How many square feet does a gallon of roof coating cover?

At one mil dry, a gallon of a 100 %-solids coating covers 1,604 sq ft, and that is a definition rather than a measurement: a US gallon is 231 cubic inches, and 231 divided by one thousandth of an inch is 231,000 sq in, which is 1,604.17 sq ft. A real product covers that times its volume solids, divided by the dry mils you want. A 95 % solids silicone at 22 dry mils therefore covers about 69 sq ft per gallon theoretically, and about 52 after a 25 % deduction for a granulated surface.

How many gallons of silicone roof coating per square?

Two different manufacturers both publish a minimum of 1.5 gallons per square for their 100 % silicone, and one of them publishes 2 gallons per square instead when the surface is granulated or otherwise rough — for the same finished film. At their published volume solids of 95 % and 90 %, 1.5 gallons per square works out to about 22.9 and 21.7 dry mils respectively. Those are minimums, not targets, and a specification or a warranty may ask for more.

What is the difference between wet mils and dry mils?

Wet mils is the thickness of the coating as it leaves the wand, carrier and all. Dry mils is what is left once the water or solvent has gone. The ratio between them is the volume solids: a 50 % volume solids product laid at 20 wet mils dries to 10 dry mils. Wet mils is the only one you can check while you can still fix it, with a notched gauge to ASTM D4414; dry mils is what the specification is written in.

Why should I use volume solids and not weight solids?

Because coverage is a volume question and weight solids answers a different one. Solids are generally denser than the liquid carrying them, so the weight percentage is always the higher number — a published acrylic is 68 % by weight and 53 % by volume, and a published silicone is 92 % by weight and 90 % by volume. Using the weight figure on that acrylic would have you ordering for 16.4 dry mils and getting 12.8. The line you need says percent by volume and is tested to ASTM D2697.

How much extra coating does a granulated cap sheet need?

About 25 % more, and that figure is not an estimate: one manufacturer publishes 1.5 gallons per square on smooth surfaces and 2 gallons per square on granulated and other rough surfaces to reach the same 22 dry mils, and one minus 1.5 over 2.0 is exactly a quarter. The coating is filling the space between granules, where it does nothing for the measured film thickness. Spray foam and other textured substrates fall in the same bracket; heavily pitted or masonry surfaces can take up to 50 % more on the first coat alone.

Can I thin roof coating to make it spray easier?

The data sheets for all three products behind this calculator say not to, and the arithmetic explains the consequence rather than the rule. A pint of thinner per gallon leaves you with 1.125 gallons at 89 % of the original solids content, so a crew spraying to the same wet-mil target puts down about 11 % less dry film. It does not reduce how much coating you must buy, because the solids that have to end up on the roof are fixed by the area and the dry mils. It reduces the film you get for them.

How many coats of roof coating do I need?

That is set by the product and the specification, not by a rule. A 100 % silicone at 90 to 95 % volume solids reaches the low twenties of dry mils in one pass, so one heavy coat is normal. An acrylic at 53 % volume solids needs two, and its own data sheet says so: typical conditions require two coats at 24 wet mils each, with the second applied perpendicular to the first to reduce pinholes and voids. Two coats also find holidays that one coat hides, and on a heavily pitted substrate they cost less material overall, because only the first coat pays the profile loss.

Do I need to prime before coating a roof?

It depends on the substrate and the answer comes from an adhesion test rather than a catalogue. Where a primer is specified, the published rates are specific: half a gallon per square for a zinc-rich metal roof primer, a gallon per square for a solvent-based multi-surface primer, and a pint per square for a TPO primer, which the same document also expresses as a gallon per eight to ten squares. No waste deduction is applied to primer in the calculator above, because those published figures are already application rates over real substrates.

What happens if I coat a roof with wet insulation underneath?

It blisters, and the manufacturer literature says why in one sentence: moisture trapped below a coating will try to get out through the coating. That is the reason a moisture survey is a precondition for a system warranty on coating work — infrared, nuclear, capacitance or core sampling, with published core-count minimums by roof size. A coating does not dry a wet roof out. It seals the water in under a new vapour-retarding membrane. Wet insulation has to come out first, which is a tear-off question, not a coating question.

Why is my coating order short when the data sheet coverage looked right?

Three usual reasons, in order of how often they bite. First, the theoretical coverage figure was used without deducting anything for waste, overspray or substrate profile — that is 10 % on a smooth roof and 25 % on a granulated one. Second, the area was taken off the roof plan without adding the parapet faces, curb sides, equipment stands and gutter interiors, all of which take material at the field rate. Third, weight solids was used where volume solids was needed. Detail work and line purging account for the rest, which is why rounding up to whole pails is not really waste.

Roof Coating Coverage Calculator

Dry mils, wet mils, gallons per square and pails on the truck — worked from the one constant the whole trade runs on and almost nobody can name

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