Independent third-party fire test on FSA FIRECOAT Exterior applied to 15/32″ OSB sheathing at 0.70 mm WFT, single coat. Results: Flame Spread Index 25, Smoke Developed Index 50 — Class A on both axes.
7 pages · 1.2 MB
Enter the surface area you're coating and we'll work out the volume of FIRECOAT Exterior required for each certified fire rating, plus a kit-level purchase recommendation.
A recent show flat returned a meaningful gap between expected and actual FIRECOAT material usage on bare plywood. After reviewing field reports from multiple application teams and reproducing the conditions in our own tests, the cause is consistent and reproducible — and it does not point to a defect in the product, the application, or the certification.
Our recommendation is to verify coverage by volume, not by film-thickness gauge. The certified Class A condition for FIRECOAT Exterior is keyed to the volume of product applied per unit area, not to the depth of film that accumulates above the surface. On bare or lightly-sealed wood, those two measurements will not agree on the first coat — the gauge will read low while the actual coverage is correct, because the substrate has drawn material into the grain before a continuous film forms above it.
This is the same volumetric basis QAI Laboratories used to certify the product to ASTM E84 Class A in November 2025: a single coat applied at 0.70 mm wet film thickness on 15/32″ OSB sheathing. The calculator below converts surface area into the corresponding volume for Class A, BAL-29, and BAL-40, and the cert and TDS are embedded in the next section for review.
No change to product or application method is being recommended. The coverage rate on the kit, the QAI test condition, and the field volume check all describe the same thing.
Base coverage rate per the FSA FIRECOAT Exterior Technical Data Sheet: 1.43 m²/L (≈ 58.3 sq ft per US gal) per coat at the certified 0.70 mm wet film thickness.
Required coats per rating: ASTM E84 Class A — 1 coat (per QAI cert QA-4152F-1a, Nov 2025 on 15/32″ OSB). BAL-29 — 1 coat at 0.70 mm WFT, 0.34 mm DFT minimum. BAL-40 — 3 coats at 0.70 mm WFT each, 1.00 mm DFT minimum (per TDS).
The recommended order adds a flat 10% over theoretical for application loss — overspray, brush and roller retention, kit-bottom residue, and standard waste. Substrate porosity is not part of the calculation because coverage is keyed to volume per unit area: the product drawn into the grain still counts toward the certified build-up. Kit recommendations round up to the nearest available container combination (2.6 gal and 1 gal in the US; 10 L and 4 L in metric markets).
The two documents the calculator is built on. Download for project files, or expand the preview to read inline.
Independent third-party fire test on FSA FIRECOAT Exterior applied to 15/32″ OSB sheathing at 0.70 mm WFT, single coat. Results: Flame Spread Index 25, Smoke Developed Index 50 — Class A on both axes.
7 pages · 1.2 MB
Manufacturer technical data sheet for FSA FIRECOAT Exterior. Covers application procedure, coverage (1.43 m²/L per coat), WFT/DFT requirements for BAL-29 and BAL-40, drying time, and certified test results.
3 pages · 740 KB
If you've measured wet or dry film thickness on bare wood and the numbers come back lower than expected for the amount of product applied, you're not alone. Here's what's happening — and a simple way to verify coverage that the substrate can't fool.
Multiple field application reports — across different teams, application rigs, and substrates — describe the same phenomenon. Material is going in, but the gauges aren't catching it on the first or second pass. Three representative data points:
Different teams, different rigs, different substrates — same pattern. That consistency points to the substrate, not to anything wrong with the technique or the equipment.
What plywood, OSB, and bare wood do to a water-based primer
FIRECOAT is a water-based intumescent primer. When it lands on bare or lightly-sealed wood, two things happen at the same time: a portion of the coating begins to form a film on top of the surface, and another portion is drawn down into the open grain and pore structure of the wood. Plywood, OSB, and birch are all highly open substrates. A water-based primer will saturate the first few thousandths of an inch of wood before any continuous film begins to accumulate above the surface.
Both the TCP wet mil comb and the PosiTector 200 ultrasonic gauge are designed to measure the film that is sitting above the substrate. They do exactly that, very accurately. What they cannot see is the material that has soaked into the wood, which on the first pass can be the majority of what was sprayed or rolled on.
