Fireproof Paint Calculator — Intumescent Coating by Fire Rating

Hp/A (the section factor) is the ratio of the heated perimeter (Hp) to the cross-sectional area (A) of a steel member. It determines how quickly the steel heats up in a fire. Low Hp/A (heavy sections like UC 254×254) heat slowly and need less intumescent. High Hp/A (light sections like CHS 114.3×3.6) heat quickly and need more. The Hp/A value is taken from the ASFP Yellow Book (Fire Protection for Structural Steel in Buildings) or calculated from steel section tables. Where the actual section factor exceeds 200 m⁻¹, the ASFP guidance caps it at 200 m⁻¹ with a 500°C limiting steel temperature.

The dry film thickness (DFT) depends on the required fire rating and the section factor. Typical values: 30 min = 250-800 microns, 60 min = 500-1500 microns, 90 min = 1000-2500 microns, 120 min = 1500-3500 microns. The exact DFT is specified in the intumescent paint manufacturer's assessment report (e.g., Nullifire, Sherwin-Williams Firetex) matched to the Hp/A value.

A complete system has three layers: (1) Anti-corrosion primer — typically 75 microns of zinc phosphate or epoxy primer. (2) Intumescent paint — applied in multiple coats to achieve the target DFT. (3) Protective topcoat — 25-50 microns to protect the intumescent from moisture and UV damage. All three components must be from the same manufacturer's tested system.

Yes, but off-site application in a controlled environment is preferred for quality. On-site application requires temperatures above 5°C, low humidity, and no rain exposure until cured. Each coat must be measured with a wet film thickness gauge. Multiple coats are needed for high DFTs — typically 500 microns per coat maximum. Drying time between coats is 4-24 hours depending on the product.

With a proper topcoat, intumescent paint lasts 25+ years in interior environments. External or high-humidity locations reduce the lifespan to 10-15 years. Regular inspection is required — check for cracking, peeling, or damage. Damaged areas must be repaired promptly as the fire protection is compromised. Fire performance of intumescent protection to structural steel is tested and assessed to BS EN 13381-8 (and the legacy UK method BS 476-20 and BS 476-21 for loadbearing elements, assessed via the ASFP Yellow Book) — not BS 8414, which is the test for fire spread over external cladding systems.