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ARMORTHANE PROTECTING YOUR WORLD

PROTECTING YOUR WORLD

ARMORTHANE PROTECTING YOUR WORLD

Roof Coating
Coverage Calculator:
Gallons, Pails, Mils

Work out how much coating a roof really needs from the dry mils you want, the solids in the pail, the texture of the surface and the pitch, then check it on the roof with a wet film gauge.
Coating quotes

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Tell us the roof type, size and the coating you are considering. We will connect you with a certified ArmorThane applicator who measures the roof, specs the dry mils for the warranty you want and quotes material and labor in writing.

  • Roof type and current surface
  • Roof size, or the building address
  • Coating you are considering
  • Warranty term you are after
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Step 1 of 3
1,604
Sq ft one gallon covers at 1 wet mil
231 cubic inches per gallon
50%
Minimum volume solids for an acrylic roof coating
ASTM D6083
16 mils
Wet film from 1 gallon spread over 100 sq ft
16.04, to be exact
100%
Solids in spray polyurea, so wet film equals dry film
No solvent or water to leave

A roof coating coverage calculator is only as good as the number it starts from. Most estimates go wrong because they start from the coverage printed on the pail, which is a wet number, instead of the dry film thickness the warranty actually asks for. Start from dry mils and volume solids and the gallons come out right.

Quick answer

Gallons = area × dry mils ÷ (1,604 × volume solids), plus texture and loss. At 20 dry mils, one gallon of a 55% solids acrylic covers about 44 sq ft, a 92% solids silicone about 74 sq ft and 100% solids polyurea about 80 sq ft. Our roof coating coverage calculator below adds roof pitch, surface texture, coats and pail counts.

What's in this guide
  1. 1The coverage formula
  2. 2Coverage calculator
  3. 3Coverage by coating
  4. 4Texture and slope
  5. 5Checking it on the roof
  6. 6Questions
The math

How roof coating coverage is calculated

A US gallon is 231 cubic inches. Spread it 0.001 inch thick (one mil) and it covers 1,604 square feet. Every coverage number in the industry comes from that one fact, adjusted for how much of the liquid is left once the coating cures.

Step 1

Wet mils from gallons

Wet mils = 1,604 × gallons ÷ sq ft. One gallon over 100 sq ft is about 16 wet mils. One and a half gallons per square is about 24.

Step 2

Dry mils from solids

Dry mils = wet mils × volume solids. Water or solvent leaves, the solids stay. 24 wet mils of a 55% acrylic dries to about 13 mils.

Step 3

Real-world extras

Add for texture, slope, overspray, hose flush and the stripe coat on seams. Rough surfaces and wind eat more than people budget for.

Volume solids, not weight solidsData sheets often list both. Weight solids run higher because pigments and fillers are heavy, and using them makes a coating look like it covers more than it does. Thickness is a volume, so use volume solids. ASTM D6083, for example, sets acrylic minimums at 60% by weight but only 50% by volume.
Estimator

Roof coating coverage calculator: gallons and pails

Pick a coating to load a typical volume solids figure, then replace it with the number from your product's data sheet. Enter the plan (footprint) area; the tool adds the extra surface from pitch and texture.

Material math

Roof coating gallons, pails and wet mils

Surface area = plan area × √(1 + (pitch ÷ 12)²) × texture factor. Gallons = surface area × total dry mils ÷ (1,604 × volume solids) ÷ (1 − loss). Wet mils per coat = dry mils per coat ÷ volume solids. Preset solids: acrylic 55% (ASTM D6083 minimum is 50%), silicone 92% and 69% and urethane 80% from published data sheets, polyurea 100%. Texture factors are our planning allowances.

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Sq ft per gallon at your mils
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Gallons to order
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Pails or drums
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Wet mils per coat to gauge

Turning gallons into a budget? See the roof coating cost guide. Deciding between products? The roof coating guide compares the chemistries side by side.

Coverage table

Coverage by coating at common thicknesses

These are theoretical numbers on a smooth, flat roof with no loss. They are useful for comparing products, because a cheap pail with low solids can cost more per finished mil than an expensive one.

Coating (example solids)Volume solidsSq ft per gal at 20 dry milsGal per 100 sq ft at 20 dry milsGal per 100 sq ft at 30 dry mils
Acrylic elastomeric55%442.273.40
Silicone, solvent-borne69%551.812.71
Urethane, single component80%641.562.34
Silicone, high solids92%741.362.04
Spray polyurea100%801.251.87
Our math from 1,604 × volume solids ÷ dry mils. Solids are examples from published data sheets; your product's sheet wins. Tap a column heading to sort.

Look at the spread at 30 dry mils: the acrylic needs 3.4 gallons per square and the high-solids silicone about 2. Price the job per finished dry mil, not per pail. Deeper product notes live in our guides to acrylic, silicone and polyurethane roof coatings.

