How We Calculate

Every calculator on RoofingWala is built to help you understand a roofing job before you pay for one. This page sets out how they work, what they assume, and where we deliberately stop, so you can judge how much weight to put on a number.

The arithmetic is shown, not hidden

Each calculator uses the standard methods a roofer or supplier would use: roof area from the footprint and the pitch multiplier, materials from the coverage printed on the product, and so on. Every calculator page explains its method in plain language and works through an example with real numbers, so you can check the result by hand if you want to.

Your numbers stay on your device

The calculations run in your browser. Nothing you type is sent to us, stored, or used to contact you, and no calculator asks for your name, email or address. The only thing remembered is whether you prefer feet or meters, and that stays in your own browser. Until you choose, the calculators start in meters if your browser is set to UK, Irish, Australian, New Zealand or South African English, and in feet otherwise. If your browser is set to Spanish and you close the suggestion to read a page in Spanish, that choice is remembered in your browser as well.

We do not invent prices

Roofing prices depend on where you live, the material, the roof, and the contractor. An average from somewhere else can be wrong for your roof by a wide margin, so our cost tools ask for your prices, from real local quotes, rather than supplying one. They multiply your numbers by your roof and show the working.

Typical figures are labeled as typical

Some calculators start from typical values: the weight of a roofing material, the R-value per inch of insulation, the density of snow. These are labeled as typical wherever they appear, they can be changed, and the page tells you where to find the real figure, which is usually on the product label or data sheet.

Quantities are rounded up

You cannot buy 42.97 sheets of plywood. Anything you order by the piece is rounded up to a whole number, and waste allowances are shown separately so you can see how much of the order they account for.

Where we stop

Some roofing questions are engineering questions. How big a rafter or beam has to be, what snow or wind load a roof must be designed for, and what size of drain a flat roof needs all depend on local building codes and on calculations an engineer signs off. A wrong answer there can mean a roof that fails, so we do not offer calculators for them. Where a calculator comes close to that line, such as rafter length or snow weight, the page says plainly what it does not tell you and who does.

How the calculators are checked

Before a calculator goes live, its formula is tested against worked examples calculated by hand, including the examples on its own page. Each calculator page shows the date it was last reviewed.

If something looks wrong

If a result does not match what you expected, or a figure on a page looks out of date, please tell us through the contact page with the calculator name and the numbers you entered. Corrections are made to the calculator and its page, and the review date is updated.