Roofing calculators

Which roofing calculator should I start with?

Start with pitch unless you already know it. US roof slope is stated as inches of rise per 12 in (1 ft) of horizontal run, e.g. 4:12. The pitch in degrees and x/12 tool converts between the ways pitch is written, which matters because UK roof tile data sheets state minimum pitch in degrees (e.g. 17.5 deg, 35 deg), not x:12.

With the pitch known, the roof squares and shingle bundles calculator multiplies the footprint by the pitch factor and converts the result to squares, then to bundles, starter, ridge cap and underlayment rolls. Metal is a different count: panels are sold by coverage width and length, not by area, so the metal roof panels and trim tool works in runs and panels instead.

Roofing pages

How much bigger is a roof than the house it covers?

The pitch factor answers that. Multiply the plan area, overhangs included, by the factor for your pitch and you have the sloped surface. The factor grows slowly on shallow roofs and quickly on steep ones, as the table shows. A frequent mistake is to price a roof from the floor area of the house: that leaves out the overhangs and the slope, and on a steep roof the shortfall is large.

The table is computed from the geometry, so it holds for any material. To read it, find your rise per foot of run in the first column and multiply your footprint by the factor in the third. The roof pitch multiplier chart (PDF) has the same figures on one printable page.

What is in a square of shingles?

Usually three bundles, but a square of bundles is not always a full hundred square feet. Three bundles of Timberline HDZ cover 98.4 sq ft, not a full 100 sq ft. Owens Corning Duration Premium is packed 4 bundles per square (98.4 sq ft); heavier shingles need more bundles. So the bundle count comes from the wrapper of the product you buy, not from a fixed rule. The calculator asks for it for that reason.

Trim waste is the other variable. ARMA says most roofs need 2% to 10% extra shingles for trim waste, more for complex roofs. Guertin: gable-end fall-off, valley trimmings and damaged shingles can add up to about 5%. Accessories are counted from edge lengths rather than area. ARMA: starter, drip edge, hip/ridge and valley flashing are estimated from eave, rake, hip, ridge and valley lengths. For example, Owens Corning ProEdge Hip & Ridge: 22 shingles (66 pieces) at 6 in exposure cover 33 lin ft (10.1 m) per bundle.

In the UK, concrete and clay tiles are counted per square metre, and the count depends on the headlap. Marley Modern (420 x 330 mm): 9.9 tiles/m2 at 75 mm headlap, 10.7 at 100 mm. UK slating and tiling is governed by BS 5534 (code of practice); Redland cites BS 5534:2014+A2:2018.

How low a pitch can each roofing material go on?

Each covering has a code or maker minimum, and below it the roof leaks however well it is laid. Under IRC R905.2.2 asphalt shingles may only be used on slopes of 2:12 (17%) or steeper. From 2:12 up to 4:12 the IRC requires double underlayment under shingles (R905.2.2 / R905.1.1). Metal can go lower, depending on the panel. IRC R905.10.2 (summary): 3:12 lapped without sealant, 1/2:12 with lap sealant, 1/4:12 standing seam; product limits may be stricter. Individual panels may be stricter than the code: Metal Sales gives a 3:12 minimum roof slope for Pro-Panel II.

UK tiles state their minimums in degrees and tie them to the headlap. Smooth Marley Modern tiles can go down to 17.5 degrees, but only with a 100 mm headlap. Marley concrete plain tile minimum pitch is 35 deg, headlap 65 mm on roofs. The roofing material comparison sets these limits side by side with weight and coverage.

How dangerous is roof work, and how often is it done?

Roofing is common and falls from roofs are serious. About 4.15 million US homeowners reported roofing replacement projects in 2023, totalling about $45.8 billion, the largest single exterior replacement category (Harvard JCHS Table A-1, 2023 AHS). For roofers at work, falls are the main cause of death by a wide margin, according to the Bureau of Labor Statistics' 2024 fatal injury census: Falls, slips and trips accounted for 96 of the 120 fatal work injuries among US roofing contractors in 2024, or 80% (BLS CFOI).

For homeowners, emergency department data give a sense of scale. An estimated 7,317 people (95% CI 5,224–9,410) were treated in US hospital emergency departments in 2025 for injuries involving falls from or onto a roof, identified from the case narrative because NEISS has no roof product code, per our analysis of CPSC NEISS data. Narratives are short, so this is a lower bound. 26.2% of people treated in US emergency departments in 2025 after a narrative-identified roof fall were admitted to hospital, about twice the 12.8% for stair injuries (1,920 of the national estimate, 95% CI 1,152–2,688), per our analysis of CPSC NEISS 2025 data. Ladders account for many more. An estimated 189,731 people (95% CI 152,918–226,543) were treated in US hospital emergency departments in 2025 for injuries involving ladders (stepladders, straight, extension and other ladders; codes 618, 619, 688, 4032, 4033, 4077, 4078), per our analysis of CPSC NEISS data. These are national estimates from a sample of US hospitals, with the margins of error shown, and the roof figure is a floor rather than a full count. The ladder and stair injury estimates page explains the method.

Prices have also moved. US producer prices for prepared asphalt roofing and siding products were 47.0% above the 2020 average in September 2026 and 2.7% higher than a year earlier (BLS PPI). See the lumber and drywall price trends page for the full set of series, and the construction waste statistics page for how many tons of shingles are torn off and landfilled each year.

Where do the roofing calculators stop?

They estimate materials for a roof whose structure is sound and whose pitch suits the covering. They cannot see rotten sheathing, the number of layers already on the roof or whether a local wind or snow rule applies. Layers matter for tear-off: IRC R908.3.1.1: no roof recover where the roof already has two or more applications of covering.

Complex roofs with many valleys, dormers and hips push waste to the top of the range and beyond, and they need accurate valley and hip lengths for flashing. Fasteners for metal panels depend on wind load. Metal Sales: fastener quantity and type must meet loading and code; use its fastener selection guide. Treat the calculator's fastener figure as a starting count and confirm it against the maker's fastening schedule.

For the sources behind each value, see formulas and data sources. Attic insulation is often done at the same time as a re-roof: the insulation calculators cover batts and blown-in fill. Copy-ready drawings of roof pitch and rafter parts are on the free roof and stair diagrams page.

Roofing calculator questions