Deck Waterproofing Calculator

Total Waterproof Area starts from the flat deck footprint (Deck Length x Deck Width), inflates it slightly for the drainage slope -- sloped surface area grows by 1/cos(angle) as pitch increases, but that correction is tiny across the declared 0-5% range: at this calculator's default 20x12 ft deck, going from 0% to the full 5% maximum slope only adds about 0.3 sq ft, so Deck Slope should not be expected to move the material takeoff much even though it matters a great deal for drainage performance and code compliance. The area total then adds post-penetration flashing (1 sq ft per post), wall counter-flashing (1 sq ft per linear foot of wall contact), and edge wrap along any open perimeter that doesn't meet a wall, before a flat 10% waste factor is applied. Deck Length has the largest measured effect on both Total Waterproof Area and Estimated Material Cost, since it enters both the flat footprint and the open-edge perimeter length.

System Type (1=liquid polyurethane, 2=sheet membrane, 3=fiberglass/resin) changes which materials -- and which unit costs -- apply, and the cost ordering does not follow the code numbers: at this calculator's default dimensions, sheet membrane (2) is actually the cheapest of the three, undercutting liquid polyurethane (1) by roughly 30%, with fiberglass (3) landing in between. Number of Post Penetrations moves Estimated Material Cost only modestly, through added flashing area and proportional sealant tube usage.