Fireproof paint thickness: how the layer is calculated for the required R limit

Clients often have an illusion: once you have bought the «right» fireproof paint, the facility is protected. In reality the can of paint itself guarantees nothing. What matters is the thickness of the applied layer, calculated for the specific structure. It is what turns the coating into R30, R60 or R90. Let us look at what this thickness depends on and why it cannot be taken «by eye».

The key parameter — the section factor of the metal

The key concept in the calculation is the section factor of the metal (sometimes called the reduced thickness). Simplified, it is the ratio of the profile cross-section area to the perimeter heated by the fire. The physical meaning is simple — how «massive» the element is relative to its heated surface.

A thin-walled profile (an angle, a thin flange of an I-beam, a bent profile) has a low section factor: little metal, large heating area, so it heats up quickly. A massive column of large cross-section is the opposite: a lot of metal that «absorbs» heat longer. The conclusion that surprises many: the thinner the metal, the thicker the fire-protection layer needed for the same limit. Not the other way round.

What else affects the calculation

Besides the section factor, the calculation includes the required fire-resistance limit (R90 requires a thicker layer than R30), the critical temperature of the steel (usually around 500 °C) and the specific type of paint. Each composition has its own «thickness table» from the fire-test report: so many microns for such a section factor for such an R. You cannot borrow data from another paint. As a result, the layer on thin elements can be several times thicker than on massive columns. See the materials on the fireproof paints page (TERM-001, TERM-007, TERM-009).

Why «by eye» is dangerous and expensive

Understate the thickness and in a real fire the coating will not provide the declared limit: it foams weakly, the metal heats faster than calculated. Formally there is paint, in fact there is no protection. Overstate it and you overpay for material, sometimes considerably. There is also the acceptance side: the dry-film thickness is checked with a gauge at several points and compared with the design. If it does not match — the hidden-works certificate is not signed and the facility is not commissioned. Recalculating and repainting afterwards is always more expensive than doing it right the first time.

How it is done properly

We take the steel schedule and drawings, determine the section factor for each profile, find the required layer thickness for the target R from the certified table of the chosen paint, compile everything into a specification and only then calculate consumption and cost. During application we control wet and dry thickness and log it.

STATERM performs this calculation for your facility: we select a TERM composition, calculate the thickness based on the section factor and the required limit, and provide a specification and estimate. Send the drawings or the steel schedule — and you will get a working fire-protection system, not just «cans of paint».

Call: +998 90 918 81 88.