How to determine intumescent coating thickness

The inputs

1. Fire rating per element (R30 … R180).
2. Critical steel temperature from the structural fire design – or the default the code allows.
3. Section shape – open (I, H, channel) or closed (hollow).
4. Exposure – 3 sides or 4 sides.
5. Section factor – Hp/A, ksm or W/D.
6. The selected product and its certification route (ETA / EN 13381-8, UL design, AS 1530.4 assessment).

Reading the loading table

Manufacturers publish certified tables of dry film thickness against section factor, for each rating, critical temperature and section shape. Find the column for the critical temperature and the row for the section factor. If the exact section factor is not tabulated, use the next higher row, or interpolate only where the assessment explicitly permits it.

Never extrapolate beyond the highest section factor or rating in the table. If the member is outside the approved range, the product cannot be used for it without further assessment.

Build a thickness schedule

The result is a schedule listing each member mark, section, section factor and required DFT. It becomes the applicator's target and the inspector's reference. Grouping members with similar thicknesses simplifies application.

Practical limits

• Maximum DFT per coat – thicker films need several coats and more drying time.
• Maximum total DFT for the product.
• Minimum DFT that the product data allows.
• Hollow sections and cellular beams often have their own tables or rules.

FAQ