“Fire mastic” is a term used loosely on site. In practice it covers several distinct products: fire-rated acrylics, fire-rated silicones and intumescent sealants, each with its own behaviour, uses and test evidence. Picking the wrong one, or using the right one outside its tested detail, can leave a fire-resisting wall or floor weaker than it looks.
This article sets out the differences in neutral, practical terms for FMs, contractors and estates teams. It is not a design guide. Which product goes where is decided by your designer or fire engineer, using the manufacturer’s tested details, and checked by building control.
What “fire-rated sealant” means
A fire-rated sealant is one that has been tested, as part of a seal, to show it can help hold back fire and smoke for a stated time. The test is not of the sealant alone. It covers a specific arrangement: product, substrate, gap size, depth, backing and service type.
Penetration seals are tested to BS EN 1366-3 and linear joint seals to BS EN 1366-4. Results are classified under BS EN 13501-2, for example EI 60 or EI 120, where E is integrity, I is insulation and the number is minutes.
So a sealant is never simply “fire-rated” in the abstract. It is fire-rated for the configurations it has been tested in.
Intumescent sealants
Intumescent sealants expand when heated. As the temperature rises in a fire, the material swells and forms a char that fills gaps, including gaps that open up as plastic pipes soften or cable sheathing burns away.
This makes them useful around services that change shape in a fire, and around small gaps where expansion helps keep the seal closed. The exact uses depend on the product’s test evidence: some are tested around certain plastic pipes up to a size, others only around metal pipes or cables. Larger plastic pipes often need collars or wraps instead.
Fire-rated acrylic sealants
Fire-rated acrylic sealants are water-based and usually paintable. They are commonly used for linear gaps in fire-resisting walls and around some service penetrations, often with mineral wool behind them where the tested detail calls for it.
Being paintable makes them popular for internal walls that will be decorated. They generally have lower movement capability than silicones, so they suit joints that don’t move much. Some acrylics also have intumescent properties; the product data will say.
Fire-rated silicone sealants
Fire-rated silicone sealants combine fire performance with higher flexibility. They are often chosen for linear joints that move, such as deflection heads at the top of partitions, joints between fire-resisting elements that shift, and some external or wet-area fire-resisting joints.
They are generally not paintable. As with all fire-rated products, the joint width, depth and backing must match the tested detail.
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Side-by-side comparison
General characteristics only. The individual product’s data and test evidence always decide.
| Intumescent | Fire-rated acrylic | Fire-rated silicone | |
|---|---|---|---|
| How it works in fire | Expands and chars to close gaps | Holds its place; may have some intumescent action | Holds its place while staying flexible |
| Movement capability | Varies by product | Generally lower | Generally higher |
| Paintable | Often | Usually | Usually not |
| Typical uses | Some pipe and cable penetrations, small gaps | Linear gaps and some penetrations in walls to be decorated | Moving linear joints, deflection heads |
| Key check | Tested for that service type and size | Tested for that gap, depth and backing | Tested for that joint and movement |
Why the tested detail matters more than the label
A seal is only as good as its match to the tested detail. That means the same:
- product (no mixing brands or types in one seal)
- wall or floor construction
- service type, material and size
- gap width and seal depth
- backing, such as mineral wool where the detail calls for it
- orientation: wall or floor
Improvising, topping up with a different product or guessing depth invalidates the evidence, even if the finished seal looks neat. Fire-rated sealant is also only one option: collars, wraps, batts, pillows and boards exist for services and openings that sealant can’t handle alone.
Rules, guidance and records
In England, Approved Document B covers fire stopping of openings and penetrations. References to BS 476 classes are being removed from it from 2 September 2029, subject to transitional arrangements, after which BS EN 13501-2 classification is the route. See our guide to the BS 476 to BS EN 13501 change.
In Scotland, clause 2.1.14 of the Scottish Technical Handbook (non-domestic), section 2.1 says penetrations should be fire stopped to at least the wall or floor’s fire resistance duration, with proprietary products tested for their end use. The ASFP publishes guidance including its “Red Book” on fire stopping.
Whatever the jurisdiction, keep the product data, tested details and a record of where each seal is. Those records help the next contractor, the next building control visit and anyone assessing the building later. Without them, nobody can easily tell whether a seal was ever right.
Checking existing fire-rated seals
On existing buildings, the question is often whether seals already in place are any good. Some signs that a seal needs looking at by someone qualified:
- new cables or pipes pushed through an existing seal
- gaps around services, or seals that have cracked or pulled away
- expanding foam or general-purpose sealant used where fire stopping should be
- mixed products in a single opening
- no label, record or data for the product used
Seeing a problem doesn’t tell you the fix. The designer or fire engineer should confirm the specified system, and remedial work should follow the manufacturer’s tested detail for that wall or floor and those services.
Our role
Mastic Doctor installs fire-rated and intumescent sealant around service penetrations and linear gaps where it is specified, to the manufacturer’s tested details. We don’t design fire stopping and we don’t certify it. If a detail calls for a collar, wrap or batt rather than sealant, we’ll tell you so the right party can be involved.
For more, see fire stopping and our overview of fire stopping explained. Tell us what records you need and we’ll confirm with the quote.
Quick answers
Is intumescent sealant the same as fire-rated sealant?
Intumescent sealant is one kind of fire-rated sealant. It expands in heat. Fire-rated acrylics and silicones are others, and each is tested for specific uses.
Can I use any fire-rated sealant around a plastic pipe?
No. Plastic pipes soften in fire, and only products tested for that pipe type and size should be used. Larger plastic pipes often need collars or wraps.
Can fire-rated acrylic be painted?
Usually, yes, which is why it is common on decorated internal walls. Check the product data for paint compatibility.
What happens if two fire sealants are mixed in one seal?
The seal no longer matches any tested detail, so its fire performance can’t be relied on. One tested product and detail should be used per seal.
Who decides which fire sealant to use?
Your designer or fire engineer, using the manufacturer’s tested details, with building control checking. We install what is specified.
Is expanding foam a fire-stopping product?
General-purpose expanding foam is not. Some fire-rated foams exist, tested for specific uses, but ordinary foam should not be used where fire stopping is required.
Related services
More advice
- Fire Stopping Explained for Building Owners
- Fire Stopping After M&E and Fit-Out Works
- BS 476 to BS EN 13501: The 2029 Change
- Fire Stopping and Scotland's Building Standards
- Building Safety Act and Fire Stopping Records
- All advice
This guide is general information about sealant work. It isn’t legal, fire-safety or structural advice; for those, speak to a qualified professional.