Methodology and Sources

Conventions

Your inputs come first. A calculator never substitutes its own figure for one you have entered. Defaults exist so the page shows a meaningful example; each is labelled and sourced in the assumption table.

Every component is a separate line. Waste, laps, overhangs, tear-off, pitch and storey factors and fees appear as their own rows in the breakdown, never folded into a single figure. If a line surprises you, you can see exactly what caused it.

Geometry first. Roof area is the footprint including overhangs times the pitch factor, √(1 + (rise ÷ 12)²); hips and valleys use the hip factor, √(rise² + 12² + 12²) ÷ 12. Pitch is entered as rise per 12 or in degrees and converted exactly.

Units are converted, not approximated. Feet and inches convert to metres and centimetres with exact factors (1 ft = 0.3048 m). Roofing squares (100 sq ft) are kept as the unit in both systems because bundles and prices are packed to them. The result of a calculation is the same whichever system you enter it in.

Rounding happens at the end, and always up for things you buy. Intermediate values keep full precision. Bundles, rolls, pieces, boxes and panels are rounded up to whole units; lengths are rounded up to the inch where they are ordered to the inch.

Ranges, not single prices. Cost calculators report a low and a high figure from dated survey ranges, because that is what the surveys support.

Averages are averages. Where a default is a typical value, it is a starting point, not a claim about your roof.

Sources for default figures

Each calculator’s assumption table names the figure it uses and the date it was last checked. When a source publishes new data, the default and the date are updated together.

What the calculators do not model

They do not assess structure: rafter, truss and deck capacity, snow loads and the effect of a heavier roof are questions for the code’s span tables, the truss maker or an engineer, and the pages say so. They do not inspect a roof, diagnose a leak or judge whether a roof is safe to walk on. They do not know your local code amendments, wind and ice-barrier maps or permit rules, which govern over the model code. Estimating factors such as hours per square, pitch and storey multipliers and waste allowances are standard trade practice, not measurements of your crew or your roof. Cost ranges are national and do not predict a particular quote.

Testing

Every calculator carries a set of test cases with hand-worked answers in both unit systems. The tests run automatically before each update to the site, and a calculator that fails any of them is not published. When an error is reported and confirmed, a test case for it is added so that it cannot come back.