Real roofs

Texture and slope eat coating

The footprint on a drawing is not the surface a sprayer has to cover. A rough surface has more area than its footprint, and a pitched roof has more area than its plan view. Both need extra material to hit the same dry mils everywhere.

Close to plan

Smooth metal and single-ply

Little extra surface, but corrugations and ribs add some. Measure a panel's stretched width against its coverage width for an exact factor.

Plan a bit more

Weathered BUR and cap sheet

Granules and a crazed, porous surface soak up the first coat. A primer or a slightly heavier base coat is common here.

Plan more

Spray foam

Orange peel texture can add a lot of surface. Coarser foam needs even more, which is one reason foam crews watch texture as closely as thickness. See spray foam roofing.

Pitch factor

Sloped roofs

True area = plan area × √(1 + (pitch ÷ 12)²). That is 1.4% extra at 2:12, 5.4% at 4:12, 11.8% at 6:12 and 41.4% at 12:12.

Do not forget the detailsSeams, fastener rows, curbs and penetrations usually get a stripe coat with reinforcing fabric before the field goes on. On a metal roof with long lap seams and thousands of screws, that detail work can be a real slice of the material order, so measure seam footage separately.
Quality control

Checking coverage on the roof

A roof coating coverage calculator gets the order right. Gauges and pail counts get the roof right. We would not sign off a coating job on the gallon count alone.

  1. Grid the roof

    Chalk the roof into known areas, say 1,000 sq ft each, and assign pails to each block. If a block uses its pails early, the film is heavy; if pails are left over, it is thin.

  2. Gauge the wet film

    Press a notch gauge (ASTM D4414) into the fresh coating every few hundred square feet and compare it with the wet mils from the calculator.

  3. Respect the per-coat limit

    Most fluid coatings have a maximum wet film per pass before they sag, blister or cure slowly. Two or three passes beat one heavy one.

  4. Confirm the dry film

    After cure, cut small samples or use the gauge the manufacturer accepts, record the readings and patch the test spots.

Coated to spec

The gallons on the quote, the mils on the roof.

A certified ArmorThane applicator can measure your roof, set the dry mils for the warranty you want and gauge the film as it goes down.

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FAQ

Roof coating coverage questions

How do I calculate how much roof coating I need?

Multiply the roof area by the total dry mils, then divide by 1,604 times the volume solids as a decimal. Add for pitch, surface texture and about 5 to 15% loss. For 10,000 sq ft at 24 dry mils of a 92% solids silicone, that is about 163 gallons before extras.

How much does a 5 gallon pail of roof coating cover?

At 20 dry mils on a smooth roof, a 5 gallon pail covers roughly 220 sq ft of a 55% solids acrylic and about 370 sq ft of a 92% solids silicone, before texture and loss. Thicker specs cover proportionally less.

What does 1.5 gallons per square mean?

A square is 100 sq ft. Spreading 1.5 gallons over it lays about 24 wet mils. At 92% volume solids that dries to about 22 mils; at 55% solids, only about 13 mils.

How thick should a roof coating be?

Most restoration specs land between about 20 and 34 total dry mils, with longer warranties calling for more. One manufacturer's 20 year silicone spec for metal roofs, for example, calls for about 34 dry mils. Follow the warranty spec for your product.

How accurate is a roof coating coverage calculator?

The math is exact for a smooth, flat surface. Real roofs add texture, slope, wind loss and detail work, so treat the result as an order quantity with allowances, then check film thickness on the roof with a wet film gauge.

Why do coverage rates on labels differ so much?

Labels often quote wet coverage, such as 1 gallon per 100 sq ft, without saying what that dries to. Two products at the same label rate can finish at very different thicknesses because their volume solids differ.

AT
Written by the ArmorThane technical team. We have manufactured polyurea, polyurethane and polyaspartic coatings, spray foam and plural component spray equipment in Springfield, Missouri since 1989. Coverage figures here are theoretical; always use the volume solids and film limits on your product's data sheet.

Sources

  1. IIBEC, ASTM D6083 Standard Specification for Liquid Applied Acrylic Coating Used in Roofing: what it says and what it means. iibec.org/publication-post/astm-d-6083
  2. Gaco, GacoFlex U91C Series product data sheet. gaco.com/wp-content/uploads/2021/08/PDS-GacoFlex-U91C
  3. ASTM International, D4414 Standard Practice for Measurement of Wet Film Thickness by Notch Gages. astm.org/Standards/D4414.htm
  4. ASTM International, D6694 Standard Specification for Liquid-Applied Silicone Coating Used in Spray Polyurethane Foam Roofing Systems. astm.org/Standards/D6694.htm

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