Roof coating calculator

Dry mils and wet mils

Coating thickness is measured in mils, thousandths of an inch. Wet mils are the thickness as the coating goes on, checked with a notched wet film gauge during application. Dry mils are what is left once it cures, and that is the number specs and warranties set.

Wet readings have to be taken right away. As the water or solvent evaporates, the film gets thinner, so a late reading comes out low. The calculator gives the wet thickness to aim for, so the crew can check it with a gauge as they go.

Why volume solids matter

Volume solids is the share of a coating, by volume, that stays on the roof once the water or solvent has evaporated. A coating with 50% volume solids loses half its wet thickness as it cures; at 100%, the wet and dry thickness are about the same. So the lower the solids, the more gallons it takes to reach the same dry mils.

Gaco's data sheets list solids both by weight and by volume, and the two differ: its GR16 silicone lists 80% by weight and 69% by volume. Use the volume figure. It varies a lot between products; Gaco lists 95% for its S42 silicone and 51% for its A47 acrylic.

Manufacturers' own rates follow the same math. Henry lists its 887 Tropi-Cool silicone at 91% volume solids and calls for at least 1.5 gallons per 100 sq ft for a minimum of 22 dry mils: 1.5 gallons over 100 sq ft is about 24 wet mils, and 91% of that is about 22. Thinning a coating lowers its solids, so a thinned coat has to go on thicker.

Why rough surfaces take more coating

On granulated cap sheets, textured spray foam and other rough surfaces, part of the coating goes into covering the granules and texture. Gaco's S42 data sheet asks for about 1.25 gallons per 100 sq ft on smooth surfaces and 1.75 on granulated or rough ones, for the same 18 dry mils. That is 40% more material.

Other makers say the same. Henry's data sheet says coverage varies with surface texture and porosity, and Neogard's guide spec says rough foam may take more coats to reach the minimum thickness. On ribbed metal roofs, Gaco adds a surface factor for the panel profile, normally 15 to 40%. Raise the allowance for a rough or porous roof.

The default 10% allowance follows Gaco, which advises adding a 10% variance factor so the application meets its minimum thickness. Overspray, wind, spills and coating left in the pail use up material too.

The manufacturer's spec sets the thickness

For a manufacturer's warranty, the dry thickness comes from that manufacturer's specification for your roof and warranty term, not from a general rule. Longer terms call for more coating: Neogard's guide spec for silicone over spray foam calls for an average of 30 dry mils for a 10-year warranty and 40 for 15 years.

The spec can also set a minimum at every point on the roof, not just an average, and the manufacturer or an independent inspector may check the work. The same Neogard spec sets a floor of 20 dry mils at any point for the 10-year system and 26 for 15 years.

Enter the spec's minimum dry mils and the volume solids from the same product's data sheet. The result is for planning and comparing bids; under the manufacturers' specs, the applicator is responsible for putting down enough coating.