Roof Coating Calculator
Calculate how many gallons of elastomeric or silicone roof coating you need for a flat or low-slope roof, plus total cost.
How this calculator works
Elastomeric and silicone roof coatings are applied at a recommended thickness that covers roughly 100 square feet per gallon for a single coat. This calculator multiplies your roof’s length by width to get total surface area, then divides by 100 sq ft per gallon.
Formula: Gallons = Roof area (sq ft) ÷ 100 sq ft per gallon (per coat).
Worked example
A 30 ft × 20 ft flat roof section (600 sq ft):
- Area: 30 × 20 = 600 sq ft
- Gallons: 600 ÷ 100 = 6.00 gallons
- At $45/gallon, that’s $270.00 for one coat.
Coverage reference
| Roof area | Gallons needed (1 coat) |
|---|---|
| 150 sq ft | 1.50 gallons |
| 600 sq ft | 6.00 gallons |
| 2,400 sq ft | 24.00 gallons |
How to measure your roof
Measure the length and width of each flat or low-slope roof section and add them together for total square footage — coating is typically used on EPDM, modified bitumen, or metal roofs rather than steep-slope shingle roofs. Double the gallons if your product spec calls for a two-coat system, and check the manufacturer’s dry-mil thickness requirement, since underapplying reduces both waterproofing and UV protection.
Frequently asked questions
How much roof does one gallon of coating cover?
About 100 square feet per gallon for a single coat of elastomeric or silicone roof coating, though the exact rate varies by product thickness and whether you're applying over a granulated cap sheet versus a smooth membrane. This calculator uses the 100 sq ft/gallon industry-standard rate for a first coat.
Do I need more than one coat of roof coating?
Most manufacturers recommend a two-coat system for full waterproofing and UV protection — typically a base coat plus a topcoat, sometimes with a reinforcing fabric embedded between them at seams and penetrations. This calculator gives you gallons for one coat; multiply by your planned coat count.
What's the difference between elastomeric and silicone roof coating?
Elastomeric (acrylic) coatings are more affordable and easy to apply but can soften in standing water over time, while silicone coatings resist ponding water and UV degradation better but cost more and are harder to recoat later since few products stick well to cured silicone. Coverage rates are similar for both at recommended thickness.
Can I coat over an existing roof without tearing it off?
Yes — roof coating is popular specifically because it can extend the life of an existing flat or low-slope roof (EPDM, modified bitumen, or metal) without a full tear-off, as long as the substrate is structurally sound, clean, and free of standing water or major ponding issues.
Why isn't there a waste toggle on this calculator?
Roof coating is applied by roller or spray at a specified wet-mil thickness across the whole surface rather than cut and pieced like a rigid material, so there's minimal cutting waste — the main variable is coat count and thickness, which you control directly rather than through a waste percentage.
Estimates only. Verify quantities with your supplier before purchasing.