Fire stopping is one of those things that catches small builders out more than almost anything else on domestic jobs. Not because it’s complicated, but because it’s invisible once you’ve boarded up. Building Control know that, which is why they’re increasingly asking to see it before you close in. Get it wrong and you’re either hacking out your own work or hoping nobody notices. Neither is a good outcome.
I’ve been on enough domestic extensions and refurbs to know that the moment services start threading through compartment walls and floors, most guys either wing it or reach for the cheapest bit of expanding foam they can find. Neither option cuts it under Part B of the Building Regulations. Here’s what you actually need to know.
What Part B of the Building Regulations actually requires on domestic jobs
Part B covers fire safety in buildings, and Schedule 1 of the Building Regulations 2010 (as amended) sets out the functional requirements. For a typical domestic extension or refurb, the bits that catch people out are compartmentation and means of escape. The requirement is that any opening made in a compartment wall or floor for a service penetration must be fire stopped to restore the fire resistance of that element.
That sounds simple enough, but the practical implication is that every pipe, cable, duct or soil stack that passes through a wall or floor rated for fire resistance needs a proper fire stop. Not a bit of mortar. Not expanding foam. A tested, rated product that actually maintains the integrity of the structure for the same period the element itself is rated for, typically 30 or 60 minutes on most domestic builds. The Approved Document B published by MHCLG is the definitive reference and is freely available on gov.uk. If you haven’t read the domestic sections, do it now.
Where fire stopping requirements actually apply on a standard domestic job
Most tradesmen assume fire stopping is only an issue on big commercial sites. It isn’t. Here’s where you’re likely to need it on everyday domestic work:
Loft conversions. Any floor between the new loft room and the floor below is a compartment floor. Services penetrating it, say a soil vent pipe or electrical cables, need fire stopping at every penetration point. The staircase enclosure is also a protected route, so any services passing through those walls need attention too.
Extensions with integral garages. An attached garage is considered a separate fire compartment from the house. The wall and floor between them need to maintain fire resistance. I’ve seen builders thread electrical conduit through that party wall with nothing more than a bead of silicone around it. That fails. Every time.
Refurbs involving chasing and re-routing services. Any time you’re opening up an existing compartment wall or floor to reroute pipework or cables, you’re creating a new penetration. That penetration needs fire stopping when you close up again.
Party walls in terraced and semi-detached houses. This is a big one. Party walls between dwellings are compartment walls by definition. If you’re doing any work that involves penetrations through them, the fire stopping requirements are non-negotiable. Anything from a flue liner to a soil pipe counts. If you’re unsure whether a wall you’re working on is a compartment wall, have a read of our guide on wet plaster versus dry lining systems, which covers wall types in detail for domestic refurbs.
Products that actually work: intumescent versus passive fire stopping
There are two main approaches and you often need both. Passive fire stopping means filling the gap around a penetration with a material that doesn’t rely on heat to do its job. Think fire-rated mortar, rockwool or mineral wool stuffed tightly around a pipe and finished with fire-rated sealant. This works fine for rigid penetrations like copper pipe or steel conduit where the pipe itself won’t burn.
Intumescent products are for plastic pipes and cables, where the material passing through the wall will melt or burn away in a fire, leaving a hole. Intumescent material expands when exposed to heat, collapsing in on itself to seal the gap the plastic pipe has left behind. The most common products for domestic jobs are:
Intumescent pipe collars, which wrap around plastic soil pipes or waste pipes at the point they pass through a floor or wall. They expand inward when heated. These are the most visible product on site and the easiest to get right if you read the manufacturer’s instructions.
Intumescent fire-rated sealant, which looks like a standard silicone but expands when exposed to heat. Used around cable bundles and small-diameter plastic pipes. Don’t use standard silicone and think it’ll pass.
Fire-rated pillow systems, which are reusable intumescent bags pushed into larger openings. Useful for cable trays or multiple service penetrations through a single sleeve. More common on commercial jobs but increasingly appearing on domestic loft conversions with complex service routes.
Every product needs to be used in accordance with its tested installation method. That means following the manufacturer’s data sheet for the specific pipe diameter, wall thickness and fire rating required. A collar rated for 110mm plastic pipe in a 100mm dense blockwork wall is not automatically rated for the same pipe in a 15kg/m3 plasterboard stud wall. These are different test scenarios and the data sheet will tell you which construction types each product is tested for.
How Building Control approaches fire stopping on domestic extensions
Building Control Officers are well within their rights to request a site visit before you close in around service penetrations. On loft conversions in particular, many BCOs now make this a standard inspection stage rather than an optional one. If you close in without an inspection, you may be asked to open up. That’s your time, your materials and your relationship with the client on the line.
Keep records. Take photos of every fire stop you install before boarding. Write on the photo what product you used, the pipe size and the wall or floor type. If you’re using a system that requires a third-party tested certificate (some proprietary systems do), have a copy on site. Some BCOs are now sophisticated enough to ask for the ETA (European Technical Assessment) or UKCA test report reference for the specific product. Firms producing detailed project documentation, including those using tools from outfits like dijitul medi for detailed service layout drawings, are better placed to show compliance from the outset.
It’s also worth noting that on a loft conversion job, the fire stopping requirements connect directly to the acoustic and thermal work you’re doing. If you’re installing mineral wool for insulation around services anyway, check whether it has a fire rating that makes it usable as part of a passive fire stop too. Rockwool RWA45 slabs, for instance, have both acoustic and fire performance credentials. Getting double use out of your materials is good practice. Have a look at our guide on Part E acoustic requirements for domestic jobs for how the two sets of requirements overlap on common build types.
Common mistakes that get builders caught out
Expanding foam is the biggest culprit. Ordinary PU foam is combustible and has zero fire rating. I’ve seen it used to fill around soil pipes through compartment floors on loft conversions, which is a straight fail. Fire-rated foam products do exist and they’re not expensive, but they need to be used for the right aperture sizes; most are only rated for small gaps, not large penetrations around 110mm plastic pipe. Read the data sheet before you reach for the can.
The second common error is treating the fire stop as an afterthought. If you’re running services and you plan to fire stop later, you’ll often find access is compromised once other trades have been in. Plan the fire stop into the installation sequence, not onto the snag list. This is especially true on jobs involving temporary works where multiple service runs are going in during a phased sequence.
Third: don’t assume a small gap doesn’t matter. Even a 5mm annular gap around a cable bundle through a timber floor is a fail under Part B if the floor is a compartment floor. The requirement is that the full fire resistance of the element is maintained. A small untreated gap can allow smoke and flame through in minutes.
Fire stopping isn’t the most glamorous part of a domestic build, but it’s one of the few things that actually saves lives if the job goes wrong. Build it in properly, document it and you’ll have no issue with Building Control. Cut corners here and you’re not just risking a BCO argument; you’re creating a genuine hazard for whoever ends up living in that house.
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