A fire rating is a performance outcome, not a product order code. The same intumescent coating can require very different dry film thicknesses depending on the steel section, the section factor and the fire test standard being applied.
Start from the section factor
The section factor (A/V) describes how quickly a steel section heats up. Heavier, more massive sections usually need less coating than thin sections with the same rating, because they absorb heat more slowly.
Provide the structural engineer’s section schedule and the required fire resistance period. Without both, a coating thickness cannot be selected responsibly.
Dry film thickness and application
Intumescent coatings are applied in multiple coats to reach a tested dry film thickness. Ambient temperature, humidity and surface preparation all affect adhesion and the final thickness.
Primers and topcoats must be compatible with the intumescent system. A decorative topcoat that is not part of the tested system can void the fire assessment.
Evidence and inspection
Request the test report for the specific steel section and rating, not a generic certificate. On site, wet film thickness should be checked during application and dry film thickness verified after curing.
Record batch numbers, application conditions and inspection results. These records are often requested by the authority having jurisdiction before occupancy.
Key takeaways
- Select coating thickness from the section factor and the required rating.
- Use only primers and topcoats included in the tested system.
- Verify wet and dry film thickness and keep application records.
