How to specify intumescent paint

How do you specify intumescent coatings?

To specify intumescent paint for steelwork, state the fire resistance period and the critical steel temperature from the fire strategy and structural engineer, then require a coating assessed to BS EN 13381-8 with a dry film thickness schedule for every member by section factor. Specify the whole system, from surface preparation and compatible primer to topcoat, for the environment it will sit in.

An intumescent clause is only as good as the thickness schedule behind it. The coating does nothing until a fire, and the dry film thickness each member needs depends on its section factor, how many sides are exposed, the fire period and the temperature at which that member would fail. None of that is visible once the steel is painted. A clause that says "60 minutes intumescent" leaves every one of those numbers to the applicator, and nobody can check the result afterwards.

This guide covers Uniclass Pr_35_31_68_43 Intumescent protective coatings, mainly as reactive fire protection to structural steel. Flame-retardant coatings that upgrade the reaction to fire of timber or boards have their own Uniclass code and do a different job; they are covered briefly in the decisions below.

What does a intumescent coating clause need to cover?

Some properties are specific to intumescent coatings. Others belong in every product clause. A clause that leaves either out gives the contractor room to substitute.

Fields marked Never guess (reaction to fire classification, fire retardant durability class and fire resistance) depend on the project and should never be filled from a default or a previous job. If the information is not available yet, the clause should say so with a TBC and a reason.

Which decisions does the clause depend on?

Most properties follow from the product you choose. These depend on the project, so they need a decision from the designer before the clause is written.

Which standards should the clause cite?

Cite the product standard the declaration of performance refers to, the code of practice for installation, and the test method behind each value you set. A value without its test method cannot be checked against a submittal.

BS EN 13381-8

Test methods for determining the contribution to the fire resistance of structural members: applied reactive protection to steel members

The test and assessment method behind a manufacturer's DFT tables for each section factor, exposure, period and critical temperature.

In the clause Require the assessment to cover the job: "Assessed to BS EN 13381-8 for the sections, exposures and periods scheduled."

BS EN 16623

Paints and varnishes: reactive coatings for fire protection of metallic substrates, definitions, requirements, characteristics and marking

Product requirements for reactive coatings, including the characteristics to be declared and project-specific validation of materials.

In the clause "Validation of materials: in accordance with BS EN 16623. Submit test results on request."

BS EN 13501-2

Fire classification of construction products and building elements: classification using data from fire resistance tests

Fire resistance classification of the protected element, such as R 60 for a loadbearing steel member.

In the clause "Fire resistance: R 60 to BS EN 13501-2."

BS EN ISO 12944 series

Paints and varnishes: corrosion protection of steel structures by protective paint systems

Corrosivity categories (Part 2) and the selection of protective paint systems for them.

In the clause "Corrosivity category: C1 to BS EN ISO 12944-2."

BS EN ISO 8501-1

Preparation of steel substrates before application of paints and related products: visual assessment of surface cleanliness

Surface preparation grades such as Sa 2½ for blast cleaning and St 2 for hand tool cleaning.

In the clause "Surface preparation: blast cleaned to Sa 2½ to BS EN ISO 8501-1."

BS EN ISO 19840

Paints and varnishes: corrosion protection of steel structures by protective paint systems, measurement of and acceptance criteria for the thickness of dry films on rough surfaces

How dry film thickness is measured on blast-cleaned steel and when readings are accepted.

In the clause "Measure dry film thickness to BS EN ISO 19840 and submit readings per member."

BS EN 13501-1

Fire classification of construction products and building elements: classification using data from reaction to fire tests

The reaction to fire class achieved by a flame-retardant coating on a stated substrate.

In the clause "Reaction to fire: Class B-s1,d0 to BS EN 13501-1 on the specified substrate."

BS EN 16755

Durability of reaction to fire performance: classes of fire-retardant treated wood products in interior and exterior end use applications

Durability classes showing whether a fire-retardant treatment keeps its performance in interior or exterior use.

In the clause "Fire retardant durability: exterior class to BS EN 16755."

Confirm the current edition of each standard at every issue. Avoice's State of UK Specifications 2026 found a withdrawn standard cited in 40% of specifications at contract issue.

For the regulations behind these choices, see Approved Document B explained, Fire strategy reports explained, or search the Avoice Regulations library.

How do you write it in CAWS?

In CAWS, intumescent coatings to steel are written in M61. The section opens with system clauses for each coating situation on the project, named by where and how the coating goes on: on-site coating to primed steel, on-site coating to galvanized steel, off-site coating, coating to existing steel. Each one bundles the preparation, primer, intumescent coat and top sealer coat into a single clause and states the finish expected in visible and non-visible areas.

