Formulas
Each calculator uses standard geometry and the published figures that tradespeople use: bag yields from manufacturer packaging, minimum slopes and spans from the International Residential Code (IRC), and material densities from industry references. The formula appears on every page under "How we calculate".
Checking
Before a calculator goes live we run every worked example on its page through the tool and compare the result with a hand calculation. We repeat this whenever a formula or default value changes.
Defaults and prices
Default prices are typical US retail figures at the time of the last update and are only there so the cost estimate isn't empty. Replace them with your local quote for a real budget.
Updates
Each page shows the month it was last reviewed. If a code requirement or product yield changes, we update the calculator and the date.
Corrections
Found something wrong? Tell us and we'll check it and fix it.