Understanding your result
The headline is the coating for the coats that product needs (or the number you chose), with the allowance, and the sub-line is the pail list with the primer added. The four figures show the roof area, the coating per coat, the pails and the primer.
The breakdown goes from the footprint through the pitch factor to the roof area, then shows the coating type with its typical rate and coats, the rate actually used, the coverage that rate corresponds to in square feet per gallon, and the usual per-coat, allowance, total and can lines, followed by the primer as its own line. The arithmetic panel shows the pitch multiplication and the rate conversion.
Coatings are bought in 5-gallon pails and the quantities are large: thirty-three gallons for a modest flat roof is normal. The spread rate and film thickness calculator is the tool for checking a data-sheet rate against the dry film the coating promises, and the film thickness guide explains why 1.25 gallons per 100 sq ft is the right amount and not a mistake. The stucco and masonry calculator handles the elastomeric that goes on walls.
How we calculate this
The pitch factor is the length of the slope per foot of run: the square root of 1 plus the rise over 12 squared, so 1.118 for 6:12 and 1.414 for 12:12. A flat roof has a factor of 1. The exterior measuring guide shows how to read the pitch from the attic or with a level and a ruler.
The rates come from the coating data sheets: acrylic elastomeric at 1.0 to 1.5 gallons per 100 sq ft per coat, two coats; silicone at 1.5 to 2.0 in one coat; fibered aluminium at 1.0 to 1.5 in one coat. The calculator uses 1.25, 1.5 and 1.0, converts each to a coverage (80, 67 and 100 sq ft per gallon) and runs the same arithmetic as every quantity calculator on the site. In metric mode the rate is litres per square metre; 1 L/m² is 2.45 gallons per 100 sq ft. The coverage rates table lists the published ranges for coatings beside the paints.
The primer rate of 0.4 gallons per 100 sq ft (250 sq ft per gallon) is typical of roof primers and bleed blockers; the allowance of 10% is higher than for walls because roofs are rough and the pails are scraped rather than measured.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Acrylic elastomeric | 1.25 gal per 100 sq ft per coat, 2 coats | Roof coating data sheets (Henry 287, GAF Unisil-free acrylics, Karnak 501): 1–1.5 gal per 100 sq ft per coat |
| Silicone | 1.5 gal per 100 sq ft, 1 coat | Silicone roof coating data sheets (Gaco, Henry 795, GAF Unisil): 1.5–2 gal per 100 sq ft |
| Fibered aluminium | 1.0 gal per 100 sq ft, 1 coat | Aluminium roof coating data sheets (Henry 555, Karnak 97): 1–1.5 gal per 100 sq ft |
| Primer | 0.4 gal per 100 sq ft | Roof primer and bleed-blocker data sheets: 200–300 sq ft per gallon |
| Pitch factors | 1.031 (3:12) to 1.414 (12:12) | Geometry |
| Allowance | 10% | Rough substrates and pail scrapings; a round figure |
Assumptions last reviewed October 7, 2026.
The calculator does not model seam tape, flashing mastic or fabric reinforcement, which are separate products at the seams and penetrations and can be a quarter of the job on an old roof. It does not decide whether the roof is sound enough to coat, which it must be: a coating over a wet or failed membrane fails with it. It is a quantity calculator; the roof safety is yours.
Two worked examples
A flat 40 × 30 ft roof, white elastomeric
Flat, 1,200 sq ft, acrylic elastomeric at the default 1.25 gallons per 100 sq ft, two coats, no primer.
- Roof area: 40 × 30 × 1.0 = 1,200 sq ft
- Coverage: 100 ÷ 1.25 = 80 sq ft per gallon
- Per coat: 1,200 ÷ 80 = 15 gallons
- Needed: 15 × 2 × 1.10 = 33 gallons, buy 6 pails + 3 gallons
Seven pails in practice. The second coat goes on at right angles to the first after it has cured, usually the next day, and both need a dry 24 to 48 hours afterwards.
A 6:12 roof, silicone over an asphalt surface
Same footprint, 6:12 pitch, silicone in one coat with a bleed-blocking primer.
- Roof area: 40 × 30 × 1.118 = 1,342 sq ft
- Coverage: 100 ÷ 1.5 = 67 sq ft per gallon
- Per coat: 1,342 ÷ 67 = 20.124 gallons
- Needed: 20.124 × 1 × 1.10 = 22.14 gallons, buy 4 pails + 2 gallons + 1 quart
- Primer: 1,342 ÷ 100 × 0.4 × 1.10 = 5.9 gallons, buy 1 pail + 1 gallon
Five pails of silicone and a pail of primer. For comparison, 2,000 sq ft of metal roof under fibered aluminium at a data-sheet rate of 1.2 gallons per 100 sq ft, no allowance, is exactly 24 gallons: four pails and four gallons.
Where to find your inputs
Footprint. Length and width of the roof including the overhangs, from the ground with a tape or from the plans. Add the sections of an L-shaped roof.
Pitch. Rise over 12 in of run. Hold a level against the roof surface or a rafter in the attic, measure 12 in along it and read the rise. Flat and low-slope roofs under 2:12 use the flat factor.
Coating and rate. The product’s technical data sheet gives the rate in gallons per 100 sq ft per coat and the number of coats; enter the rate if it differs from the default.
Primer. The same data sheet names the primer for your substrate, if any.
Common mistakes
- Dividing by wall-paint coverage. A coating goes on five times thicker. Use the data-sheet rate.
- Spreading the coating thin to make the pails last. The film is the waterproofing; thin, it fails.
- Coating a wet roof. Moisture under the coating blisters it off. Dry substrate, dry forecast.
- Silicone over something else, or something else over silicone. Only silicone sticks to silicone.
- Forgetting the seams. Tape and mastic at every seam and penetration are separate products.
- Coating shingles. Most coatings are not approved for them and the warranty goes.
Questions people ask
- How much roof coating do I need?
- A 40 × 30 ft flat roof is 1,200 sq ft. Acrylic elastomeric at 1.25 gallons per 100 sq ft per coat is 15 gallons per coat, and two coats with a 10% allowance is 33 gallons, so six 5-gallon pails and three gallons. Silicone at 1.5 gallons per 100 in one coat is about 20 gallons.
- How do I measure a pitched roof?
- Measure the footprint, length times width including the overhangs, and multiply by the pitch factor. A 6:12 roof is 1.118 times its footprint; a 12:12 roof is 1.414. The calculator has the factors for the common pitches; for a flat roof the footprint is the area.
- Why gallons per 100 sq ft?
- Because coatings are applied to a thickness, not spread until the can runs out. A rate of 1.25 gallons per 100 sq ft is 80 sq ft per gallon, about a fifth of what wall paint covers, and that thickness is what gives the coating its waterproofing and its reflectivity. Thinner is not cheaper; it is a failed roof.
- One coat or two?
- Acrylic elastomeric goes on in two coats at right angles, the second after the first has cured. Silicone is one heavy coat; nothing but silicone will stick to it afterwards. Fibered aluminium is one heavy coat. The calculator defaults to the usual number and you can change it.
- Does the roof need a primer?
- Asphalt roofs under a white coating often need a bleed-blocking primer so the asphalt does not stain through; rusted metal needs a rust primer; chalky old coatings need a bonding primer. A sound, clean previous coating of the same type does not. The data sheet for the coating names the primer.
- Can I coat shingles?
- Usually not. Most roof coatings are for flat and low-slope roofs on modified bitumen, EPDM, metal and sprayed foam. Coating asphalt shingles is not recommended by the manufacturers and can void the shingle warranty. The calculator gives a quantity for any pitch, but check the substrate.