What Intumescent Paint Thickness for a Fire Rating?

Quick Answer

There is no single figure: DFT depends on the fire rating, the section factor and the certified product

Required dry film thickness (DFT) for intumescent paint on structural steel is set by three things together: the fire resistance period (30, 60, 90 or 120 minutes), the steel section’s Hp/A section factor, and the manufacturer’s certified loading table for the exact product chosen. As a rough sense of scale, thin-film DFT typically spans about 0.3mm on a heavy section at 30 minutes up to several millimetres on a slender section at 120 minutes, but the number for your job comes from the product’s test data, not a rule of thumb.

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Typical DFT by Fire Rating (Indicative Only)

Indicative dry film thickness for intumescent steel paint, by fire resistance period
Fire rating	Typical DFT	Note
30 minutes	~0.3–0.9mm	~0.46mm at Hp/A 150 (Nullifire SC902 loading table, 3-sided beam)
60 minutes	~0.5–1.4mm	~0.72mm at Hp/A 150
90 minutes	~1.1–3.0mm	~1.51mm at Hp/A 150
120 minutes (2 hour)	~1.7–4.7mm, more on slender sections	~2.37mm at Hp/A 150; the model below is not calibrated past Hp/A 300

The middle column comes from one certified product’s loading table (Nullifire SC902, tested to BS 476 Part 21 and BS EN 13381-8), scaled across the Hp/A range our intumescent paint calculator covers (roughly Hp/A 105 to 300 m⁻¹). A different product, section orientation (3-sided vs 4-sided) or design temperature will give different numbers. Treat this table as a sense of scale, not a specification.

Why DFT Is Not One Number

Dry film thickness (DFT) is how much cured intumescent coating is on the steel once it has dried, usually measured in microns or millimetres. It is the figure a certified fire protection system is built around, and it moves with three separate variables:

Fire rating period. Longer protection needs a thicker char layer: 120 minutes needs several times the DFT of 30 minutes on the same section.
Section factor (Hp/A). A thin, light section with a lot of exposed perimeter relative to its cross-sectional area heats up faster in a fire and needs more coating than a heavy, chunky section for the same rating and the same period.
The certified product and its design temperature. Every intumescent system is tested and assessed as a whole (primer, intumescent coat, topcoat) to a specific critical or design temperature for the steel. Two products rated for the same fire period on the same section can call for different DFT because their assessments used different design temperatures.

Change any one of those and the DFT changes with it, which is why a single quoted figure such as “2mm for two hours” is only ever true for one specific product on one specific section.

The Steel Section Factor (Hp/A)

Hp/A is the ratio of the heated perimeter of a steel section to its cross-sectional area, in m⁻¹, published in the Steel Construction Institute’s Blue Book alongside the section properties. A slender section such as a 152×89 UB has a high Hp/A (around 280 m⁻¹) and heats quickly, so it needs more DFT. A heavy section such as a 305×305 UC has a low Hp/A (around 85 m⁻¹) and holds its temperature longer, so it needs less. Our intumescent paint calculator has a lookup for common UK beam and column sections, or you can enter Hp/A directly from the section’s structural drawings.

What a 2-Hour (120-Minute) Rating Actually Means

A 120-minute structural fire resistance period is not the default for a house extension. Per Approved Document B Volume 1, Table B3 (see our Part B fire resistance reference), 120 minutes applies to dwellings with a top storey more than 30m above ground, roughly 11 storeys and above. It also shows up in taller commercial and mixed-use buildings under the relevant fire strategy. At this end of the scale the swing between a heavy column and a slender beam gets large: in a published US structural-engineering comparison of standard sections at a 120-minute rating, required DFT for different section shapes ranged from roughly 4.1mm up to 7.8mm on the same fire period (161 to 309 mils, converted at 1 mil = 0.0254mm; StructureMag, passive fire protection coatings). The exact test regime and design temperatures differ from UK assessments, so treat that as an order-of-magnitude check rather than a UK figure, but the pattern holds here too: at 120 minutes, section choice moves the answer by a large margin, not a small one. This is fire engineer and certified-assessment territory, not a DIY estimate.

Getting the Number You Actually Need

For a real project, the sequence is: the fire strategy or Building Control sets the required period (30/60/90/120 minutes) for that element; the structural engineer confirms the section and its Hp/A and design temperature; the paint manufacturer’s certified assessment (their loading table for that specific product, tested to BS 476 Part 21 and BS EN 13381-8) gives the DFT for that exact combination. That certified figure, not a website range, is what gets specified and signed off. Our intumescent paint quantity answer walks through turning a known DFT into litres and tins once you have that number; it quotes coverage by weight (kg/m²) alongside the same DFT-by-rating bands used on this page, so treat the two as different units on the same idea, not competing figures, and always use the certified DFT from your product’s loading table as the figure that actually governs.

How DFT Is Checked on Site

Thickness is controlled at two stages. During spraying or brushing, applicators check wet film thickness (WFT) with a simple comb gauge against the manufacturer’s wet-to-dry conversion, since WFT is what they can measure while the coating is still workable. Once cured, dry film thickness is measured with a non-destructive magnetic or ultrasonic DFT gauge at multiple spot checks across every protected member, following the current industry code of practice, the Association for Specialist Fire Protection’s TGD 011 (specification and on-site installation of intumescent coatings for fire protection of structural steelwork), which superseded the older TGN 3:P1 note on measuring intumescent coatings. The results go into a QA report. Coating that falls short of the specified thickness anywhere on a member is one of the most common reasons passive fire protection fails inspection, which is another reason to work from the certified figure rather than an estimate.

Figures cross-checked against the site’s own intumescent paint calculator (Nullifire SC902 loading table), fireproof.co.uk and StructureMag guidance on passive fire protection coatings, July 2026. Intumescent fire protection for structural steel must be specified by a fire engineer as part of the approved fire strategy, using the certified assessment for the exact product, section and rating, and applied by a certified contractor. Nothing on this page is a specification for any project. Last updated: July 2026.