The certification basis and the practical field check should agree. The volumetric method — gallons (or litres) of product per unit area — is what the lab measures, what the kit label specifies, and what the calculator on this page computes. We recommend it as the primary coverage check. Film thickness gauges remain useful, especially on later coats and sealed substrates where they read accurately, but they should be treated as a secondary indicator rather than the deciding measurement.
How much product has actually gone onto this surface?
What is the depth of the coating that has accumulated above the surface?
The current ASTM E84 certificate on FIRECOAT Exterior tests the material applied to 15/32″ OSB sheathing in a single coat at a wet film thickness of 0.70 mm. Under those conditions the panel returned a Flame Spread Index of 25 and a Smoke Developed Index of 50 — the upper edge of Class A on both axes. That wet film thickness corresponds to a specific volume of product per square foot, which is the same number printed on the kit as the coverage rate, and the same number the calculator on this page computes.
Two points worth surfacing for the record. First, the QAI cert was achieved in a single coat — not two, not three. Reaching Class A does not require building thickness through multiple passes; one coat at the certified WFT is what was tested and certified. Second, QAI specified the test condition volumetrically, the same way the kit, the TDS, and this calculator do. The field measurement and the lab measurement are describing the same thing.
The volumetric expression is the one that travels safely from a test panel to a production substrate. On a primed or sealed wall, the WFT gauge will land near 0.70 mm at the labeled coverage rate. On bare birch or plywood, the gauge will read lower for the first coat or two while the substrate fills in — but the volume of product on the wall is what determines whether the certified condition has been met.
The burnability of the test panel versus the rest of the wood-product world
The substrate used in the ASTM E84 test isn't arbitrary. OSB sits at the higher-burnability end of common wood-based building products for two reasons: the synthetic resins that bind the strands carry their own energy when burned, and the strand structure exposes a large amount of small surfaces to flame at once. Published untreated values for OSB land in the upper Class C range — around 150, with some products reading higher.
For context, here are typical untreated ASTM E84 flame spread indices for substrates a project might encounter, sourced from USDA Forest Products Laboratory data compiled by the American Wood Council:
Two things follow from this picture. First, the Class A result on OSB isn't a "best case" number — it's the result against one of the harder substrates to treat. Reducing an FSI of roughly 150 down to 25 is about a six-fold drop, achieved with a single 0.70 mm WFT coat. Second, for substrates with lower native FSI — softwood plywood, oak, birch, cedar, fir — the same applied volume of FIRECOAT delivers protection against a fire load that is already lower to begin with.
The TDS reflects this by listing FIRECOAT Exterior as approved for natural and composite timber, bricks, concrete, plasterboard, masonry, and structural steel. The certification did the work on the hardest case in the wood-product family. The same product carried forward to other substrates is moving in the easier direction.
Keep doing the gauge measurements — they are still useful, especially on the second and third passes where the substrate is sealed and the readings become more representative. Alongside them, add a volumetric coverage check. It takes a few minutes per flat and removes the substrate variable from the question.
If the gauge still reads low after the certified volume has been applied, that tells us the substrate is absorbing more than typical — useful data, but it doesn't change whether the coverage was met. If the gauge lines up, even better. Either way, you're no longer guessing — you're working from the same basis the ASTM test uses.
A second-quality ultrasonic dry-film gauge designed for non-magnetic coatings on metal substrates can be purchased for well under US$200 — about 5% of the cost of a PosiTector 200. To use one, place a small piece of sheet metal (a 2 ft × 2 ft offcut is plenty) flush on the substrate before spraying. Coat the metal in the same passes as the flat. After cure, lift the witness panel and read its DFT with the metal-substrate gauge.
Because the metal panel doesn't absorb, the film built on it is the same film a sealed substrate would hold — a reliable proxy for the coverage that landed on the rest of the show flat. Combined with the volumetric check, this gives a vendor a defensible, inspector-friendly DFT number without the cost of a PosiTector 200.
Product usage and application direction remains unchanged. The items below are the variables most worth verifying with the application team on active and future projects, so that field practice and the certification basis stay aligned:
What sealer or primer (if any) is in use under FIRECOAT, and how long is it given to cure. A fully-sealed face changes the absorption picture significantly and brings the gauge readings closer to the lab condition.
Track gallons or litres used against the surface area coated. The calculator above gives you the target volume. If you've applied that volume, you've delivered the certified coverage — regardless of what the gauge says.
For inspections, keep the ASTM E84 certificate and the TDS together (links above). The certificate documents the certified condition; the TDS documents the coverage rate. Both speak to volume per area as the basis for compliance.