Question 1

What is intumescent coating?

Accepted Answer

An intumescent coating is a reactive passive-fire material applied to structural steel members. At ambient temperatures it appears as a paint layer. When exposed to sufficient heat — typically above 150–200°C, varying by product chemistry — it undergoes a chemical reaction causing it to expand and form a low-conductivity insulating char layer many times thicker than the original coating. This char insulates the steel surface and slows the rate of steel temperature rise, allowing the structure to maintain its required load-bearing function for the specified fire-resistance period.

Question 2

How does intumescent coating protect structural steel?

Accepted Answer

The char layer formed by the expanding intumescent material acts as a thermal barrier between the fire and the steel surface. By slowing the rate of heat transfer to the steel, the intumescent system delays the point at which the steel reaches its critical temperature — the temperature at which its load-bearing capacity reduces to the level where structural failure may occur. The fire-resistance period is the time between the start of the standard fire test and the point at which the steel reaches its critical temperature in the tested configuration.

Question 3

Is intumescent paint the same as fireproof paint?

Accepted Answer

No. Intumescent coating is not fireproof and the term 'fireproof paint' is misleading. No coating makes steel permanently immune to fire. An intumescent system provides a tested period of delay — typically 30, 60, 90 or 120 minutes in a standard fire test — during which the steel maintains its specified structural capacity. After the specified period, the protection is no longer guaranteed. The system is a means of meeting a defined fire-resistance requirement, not a guarantee of permanent fire immunity.

Question 4

What fire-resistance periods can intumescent coating provide?

Accepted Answer

The fire-resistance periods achievable depend on the specific tested system, the steel member type, the section factor and the critical steel temperature. Common tested periods are 30, 60, 90 and 120 minutes in a standard fire exposure (ISO 834). The achievable period for a specific member on a specific project must be confirmed from the product's fire-resistance assessment documentation — not assumed from a general product description. Different members on the same project may achieve different periods at the same DFT if their section factors and critical temperatures differ.

Question 5

What is the difference between reaction to fire and structural fire resistance?

Accepted Answer

Reaction to fire (EN 13501-1) measures how a product contributes to fire growth, smoke and flaming droplets if it becomes involved. A2-s1,d0 is a reaction-to-fire classification for surface linings — walls, ceilings, soffits. It does not provide structural fire resistance. Structural fire resistance (EN 13501-2, BS EN 13381) measures how a construction element maintains its integrity, insulation and load-bearing capacity during a standard fire exposure for a stated period. An intumescent coating must have appropriate fire-resistance evidence under BS EN 13381-8 for the steel configuration specified. These are different characteristics, different test standards and different specification routes.

Question 6

What is a section factor?

Accepted Answer

Section factor (also called profile factor) relates the heated perimeter of a steel section to its cross-sectional area. It is expressed as Hp/A (in m⁻¹) where Hp is the heated perimeter in metres and A is the cross-sectional area in m². More slender sections — with a higher section factor — heat more rapidly under fire conditions and generally require greater protection thickness than more compact sections. The section factor changes depending on whether the member is exposed on three or four sides, whether it is a column or beam, and whether the section is open (universal beam, universal column) or hollow (CHS, RHS). The value must be calculated for each actual member and exposure condition, not estimated from a generic table.

Question 7

What is critical steel temperature?

Accepted Answer

Critical steel temperature is the temperature at which a steel member's load-bearing capacity reduces to the level corresponding to the design load under fire conditions. It is not universally fixed at a single value. It depends on the structural utilisation of the member, the load ratio, the design assumptions and the applicable structural fire design standard (EN 1993-1-2). A lower critical temperature — where the member is more highly loaded relative to its capacity — may require a greater protection thickness. The critical temperature must come from the structural fire design, not from a default assumption. Product loading schedules must be selected using the correct critical temperature for each member.

Question 8

How is the required dry-film thickness calculated?

