares talk to the supplier, bundles ride the truck. The yard quotes per square; the pallet is built in bundles. Bundles per square is printed on the wrapper and changes with the product — a heavy laminate at four per square is a third more bundles for the same roof. Read the wrapper and put that number in, not the one you remember from the last job.
Cut-up roofs eat shingles. Every hip and valley is a diagonal cut, and the offcut on one side rarely fits the other. Ten percent covers a simple gable; a roof with dormers and valleys can want 15% or more. Starter and ridge cap are a separate line on the order — the field count here does not include them.
What went wrong?
Roofing takeoffs miss in three places: a flat area ordered as if it were sloped, a bundle count rounded the wrong way, and a metric pitch read as a ratio.
Quick referencefor roofing
- A square
- 100 ft² of roof surface (9.29 m²)
- Pitch factor
- √(rise² + 12²) ÷ 12
- 4/12 · 6/12 · 8/12
- 1,054 · 1,118 · 1,202
- 9/12 · 10/12 · 12/12
- 1,250 · 1,302 · 1,414
- Degrees to /12
- tan(angle) × 12 — 30° ≈ 6.9/12, 45° = 12/12
- Bundles
- squares × your manufacturer's bundles per square, rounded up
- Perte
- 10% simple gable · more with hips, valleys and dormers
Waste & overagewhat to add, and why
- Roofers add 10% on a plain gable and 15–20% on a hip roof or one cut up with dormers and valleys, because every angled cut wastes a piece of shingle. Starter, ridge caps and the hip caps are extra items on top of the squares here. The waste field starts at 10%.
- Bundles per square is your manufacturer's number — three is the common figure for three-tab, but architectural and designer shingles differ, so read the wrapper.
- Buy whole bundles from one lot number; a shade difference between lots shows on a roof from the street.
Mistakes to catchbefore you order
- Measuring the footprint and forgetting the pitch multiplier — a 6/12 roof has 12% more surface than its plan area, and a 12/12 has 41% more.
- Typing the footprint width to the outside of the walls when the eaves overhang by a foot each side; roofing covers to the drip edge.
- Adding the two sides of a gable as if each were the full footprint — the tool wants the plan area once; it covers both slopes.
What an inspector looks forthe building inspector at the roof call, and the manufacturer's technical rep behind the warranty
On this part of the job
- The deck: sheathing thickness, gaps, edge support, fastening, and that the deck is dry and sound before anything is laid over it.
- Underlayment and any ice-and-water protection — where it goes, how far up from the eave, and how the laps run — all from the covering's own instructions.
- Flashing at every penetration, wall, valley and chimney, and the drip edge at eave and rake. Flashing is where roofs fail and where inspections fail with them.
- Fastening: the number of fasteners per unit and their position, against the printed instructions. High-wind areas have their own pattern, and it is the manufacturer who sets it.
- Valley method, starter course and hip-and-ridge detail matching the system, not mixed between brands.
- Layers: whether a second covering over the first is allowed here at all.
Have this ready
- The bundle wrapper or the product data sheet with the installation instructions.
- Your square count and the waste you allowed, so a short order is caught on the ground.
- Enough of a valley and a penetration left visible to be looked at.
Every figure an inspector measures against comes from the code your jurisdiction has adopted, the stamped drawing, or the manufacturer's printed instructions — so none of them are printed here. Look the number up in your own book, type it into the tool, and the arithmetic is yours.
La formule
roof area = length × width × √(rise² + 12²) ÷ 12
Multiply the flat footprint by the pitch factor for the true area, add the waste, then divide by 100 for squares and multiply by bundles per square for bundles — rounded up to whole bundles. In metric the area stays in square metres; the squares line is shown for talking to a supplier who quotes that way.
What this tool assumes
That the whole footprint carries one pitch, that the roof is a plain plane or a set of identical ones, and that bundles per square is what your wrapper says. The default of 3 is a common figure for 3-tab shingles and nothing more; the waste default of 10% suits a simple gable. Ridge, hip, starter and valley material are not counted here.
Questions
- How do I calculate roofing squares?
- A square is 100 square feet of roof surface. Measure the footprint of the roof from the ground or the plans — length × width of the area it covers — multiply by the pitch factor to get the true sloped area, then divide by 100. A 40 × 30 ft footprint at 6/12 is 1,200 ft² flat, 1,342 ft² on the slope, or 13.4 squares before waste.
- What is the pitch multiplier and why do I need it?
- A roof is longer up the slope than it is across the ground beneath it, and the multiplier is that stretch. For a pitch of x/12 it is √(x² + 12²) ÷ 12 — 1.118 for a 6/12, 1.25 for a 9/12, 1.414 for a 12/12. Skip it and a 6/12 roof comes out 12% short. The tool applies it to the footprint you type.
- How many bundles of shingles per square?
- That depends on the product — three bundles per square is the long-standing figure for 3-tab and many architectural shingles, but heavier laminates run four or more and some run less. The number is printed on the wrapper or the spec sheet; type your manufacturer's figure and the tool counts whole bundles from it.
- How much waste should I add for a roof?
- Ten percent is a fair start for a simple gable with few cuts. Hips, valleys and dormers waste more off the angled cuts — many roofers go to 15% on a cut-up roof. Starter strip, ridge cap and the offcuts at rakes are not in the field count, so budget those separately or let the waste factor carry them.
- Do I measure the sloped roof or the flat footprint?
- Either works, as long as you tell the tool which. The footprint (from the ground or the plans) is easier and safer to get, and the pitch factor stretches it for you. If you have already measured up the slope with a tape, enter the pitch as 0/12 so the factor is 1 and the area is used as typed.
- What about ridge, hips, valleys and starter?
- This page counts field shingles by area. Ridge and hip cap are sold by the linear foot, starter strip by the roll or bundle, and valley metal by the length — measure those lines and price them from your supplier. They are deliberately outside this tool so the field count stays honest.
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