Waterproof Membrane Calculator

How the waterproof membrane calculator works

Behind the tiles of a shower or wet room there should be a waterproof layer: a sheet membrane bonded to the backing board, or a liquid membrane rolled or troweled on. Either way it is sold by coverage, so the order is a matter of dividing the area by what one roll or pail covers. The calculator adds an allowance for seam overlaps and waste, rounds up to whole units, and shows the spare left over and the cost.

Units = ⌈A × (1 + w) ÷ C⌉, where A is the area to waterproof, w the allowance as a decimal and C the coverage of one roll or pail. For a liquid, C must be the area one pail covers at the full number of coats, not the area of a single coat.

Liquid membranes: coverage at two coats

Custom Building Products' RedGard technical data sheet (TDS-104) tells installers to apply the membrane at 110 ft² per gallon for each coat, in two coats, so a 1 gallon (3.78 L) pail waterproofs 55 ft² (5.1 m²) and a 3.5 gallon (13.2 L) pail 192 ft² (17.8 m²), the figures in the sheet's own coverage chart. Those are the defaults here: 55 ft² on the US side and 5.1 m² on the metric side. Used as the shower pan liner itself, to the IAPMO requirement, RedGard goes on at 80 ft² per gallon per coat, so a gallon covers only 40 ft² (3.7 m²); enter that coverage for a floor done that way. Other liquid membranes have their own rates, and the data sheet is where to find them. Applying the product thinner to make a pail go further leaves a film below the thickness the product was tested at, which defeats the point.

Sheet membranes: seams overlap

Sheet membranes such as Schluter KERDI are joined by lapping one sheet over the next. The Schluter Shower System Installation Handbook calls for a 2 in (50 mm) overlap at seams, bonded with thin-set mortar, and notes that a 2 m wide roll covers a wall to showerhead height without a horizontal seam. Each seam uses up coverage: a 50 mm lap on a sheet 1 m wide takes 5% of the sheet, on a 2 m wide roll 2.5%. Add cutting waste around the niche, bench and valve, and the default 10% allowance is a reasonable starting point for a sheet; a liquid needs less, only what is left on the roller and in the tray.

What to measure

Measure every surface that gets membrane: the shower floor, each wall of the enclosure to the height you are waterproofing, the curb or upstand, and the niche and bench faces. In the United States the International Residential Code requires the finish on shower walls to be nonabsorbent to at least 6 ft (1,829 mm) above the floor (IRC 2024, section R327.2), which covers the finish, not the membrane. Schluter's handbook sets the membrane minimum as showerhead height, and in an enclosed shower, such as a steam shower, every surface including the ceiling, door jambs and door header. Wet rooms usually waterproof the whole floor and the walls around the shower zone. Your local rules and the membrane maker's instructions set the minimum.

Worked examples

A 5 ft by 3 ft alcove shower waterproofed to 7 ft has a back wall and two end walls: (5 + 3 + 3) × 7 = 77 ft², plus the 15 ft² floor, 92 ft² in all. With 10% allowance that is 101.2 ft², and 101.2 ÷ 55 = 1.84, so two 1 gallon pails. On the metric side, a 1.5 m by 0.9 m shower to 2.1 m has (1.5 + 0.9 + 0.9) × 2.1 = 6.93 m² of wall and 1.35 m² of floor, 8.28 m². With 10% that is 9.11 m², and 9.11 ÷ 5.1 = 1.79, so again two pails. A 100 ft² job on a single 3.5 gallon pail leaves 192 − 110 = 82 ft² unused, which is why the result shows the spare.

Details the area does not cover

The weak points of any shower are the changes of plane and the holes. Inside corners, the joint between wall and floor, pipe and valve penetrations and the drain all need their own pieces: pre-formed corners, pipe seals, seam band or embedded fabric, and a drain collar or bonding flange. RedGard's data sheet, for example, asks for a heavy precoat at corners extending 6 in (15 cm) each side and fabric embedded over gaps of 1/8 in (3 mm) or more. These are bought separately, which is why the result lists them as not included.

Curing before tiling or testing

Liquid membranes need to dry between coats and before tile goes on, and a flood test on the floor before tiling catches a leak while it is still cheap to fix. Drying times depend on temperature and humidity, so follow the product's own data sheet.

Using this with other BuildMetricLab tools

The bathroom tile calculator works out the tiles for the same walls and floor, and the shower slope calculator the fall of a shower floor to its drain. All BuildMetricLab tools run in your browser with no sign-up, and the formula is on the page so the maths can be checked.