The fire period and the exposure conditions are usually written once in the performance clauses of M61 and referred to from each system clause, so a project with internal and external steel shares one fire performance clause. The general clauses then cover validation of materials, working procedures, working conditions, inspection by the manufacturer and surface preparation. The DFT per member sits in a schedule, not in the clause.

Clauses the type clause relies on

M61/202

Fire performance to BS EN 13501-1 and -2 The fire resistance classification for the protected steel, written once for the section.

M61/204

Exposure and durability The environmental exposure and durability the coating system must suit.

M61/205

Validation of materials Project-specific evaluation of the coating materials in accordance with BS EN 16623.

M61/210

Working procedures Notice before preparation and application, and recorded quality control procedures.

M61/215

Working conditions Temperature, humidity and surface condition during application and drying.

M61/270

Inspection Access for the manufacturer to inspect work, check records and take DFT readings.

Clause numbers vary between NBS versions and practice masters. The general clauses here follow the M61 section of a published NBS-format tender specification; check that each clause your system clause refers to exists in your own M61.

For more on clause structure and checks, see NBS clause writing: common mistakes.

How do you write it in Uniclass?

In Uniclass the protected steelwork is written as a system and the coating as a product. Ss_40_90_40_42 Intumescent coating systems describes the whole treatment: what it is applied to, preparation, primer, intumescent and top coat, the fire resistance the protected member must achieve and how thickness is measured and recorded. The intumescent itself is a product clause under its Pr code.

The split puts the performance in the right place. The product clause holds what the manufacturer declares about the coating: its assessment, binder type, colour and gloss of the system. The fire period, critical temperatures and thickness per member belong to the system clause and the schedule, because they change with the steel, not the paint. Flame-retardant treatments to timber sit in a different system, Ss_40_90_95 Wood preservative and fire retardant treatment systems.

In NBS Chorus, Uniclass product clauses also carry an NBS reference (for intumescent coatings, 45-35-84/364). Use whichever your practice's masters use, but do not mix CAWS and Uniclass references in one specification.

What goes in the spec, and what goes in the schedule?

The specification defines each type once. The schedule and drawings say where each type goes.

What are the common mistakes?

Most problems are a missing property, a property filled with the wrong kind of value, or a decision left silently open.

• !
  A period with no DFT schedule. "60 minutes intumescent" cannot be checked on site. Require a member-by-member DFT schedule from the manufacturer's assessment.
• !
  A default critical temperature. Assuming one temperature for every member can under-protect heavily loaded steel. Take the values from the structural engineer.
• !
  An incompatible shop primer. Fabricators apply a standard primer before the intumescent is chosen. Specify the primer as part of the system, or require a compatibility statement.
• !
  Internal coatings used outside. Water-based thin films in damp or exposed locations can fail. Match the product and topcoat to the corrosivity category.
• !
  No measurement or records. Without DFT readings to a stated method, nobody can prove the steel is protected. Require readings and hand them over.
• !
  Fire retardant and fire resistance confused. A flame-retardant varnish on timber improves its reaction to fire class. It does not give steel fire resistance. Name which one is needed.

How Avoice helps

Avoice drafts intumescent clauses from the steelwork and fire strategy information on the project, and records what still has to come from the engineers.

1. 1
   Drafts from the fire strategy and steel drawings

   It reads fire periods from the fire strategy drawings and exposed steel from the structural and architectural drawings, writes the M61 or Uniclass clause for each coating situation, and fills in what the information supports.

2. 2
   Leaves a TBC for the engineers' numbers

   If critical temperatures have not come from the structural engineer, or the period for a mezzanine is unclear, the clause says TBC with the reason. Each one is listed in the project TBC register to be closed before tender.

3. 3
   Writes in your format

   CAWS M61 system clauses or Uniclass system and product clauses, adapted from your practice's own masters rather than generic text.

4. 4
   Checks against drawings and schedules

   QA/QC flags a steel member on the fire strategy drawing with no protection specified, a clause that refers to a performance clause missing from your M61, a period in the clause that differs from the schedule, or a withdrawn standard.

5. 5
   Keeps product data with the clause

   Data sheets and declarations of performance sit in the Product Room, linked to the clause. Edit the linked coating once and every system clause that uses it updates. Approved Document B is in the Avoice Regulations library for checking the periods behind the schedule.

Related guides: How to specify fire doors, How to specify plasterboard.

Software can draft and check. The designer still chooses the product and is responsible for the specification.

For more on the evidence behind product choices, see the role of manufacturer product data in specification writing.