Understanding Dry Film Thickness (DFT) in Passive Fire Protection
When applied correctly, intumescent coatings provide vital protection to structural steel and other materials during a fire. These coatings expand when exposed to high heat, forming a char that insulates the substrate and helps delay collapse.
However, the effectiveness of an intumescent coating depends heavily on one critical factor: film thickness. Too little, and the fire resistance may be inadequate. Too much, and the product may fail to perform as tested—or even suffer from cracking and delamination.
In this article, we explain why intumescent coating thickness matters, how it’s measured, and what can go wrong if it isn’t applied to the correct specification.
What Is Dry Film Thickness (DFT)?
Dry Film Thickness (DFT) refers to the thickness of the intumescent coating after it has dried and cured. It’s measured in microns (μm) and is a key factor in determining how long the coating will protect the structure in a fire—typically 30, 60, 90 or 120 minutes of fire resistance.
The required DFT is influenced by several factors, including:
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The type of intumescent product used
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The size and shape of the steel section (its “section factor” or Hp/A value)
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The fire resistance rating specified in the building design
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The orientation of the member (e.g. column, beam, hollow section)
Manufacturers provide detailed DFT loading tables or software to calculate the required thickness for each specific scenario.
Why Is Coating Thickness So Important?
Intumescent coatings work by expanding many times their original thickness to form a protective char. This insulating layer slows the transfer of heat to the steel beneath, helping to prevent structural failure for a specified period.
If the coating is too thin, the char layer may not reach the required depth or insulation performance—meaning the steel heats up too quickly in a fire.
If the coating is too thick, the product may not behave as it did in fire testing. Excessive film thickness can lead to:
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Cracking or delamination during drying or expansion
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Inconsistent expansion during a fire
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Failure to meet the tested fire resistance rating
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Increased material cost and unnecessary weight
Getting the DFT right is essential for both performance and compliance.
How Is Intumescent Coating Thickness Measured?
1. During Application (Wet Film Thickness)
Installers often measure Wet Film Thickness (WFT) during application using a simple wet film gauge. Since coatings shrink as they dry, a conversion is applied (usually provided by the manufacturer) to calculate expected DFT.
This helps installers apply the correct amount in each coat to avoid under- or over-application.
2. After Application (Dry Film Thickness)
Once the coating is fully dry, Dry Film Thickness is measured using a non-destructive digital thickness gauge (often magnetic or ultrasonic). This provides an accurate reading in microns and is used to confirm compliance with the specification.
3. Spot Checks and QA Reports
Professional contractors will carry out multiple spot checks across all protected members, record the data, and produce a quality assurance (QA) report. This documentation is often required by:
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Building control officers
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Fire risk assessors
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Main contractors and specifiers
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Insurance providers
Who Is Responsible for Checking Coating Thickness?
The applicator (installer) is responsible for applying the correct DFT according to the project specification and manufacturer guidance. However, the building owner, developer or main contractor should always ensure:
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The applicator is qualified and experienced
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All coatings are tested and certified products
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QA checks are performed and documented
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Any deviations are addressed before the handover
In some cases, a third-party passive fire protection inspector may be brought in to verify application and testing, especially on large or high-risk projects.
What Happens If the Coating Is Too Thin or Too Thick?
Incorrect film thickness is one of the most common reasons intumescent protection fails inspections. The consequences include:
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Failed building control sign-off
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Voided product warranties or certifications
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Compromised fire resistance in real-world conditions
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Costly remedial works, including recoating or reapplication
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Legal liability if protection fails during a fire
To avoid these issues, always ensure coatings are installed to the correct DFT—and that records are kept throughout the process.
Final Thoughts
Intumescent coatings are only effective when applied correctly, and dry film thickness is at the heart of that performance. Whether you’re a contractor, project manager, or building owner, it’s vital to understand the importance of DFT, how it’s measured, and why it can’t be left to guesswork. Contact Martyn Young Fireproofing Consultancy today for expert advice on intumescent products.