Liquid Rubber / Roof Coating Calculator

How the liquid rubber and roof coating calculator works

Enter the area to coat, the coverage of one coat from the product's data sheet, the number of coats, the container size and the price of a container. The calculator divides the area by the coverage, multiplies by the number of coats, and rounds the total up to whole containers. It also shows the wet film thickness each coat gives, so the result can be checked against the thickness the maker asks for.

On the US units side coverage is in square feet per gallon per coat, the way US labels print it, so the volume is V = A × c ÷ q. On the metric side it is litres per square metre per coat, so V = A × q × c. The two are linked by simple geometry: one litre spread over one square metre makes a film 1 mm thick, and one US gallon (231 in³) spread 1 mil (0.001 in) thick covers 231,000 in², or 1,604 ft². So a coverage of q ft² per gallon gives a wet film of 1,604 ÷ q mils.

Where the default comes from

Coating standards leave coverage to the maker. ASTM D6083, the US specification for water-based acrylic elastomeric roof coatings, sets performance requirements but states that it does not give guidance on application. The default here is taken from the Rust-Oleum 710 Elastomeric Roof Coating data sheet: 1.5 gallons per 100 ft² for each of two coats, applied 24 wet mils thick to dry at 12 mils per coat. That is 66.7 ft² per gallon per coat, or 0.61 L/m², and the calculator's wet film line shows 24.1 mils for it. The same sheet raises the rate to 2 gallons per 100 ft² per coat on built-up and modified bitumen roofs, and tells users to multiply length by width by at least 1.15 on corrugated or irregular metal.

Per coat or for the whole membrane

Data sheets state coverage in two ways, and mixing them up gives a large error. Some, like the Rust-Oleum sheet, give coverage per coat and say how many coats to apply. Others give coverage for the finished membrane. EPDM Coatings, for example, lists its liquid butyl rubber at 250 ft² per 5-gallon pail at 20 mils dry film, which is 50 ft² per gallon for the whole thickness however many coats it takes. For a product quoted that way, set coats to 1 and enter the total figure. Entering a whole-membrane coverage with two or more coats would double or treble the order.

A worked example

A 215 ft² flat roof at 66.7 ft² per gallon per coat, with two coats, needs 215 × 2 ÷ 66.7 = 6.45 gallons, which is two 5-gallon pails with about 3.5 gallons over. On a modified bitumen roof at 2 gallons per 100 ft² per coat, the same area needs 215 × 2 × 0.02 = 8.6 gallons. On corrugated metal, the area to enter is 215 × 1.15 = 247 ft², which at the standard rate needs about 7.4 gallons.

In metric, 20 m² at 0.61 L/m² per coat with two coats needs 20 × 0.61 × 2 = 24.4 litres, which is five 5-litre containers. The same job with a whole-membrane coverage of 1.2 L/m² and coats set to 1 needs 24 litres.

Wet film and dry film

The film shrinks as the water or solvent leaves it. The dry thickness is the wet thickness times the coating's volume solids, which the data sheet gives: Rust-Oleum lists 51% solids by volume, so 24 wet mils dries to about 24 × 0.51 = 12.2 mils, which matches its 12 mil target. Two coats give about 24 dry mils in all. A coating spread thinner than the sheet asks for dries thinner than the film the sheet's figures are based on.

Practical points

The data sheet's coverage assumes a clean, sound, primed surface where the maker calls for primer. Rough and uneven surfaces take more than a smooth one: the Rust-Oleum sheet raises the rate on built-up and modified bitumen roofs, and adds at least 15% to the area of corrugated metal. Rust-Oleum's practical coverage figure allows for a 15% material loss, so check whether the coverage you are using already includes a loss allowance before adding more. Seams, penetrations and drains are usually reinforced with fabric or extra coating, which uses material beyond the field rate.

What this tool does not do

It does not choose a coating, check that it suits the roof or wall, or allow for primer, fabric or sealant. It turns a coverage rate, a number of coats and an area into a volume and a number of containers.

Using this calculator with other BuildMetricLab tools

The foundation waterproofing calculator and crystalline waterproofing calculator cover below-ground work, and the roof pitch and roof area tools help measure the surface. All BuildMetricLab tools run in your browser with no sign-up, and every formula is shown on the page so the maths can be checked.