Intumescent coatings
for structural steel

A coating delivers R, and only R

An intumescent coating delivers R, loadbearing capacity. It keeps the section below its critical temperature so the member carries its load for the rated period. It does not deliver E, integrity, or I, insulation. A coated column is not a fire barrier, and an enquiry asking for EI 120 on bare steelwork is asking a coating to do something no coating does.

That distinction decides the product before any thickness is discussed. If the requirement is to keep a structure standing, it is a coating. If the requirement is to stop fire crossing a plane, it is firestopping or a rated wall.

Four inputs, and a specification that omits them will come back as three quotes that do not compare.

The critical temperature of each member, which is its true limiting temperature under the actual load, not the 550°C most tables assume by default. The section factor, Hp/A, and the exposure it is calculated for, since a three-sided beam and a four-sided column of the same section resolve to different thicknesses under the same rating. The environment, internal, external or corrosive, which decides the primer and topcoat system and the corrosivity category the build-up has to meet. And whether application is off site or on site, which changes the programme more than it changes the product.

From those we return a dry film thickness per member, the certified table it is read from, and the evidence behind it. Thickness may be interpolated inside a certified range. It is never extrapolated beyond one.

Intumescent coating, intumescent paint, fireproof paint

Intumescent coating, intumescent paint and fire protection coating describe the same product. On Indian tenders you will also see fireproof paint, fire retardant paint and steel fireproofing, and the BOQ line is often written as supply and application of intumescent fire protection coating to structural steel.

Two of those names describe something different and get confused with it. A fire retardant paint in the strict sense improves a surface’s reaction to fire, its spread of flame, and does not give a structural member a fire resistance rating. Sprayed cementitious or vermiculite protection also delivers R, but as a thick applied layer rather than a char-forming film, with a different finish, thickness and cost profile.

Getting the vocabulary right matters commercially because the evidence is product-specific. A thickness read from one manufacturer’s table is not valid for another’s, whatever the two tins say on the label. The same logic runs through the rest of the passive layer, including cavity barriers, where the certificate covers a tested configuration rather than the nominal opening on the drawing.

Structural steel coatings are tested and classified under [EN 13381-8], [BS 476 Part 21] and [ISO 834], with [UL 263] for cellulosic exposure and [UL 1709] for hydrocarbon. Tunnel work is assessed against the RWS curve.

Those certificates are held by the manufacturer and cover the tested system, which is the primer, the intumescent and the topcoat as a build-up. Agnimatic supplies [product families] against that evidence and provides the certificate, the thickness table and the classification report on request. Agnimatic’s own scope on this line is [Supply, or SITC].