Accepted Answer

The required DFT for each member is determined by entering the section factor and critical steel temperature into the approved product loading schedule or fire-resistance assessment software. The loading schedule is derived from the fire-resistance assessment documentation (BS EN 13381-8 test evidence or ETA) for the specific product. Different members on the same project will typically require different DFTs if their section factors or critical temperatures differ. The DFT values produced by the loading schedule must be applied and verified by measurement — they cannot be substituted by a universal figure or a generic coating thickness.

Question 9

Does every steel member on a project need the same coating thickness?

Accepted Answer

No. The required DFT is member-specific. Members with different section factors or critical temperatures require different thicknesses from the same product to achieve the same fire-resistance period. A column with a low section factor (compact section, exposed on four sides) may require a different thickness from a beam with a high section factor (slender section, exposed on three sides) even if both are specified for the same resistance period. The member schedule must record the required DFT range for each member and the inspection records must confirm that the applied DFT fell within that range.

Question 10

Does structural steel require a primer before intumescent coating?

Accepted Answer

In most cases yes. The approved system documentation will specify compatible primers and the preparation standard required before priming. Only primers included in or accepted by the system evidence should be used. An existing or generic primer cannot automatically be retained — its identity must be established and its compatibility confirmed against the intumescent system assessment documentation. Primer thickness may affect the approved build-up. Overcoating windows between the primer and intumescent layer matter. On galvanised steel, specialised treatment or primer is typically required. Incompatible primers can cause delamination or invalidate the specification.

Question 11

Can intumescent coating be applied over existing paint?

Accepted Answer

Only if the existing coating can be identified and confirmed as compatible with the selected intumescent system from the system assessment documentation. An existing coating of unknown composition must be identified by testing or removed. Applying an intumescent system over an incompatible or poorly bonded existing coating creates a risk of delamination under fire conditions — potentially invalidating the specified fire resistance. Adhesion testing before committing to a full programme is standard practice on refurbishment projects.

Question 12

Can existing intumescent coating be repaired or overcoated?

Accepted Answer

Possibly — subject to the existing system being identified, the original loading schedule being available, the applied DFT being confirmed, and the compatibility of the repair material being established from the original or replacement system documentation. Existing intumescent that has been water-damaged, physically damaged, cracked, detached or improperly overcoated may not be performing within its original specification. Visual appearance alone cannot confirm continued fire performance. Where the existing system cannot be identified or justified, complete removal and replacement may be required. Any repair must be documented and must restore the member to the required DFT confirmed in the member schedule.

Question 13

Is a topcoat required?

Accepted Answer

Topcoat requirements depend on the environment, the exposure category and the requirements of the approved system documentation. In dry internal environments, a sealer or decorative finish may be optional. In humid internal environments, external conditions or where chemical exposure or mechanical damage is a risk, a compatible topcoat or sealer is typically required — confirmed from the system assessment documentation. Only topcoats confirmed as compatible with the intumescent system should be used. Applying a standard decorative paint over an intumescent layer without confirming compatibility risks impairing the intumescent reaction by restricting expansion. Maximum and minimum recoat windows between the intumescent and topcoat layers must be respected.

Question 14

Can intumescent coating be used externally?

Accepted Answer

Some intumescent systems have external-exposure grades or assessment categories. External exposure creates additional demands — UV degradation, moisture cycling, freezing, thermal movement and greater mechanical risk. An internal-only system must not be applied externally. The system assessment documentation must confirm the applicable environmental category and whether external use is within the assessed scope. External applications typically require a compatible UV-stable topcoat and a higher level of ongoing inspection and maintenance.

Question 15

How is intumescent DFT inspected?

Accepted Answer

DFT is measured using calibrated electronic film-thickness gauges on the cured intumescent film. Readings must be taken at a number and distribution of positions confirmed in the inspection plan. The gauge must be calibrated on a reference surface of the same curvature and material as the substrate. Wet-film thickness checks during application are used to manage application but are not a substitute for DFT measurement on the cured film. All DFT readings must be recorded against the member identification in the member schedule so that compliance can be verified for each individual member. Readings below the minimum required DFT indicate a non-conformance and must be remediated and reinspected.

