What is intumescent paint and how does it work?

Intumescent paint is a fire protection coating that looks and feels like normal paint at room temperature. When exposed to heat (typically above 200°C), it expands to form a thick char layer (up to 50 times its original thickness) that insulates the underlying material from fire. For steel, this prevents the steel from reaching its critical failure temperature (around 550°C). For timber, it slows the rate of charring. It is tested to BS 476, BS EN 13381-8 (steel) and BS EN 13381-7 (timber).

What fire rating do I need for my building?

The required fire rating depends on the building type, size and use, as set out in Approved Document B (England & Wales). Typical requirements: domestic houses need 30 minutes; flats, offices and shops up to 18m height need 60 minutes; buildings 18-30m need 90 minutes; buildings over 30m need 120 minutes. Your structural engineer or building control officer will specify the exact requirement.

What is Hp/A and why does it matter?

Hp/A is the ratio of the heated perimeter (Hp) to the cross-sectional area (A) of a steel section, measured in m⁻¹. It indicates how quickly a steel section will heat up in a fire. A higher Hp/A means a thinner, lighter section that heats faster, requiring more intumescent paint. Thick heavy sections (low Hp/A, e.g. 80-100) need less paint than thin light sections (high Hp/A, e.g. 250-300). The Hp/A value is published in the Blue Book (Steel Construction Institute tables).

How many coats of intumescent paint do I need?

The number of coats depends on the required dry film thickness (DFT), which in turn depends on the fire rating and the steel section's Hp/A value. For 30-minute fire ratings, 1-2 coats is often sufficient. For 60 minutes, 2-3 coats are typical. For 90-120 minutes, 3 or more coats may be needed. Each coat should be allowed to dry before the next is applied. The manufacturer's data sheet will specify the maximum WFT (wet film thickness) per coat.

Does intumescent paint need a primer or topcoat?

For steel, an anti-corrosion primer (such as a zinc phosphate primer) is almost always required before applying intumescent paint. A compatible decorative topcoat can be applied over the intumescent coating if needed for aesthetic purposes, but it must be compatible — check with the paint manufacturer. For timber, a suitable wood primer may be needed on bare timber. Always follow the specific product's data sheet for the full system specification.

Intumescent Paint Calculator — Fire Protection for Steel & Timber

Calculate intumescent fire protection paint quantities for steelwork and timber. Coverage rates for 30, 60, 90 and 120 minute fire ratings per UK Building Regulations Approved Document B.

Surface & Fire Rating

Surface Type

Fire Rating Required

Per Approved Document B

Surface Area to Paint

Total surface area of all steel/timber to protect

Number of Coats

2 coats typical for 60 min fire rating

Steel Section (Hp/A)

Steel Section

Select a common section or enter Hp/A manually

Hp/A Factor

Heated perimeter / cross-section area (from Blue Book)

Options

Tin Size

Include primer

Anti-corrosion primer for steel (~10 m²/L)

Price Per Tin (optional)

Intumescent paint price per tin for cost estimate

How We Calculate This

This calculator estimates the volume of intumescent fire protection paint needed for steel or timber surfaces based on the fire rating required, surface area, and the steel section's Hp/A factor.

Steel calculations

Paint volume = Surface area ÷ Coverage rate × Number of coats

The coverage rate and required dry film thickness (DFT) depend on the fire rating and the Hp/A value of the steel section. Higher Hp/A values (thinner sections) require thicker coatings for the same fire rating.

Typical DFT and coverage rates

30 min: ~0.4mm DFT, coverage 8-10 m²/litre
60 min: ~1.0mm DFT, coverage 4-5 m²/litre
90 min: ~1.5mm DFT, coverage 2.5-3 m²/litre
120 min: ~2.0mm DFT, coverage ~2 m²/litre

These are baseline rates adjusted by the Hp/A factor. Actual rates vary by manufacturer — always refer to the specific product's assessment and data sheet.

Timber calculations

Timber intumescent paint is typically applied at 400-500 microns WFT (wet film thickness) per coat, with a coverage rate of approximately 6 m²/litre. One or two coats are standard depending on the manufacturer's specification and the fire rating required.

UK standards

Intumescent coatings for structural fire protection are assessed to BS 476-21, BS EN 13381-8 (steel) and BS EN 13381-7 (timber). Fire rating requirements are specified in Approved Document B of the Building Regulations.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.