Solve backwards
How much flat roof will my liquid waterproofing membrane cover?
- 5 galThe example worked below
- Cold-Applied Liquid Waterproofing Membrane Volume CalculatorEvery figure is its own
You have
How much liquid waterproofing membrane?
That is 5 gal(18.9 L). Worked by the Cold-Applied Liquid Waterproofing Membrane Volume Calculator at the settings below.
The settings it uses
- Target Dry Film Thickness (mils)
- 60
- Product Solids Content (%)
- 80
What it will do
107 sq ft of waterproofed surface
9.94 m²
One flat roof. Enough for a flat roof about 10.3 × 10.3 ft.
Check it forwards
- Area to Waterproof
- 107 sq ft
- It asks for
- 5 gal
- You have
- 5 gal
The Cold-Applied Liquid Waterproofing Membrane Volume Calculator asks for 5 gal at that size.
If you had more, or less
Around your amount
Each worked by the Cold-Applied Liquid Waterproofing Membrane Volume Calculator at the same settings.
Half
2.5 gal
53.5 sq ft
a flat roof about 7.3 × 7.3 ft
Yours
5 gal
107 sq ft
a flat roof about 10.3 × 10.3 ft
Half as much again
7.5 gal
160 sq ft
a flat roof about 12.7 × 12.7 ft
Double
10 gal
214 sq ft
a flat roof about 14.6 × 14.6 ft
Common amounts
What each amount of liquid waterproofing membrane covers
The sizes it is usually sold in, each worked by the Cold-Applied Liquid Waterproofing Membrane Volume Calculator at the settings above.
| You have | It covers | As one flat roof |
|---|---|---|
| 1 gal(3.8 L) | 21.4 sq ft(1.99 m²) | a flat roof about 4.6 × 4.6 ft |
| 5 gal(18.9 L) | 107 sq ft(9.94 m²) | a flat roof about 10.3 × 10.3 ft |
| 10 gal(37.9 L) | 214 sq ft(19.9 m²) | a flat roof about 14.6 × 14.6 ft |
| 25 gal(94.6 L) | 535 sq ft(49.7 m²) | a flat roof about 23.1 × 23.1 ft |
Dry film from wet material
Wet film is the dry film divided by the solids content: 60 mils dry at 80% solids is 75 mils wet, 1.905 mm, which is 1.905 litres a square metre. Read backwards, a gallon covers about 21 square feet and 20 litres about 10.5 m². A 50% solids product at the same dry film needs 120 mils wet, so the same pail covers under two thirds as much — solids content is worth reading before price.
Fleece and details drink more
A cold-applied system is resin plus reinforcement, and the reinforcement drinks far more than the field coat does. Polyester fleece has to be wetted out from below and above to be properly embedded, and every upstand, internal corner, outlet, penetration and movement joint takes a reinforced detail band laid before the field goes down. Detailing uses material fast, so a busy roof covers less than this.
The roof is not flat
The area entered is a flat surface, and a waterproofed roof is not one. Upstands of 150 mm (6 in) or more right round the perimeter, curbs to every rooflight and plant base, gutter cheeks and the flange of every outlet are all additional surface, and on a small, busy roof that detail area can rival the field it surrounds. Measure them and add them before setting what you have against the roof.
How the answer is worked
Nothing here is a second formula. The Cold-Applied Liquid Waterproofing Membrane Volume Calculator runs forwards — a size in, a quantity out — and this page grows its own starting flat roof in proportion until the quantity it asks for is what you have. Where it rounds up to whole containers, the answer is the largest size before the next one would be needed.
Where the calculator adds an allowance for cuts or breakage when you order, the same allowance comes off here, because it is the same calculator: a stack you already own finishes less than its face value. Where it adds none, neither does this page — the settings beside the answer say which.
Cold-Applied Liquid Waterproofing Membrane Volume Calculator
runs it forwardsCalculate the liquid waterproofing material needed to achieve a target dry film thickness, accounting for solids content.
Work out what to buy insteadSolve backwards
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Solve backwards
Start from what is already in the yard
Paint, sheets, rolls, bags and pieces, each read the other way by the calculator that orders it.
The calculator's own figures
Nothing is estimated twice: the page runs the same calculator you would order with.
The largest size that fits
Where a calculator rounds up to whole containers, the answer stops before the next one.
Both unit systems
Gallons and litres, square feet and square metres, side by side.
Frequently asked questions
- How much waterproofed surface does 5 gal of liquid waterproofing membrane cover?
- At the Cold-Applied Liquid Waterproofing Membrane Volume Calculator's own settings, 107 sq ft (9.94 m²) — a flat roof about 10.3 × 10.3 ft. Every figure follows the settings, which you can change in the calculator.
- How much waterproofed surface does 20 L of liquid waterproofing membrane cover?
- At the Cold-Applied Liquid Waterproofing Membrane Volume Calculator's own settings, 10.5 m² (113 sq ft) — a flat roof about 3.2 × 3.2 m. Every figure follows the settings, which you can change in the calculator.
- How much does a gallon of liquid waterproofing cover?
- It depends on the dry film required and the solids content. At 60 mils dry and 80% solids, a gallon covers about 21 square feet (2 m²) at full thickness; at 40 mils dry it covers about 32 square feet. The data sheet's minimum dry film thickness is the figure to work from.
- What is the difference between wet and dry film thickness?
- Wet film is what goes on; dry film is what stays after the water or solvent evaporates. Dry equals wet times the solids fraction, so an 80% solids product applied 75 mils wet cures to 60 mils dry. Performance rests on the dry figure, which is why the wet build has to be worked out from it.
- How thick should a liquid roof membrane be?
- As thick as the manufacturer's system requires for that use — the calculator opens at 60 mils (1.5 mm) dry and accepts 20 to 120 mils (0.5 to 3 mm). Reinforced systems embed a fleece and need more material. The product's technical data sheet gives the minimum dry film thickness to meet.