Question 16

Can intumescent coating be applied in occupied buildings?

Accepted Answer

In some cases, subject to zone management, solvent and VOC controls, ventilation, occupancy controls and the applicable COSHH or safety assessment. Water-based intumescent systems typically have lower solvent emissions than solvent-based systems and may be more suitable for occupied environments. The applicable safety data sheet, COSHH assessment, method statement and relevant workplace-safety requirements must be confirmed before work begins. Application areas must be isolated from occupied zones during application and through any required ventilation period. Programme and access must allow adequate cure before re-occupation.

Question 17

Who should apply and inspect intumescent coating?

Accepted Answer

Application should be by competent operatives who understand the product, the application method, the member schedule, the DFT requirements, the environmental conditions, the required inspection procedures and the applicable safety requirements. Many intumescent product suppliers require or recommend that application is by contractors within an approved applicator scheme — this should be confirmed from the product assessment documentation and the specific project requirements. Inspection should be carried out by a competent person able to operate DFT gauges, read the member schedule, identify non-conformances and document results traceable to individual members.

Question 18

What documentation is required?

Accepted Answer

Required documentation typically includes: the approved fire-resistance assessment or ETA for the selected system; the member schedule with section factors, critical temperatures and required DFT ranges for each member; product batch and delivery records; surface preparation records; primer application records, including batch, DFT and environmental conditions; intumescent application records, including DFT readings per member, environmental conditions and batch information; non-conformance records and remediation; topcoat records where applicable; photographs; final inspection certification; handover documentation including maintenance requirements; and the original classification and assessment documents. Records must be traceable to individual members and must be available for building control, the building safety case and future inspection or repair.

Question 19

How is damaged intumescent coating repaired?

Accepted Answer

Local damage — mechanical impact, cuts, abrasion — can be repaired using compatible materials confirmed from the original or replacement system documentation. The repair area must be prepared to the required standard, primed if required, built up to the required DFT in the repair zone, and reinspected and recorded. Using an incompatible repair material or applying an insufficient repair thickness creates a gap in the protection for that member. Widespread damage, water ingress, corrosion breakthrough, cracking or detachment may require complete removal and replacement rather than local repair. All repairs must be documented and the member schedule updated.

Question 20

Are warranties available?

Accepted Answer

Warranty terms for intumescent coating systems depend on the product, the system build-up, the substrate condition, the environmental exposure category, the preparation standard, the application standard and the inspection record. Warranties are typically conditional on: application by approved or competent contractors; preparation to the required standard; DFT achieved within the confirmed range per member; compatible primer and topcoat applied where required; documentation completed and retained; and ongoing inspection and maintenance. The specific warranty terms for any system must be confirmed from the current product manufacturer documentation — not from a general product description. No warranty applies automatically on completion.

Question 21

When is another passive-fire system more appropriate than intumescent coating?

Accepted Answer

Intumescent coating may not be the most appropriate passive-fire system where: the member type, section factor or required resistance period falls outside the available assessment evidence; the environmental exposure or service conditions exceed the intumescent system category; access for application and inspection is inadequate; the programme does not allow adequate application and cure; the steel is significantly corroded or structurally compromised; or the overall cost, programme, aesthetics or maintenance requirements of a board, cementitious spray or encasement system are better suited to the project. Boards, vermiculite or cementitious sprays and concrete encasement are alternative passive-fire protection methods — each with different aesthetics, performance ranges, cost profiles and maintenance requirements. The selection between systems should be made by the fire engineer and structural engineer for the specific project.

Intumescent Coatings for Structural Steel

The Right System for This Application

Project-Specific Intumescent System Selection

Intumescent Coating — Specification Decision Paths

Specification Questions