How is wet film thickness measured on site?

Immediate, in-process measurement is conducted. Readings are taken at multiple points across each steel member, measurements are recorded and logged as part of the quality process, and results are checked against the specified WFT requirements. This approach ensures consistency across complex steel profiles and prevents compliance issues being discovered after curing.

What factors influence achieved WFT?

Four main factors influence achieved WFT: 1) Surface condition and preparation, 2) Spray application consistency using airless spray equipment, 3) Environmental conditions including ambient temperature, substrate temperature, relative humidity and dew point, and 4) Manual application considerations where brush or roller application is required.

Wet Film Thickness (WFT) Explained

Q: Why does thickness determine fire performance?

Fire resistance is thickness-dependent. Wet Film Thickness (WFT) directly determines whether an intumescent coating will perform as tested, achieve the required fire-resistance period, and meet regulatory approval.

Q: Why does WFT logging matter?

WFT logs demonstrate application to tested specification, building control and insurers require documented proof of compliance, and certification cannot be issued without complete and accurate records. WFT documentation provides clear, auditable evidence that the installed system matches the fire-rated design.

The Foundation of Fire Protection Performance and Compliance

Wet Film Thickness (WFT) is one of the most critical controls in structural steel fire protection. It directly determines whether an intumescent coating will perform as tested, achieve the required fire-resistance period, and meet regulatory approval. Without accurate WFT control, even the best fire-protection products cannot deliver their certified performance.

Hand wearing a blue glove holding a brown notched paint thickness gauge against a construction site background.

Why Thickness Determines Fire Performance

Fire Resistance Is Thickness-Dependent

Fire resistance ratings — such as 30, 60 or 90 minutes — are achieved through independent laboratory testing of intumescent coating systems applied at specific thicknesses. These tests establish how long coated steel remains below its critical failure temperature during a fire.

The specified WFT for a 30-minute rating is fundamentally different from that required for 60 or 90 minutes. If the coating is under-applied, the steel will heat faster than the tested system allows. Over-application can also create defects, cracking or adhesion issues. Fire performance is only achieved when coatings are applied exactly to the tested specification.

Close-up of a metal paint thickness gauge with measurements from 175 to 750 held by a hand in a blue glove.

How Wet Film Thickness Is Measured on Site

Immediate, In-Process Measurement

WFT is measured immediately after application using calibrated wet film gauges. Measurements are taken while the coating is still wet, allowing adjustments to be made in real time to ensure compliance before curing begins.

Readings are taken at multiple points across each steel member

Measurements are recorded and logged as part of the quality process

Results are checked against the specified WFT requirements

This approach ensures consistency across complex steel profiles and prevents compliance issues being discovered after curing.

influence

Factors That Influence Achieved WFT

Surface Condition and Preparation

Rust, mill scale, existing coatings and surface contamination directly affect adhesion and coating build. These must be removed through appropriate surface preparation to ensure the applied coating forms a continuous, stable film capable of achieving the required thickness.

Spray Application Consistency

Airless spray equipment allows intumescent coatings to be applied evenly across complex steel sections. Correct equipment setup, material control and spray technique are essential to achieving uniform WFT without excessive build or voids.

Environmental Conditions

Ambient temperature, substrate temperature, relative humidity and dew point all influence coating behaviour. Cold, damp or unsuitable conditions can slow drying, affect film formation and compromise thickness control. These conditions must be monitored and managed throughout application.

Manual Application Considerations

Where brush or roller application is required, greater skill and control are needed to maintain even thickness across all surfaces. Manual methods demand experienced operatives and close monitoring to avoid uneven build.

Worker wearing a respirator and high-visibility vest applying fire-resistant intumescent coating to a structural steel beam indoors.

Verification and Compliance Records

Why WFT Logging Matters

Every WFT measurement is recorded and cross-referenced to the project specification. These records form a critical part of the fire-protection compliance pack.

WFT logs demonstrate application to tested specification

Building control and insurers require documented proof of compliance

Certification cannot be issued without complete and accurate records

WFT documentation provides clear, auditable evidence that the installed system matches the fire-rated design.

Specialist Application Ensures Fire Safety

WFT measurement and verification are non-negotiable in compliant fire protection. Accurate thickness control, environmental monitoring and structured records are what separate verified fire performance from unproven application. This level of control can only be delivered by trained, competent contractors operating within defined quality systems.

Graco 695 XT airless sprayer with a yellow bucket in an unfinished industrial interior with metal framing and construction equipment.

From the founder

“After a decade in the protective coatings industry, I built FireCoatings to deliver compliant fire protection without unnecessary noise — just clear documentation, a controlled application process, and professional site practice.”

Myles Howson

Managing Director

Person wearing protective suit, gloves, and respirator applying fireproof coating to metal pipes and beams in industrial setting.

Assurance

Compliance & Accountability at Every Stage

We prioritise wet film thickness control, environmental logging, product traceability and layer-by-layer inspection throughout application. Comprehensive documentation ensures building control approval, insurance acceptance and assured project sign-off.

Hand in black glove holding a blue infrared thermometer showing 20.1°C and a black temperature and humidity meter showing 18.5°C, 73.2% relative humidity, and 13.6°C dew point.

Interior construction site with yellow steel support beams surrounded by red safety barriers and exposed ceiling ducts and wiring.

Featured Projects

Steel Fire Protection Delivered

Representative projects demonstrating our methodology across sectors and steel types.

View Projects

Intumescent Fire Protection & Structural Steel Coating

297 Oxford Street, London

297 Oxford Street – Structural Steel Fire Protection Upgrade

Fire protection upgrade to exposed structural steel using intumescent coatings, delivered during night shifts to maintain programme continuity.

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Intumescent Fire Protection – Structural Steel

P&G Manufacturing Facility, Tilbury / Grays, Essex

Procter & Gamble – Structural Steel Fire Protection Upgrade

Application of a 60-minute intumescent fire protection system to structural steel within a live manufacturing environment.

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Intumescent Fire Protection & Decorative Coating

Devonshire House, 1 Mayfair Place, London, W1J

Devonshire House – Structural Steel Fire Protection Upgrade

Intumescent fire protection applied to exposed structural steel following mechanical preparation, delivering a 120-minute fire rating within a live office environment.

Show project

Intumescent Fire Protection & Structural Steel Coating

297 Oxford Street, London

297 Oxford Street – Structural Steel Fire Protection Upgrade

Fire protection upgrade to exposed structural steel using intumescent coatings, delivered during night shifts to maintain programme continuity.

Show project

Intumescent Fire Protection – Structural Steel

P&G Manufacturing Facility, Tilbury / Grays, Essex

Procter & Gamble – Structural Steel Fire Protection Upgrade

Application of a 60-minute intumescent fire protection system to structural steel within a live manufacturing environment.

Show project

Intumescent Fire Protection & Decorative Coating

Devonshire House, 1 Mayfair Place, London, W1J

Devonshire House – Structural Steel Fire Protection Upgrade

Intumescent fire protection applied to exposed structural steel following mechanical preparation, delivering a 120-minute fire rating within a live office environment.

Show project

eyebrow

"Thank you Myles and team for all your hard work on this one. It wasn't easy working around the schedule but you got it done without causing any further issues. It was a pleasure to work with you."

Chris

Project Manager, Millcroft Services

Interior view of a construction site featuring steel beams and columns painted with white protective coating and temporary support props.

eyebrow

"NuColour worked efficiently around our programme and delivered a on time at late notice. The team were organised, flexible, and professional throughout the works."

Ciprian

Site Manager, Principal Contractor – GEMCO