Intumescent Strips: FD30, FD60, and How to Get Past Building Control

The building control officer turns up to sign off your loft conversion. He looks at the new FD30 fire door at the top of the stairs, runs a finger along the gap between door and frame, and asks where the intumescent strips are. You point at the door. He shakes his head, points at the bare timber edge with no groove and no strip, and tells you the door fails. The certified door leaf you paid for is fine. The frame isn't. You're now looking at routing grooves into an installed lining (which is nearly impossible because the router can't reach the corners) or replacing the entire frame. That's a frame-replacement bill on top of the cost of the door itself, all because nobody told you a fire door is only as good as the assembly it sits in.

What an intumescent strip actually does

An intumescent strip is a thin plastic-cased seal, typically 15mm wide and 4mm deep, that sits in a routed groove on the frame (lining) (preferred) or the door edge (acceptable) of a fire door. At room temperature it's inert. Heat it past around 150 degrees Celsius and the chemical core inside expands rapidly, swelling up to thirty times its original volume in some formulations, completely filling the gap between door and frame. That sealed gap is what stops fire and hot smoke pushing past the door for the rated time (30 minutes for FD30, 60 minutes for FD60).

That rating isn't a guess. It comes from a destructive test under BS 476 Part 22 or BS EN 1634-1, where the complete assembly is mounted in a furnace and burned. The door, the frame, the hinges, the latch, the strips, all of it tested together. The certificate that comes back applies to that exact combination. Swap any component and the certificate is technically void.

This is the part most homeowners don't realise. A "fire doorset" is the complete tested assembly, not the door leaf. Buying a Certifire-listed FD30 door and fitting it into the existing standard internal door frame creates a non-compliant assembly, even if the door is genuinely fire-rated. Building control inspectors check the whole set. Three out of every five rejections at FD30 inspection trace back to this single mistake.

The TGN9 revocation: why old advice is wrong

If you've read older DIY forum threads or pre-2022 articles, you'll find a recurring claim that intumescent strips are optional in single-family dwellings. That advice came from Technical Guidance Note 9, published by the Building Control Alliance, which created a concession allowing FD30 doors to be installed without strips in domestic settings as an "FD20 equivalent."

TGN9 was revoked in January 2022. No replacement, no explanation, just gone. Since that date, intumescent strips are required to achieve any FD rating in a tested assembly. Door manufacturers stopped testing at FD20 long before 2022 anyway, so FD30 is now the practical minimum for any internal fire door application.

If your guide, your forum source, or your builder is quoting pre-2022 rules, treat it as outdated. Approved Document B, the Fire Safety (England) Regulations 2022, and current BWF Fire Door Alliance guidance all align: strips are not optional.

Click to enlarge

Where you actually need them

Approved Document B Volume 1 (2019 edition with 2025 amendments) sets out the scenarios where FD30 doors are required in a single-family dwelling. The big four:

Loft conversions. Once you create a third storey, every habitable room opening onto the protected stairwell needs an FD30 door. That covers the new loft bedroom and also every bedroom on the first floor, every living room, and every habitable space whose door opens onto the stairs. Bathrooms and WCs without fuel-burning appliances are usually exempt, but check with your inspector. Loft conversion fire-door requirements regularly catch homeowners out: budget for eight or nine doors, not one.

Three-storey houses. Same rule. Habitable rooms onto the protected escape route get FD30 doors.

Integral garages. The door from a garage into the dwelling needs to be FD30S (the S means smoke seal added). Fumes and fire from a vehicle fire have to be held back from the house.

Houses in multiple occupation (HMOs). Different regulations apply, generally stricter. Out of scope for a single-family extension but worth flagging if your conversion changes the property's use class.

For a standard two-storey extension or a kitchen-extension that doesn't change the storey count, you're usually outside FD30 territory. The strips and the doors aren't part of your build. But the moment you go up a storey or touch a garage-to-dwelling junction, plan for them from day one.

FD30 vs FD30S: the smoke seal question

You'll see two ratings on every product label. FD30 means 30 minutes of fire integrity. FD30S adds an "S" for cold smoke seal, which is a separate brush or fin seal that stops smoke leaking through the gap before the door gets hot enough to activate the intumescent. The intumescent strip needs around 100 to 120 seconds at fire temperature before it expands enough to seal. During those two minutes, cold smoke pushes through the gap freely unless you have a smoke seal.

Most modern combination products do both jobs in one strip: a thin plastic carrier with the intumescent core down the middle and a brush or fin seal along one face. Buy one of these and you've covered FD30S in a single product. The Astroflame and TIMCO door packs sold at Toolstation and B&Q are exactly this format.

For an integral garage door, FD30S is mandatory. For a loft-conversion stairhead, FD30 is the legal minimum but FD30S is strongly recommended and many building control officers will require it. The cost difference is negligible (a couple of pounds per door), so just buy the combined product.

The three intumescent compounds (and why mixing them is a fail)

This is technical but worth understanding because it determines which product to buy and why you should not mix brands around the same door.

Sodium silicate. Activates above 100 degrees Celsius. Expands around 10 times. Forms a hard, rigid foam. The original intumescent material. Hygroscopic, meaning it absorbs moisture, so not the best choice for bathrooms or external doors.

Intercalated graphite. Activates around 150 to 190 degrees. Expands 15 to 20 times. Forms a fluffy, sponge-like char. Non-hygroscopic, which makes it the right choice for humid environments. The most common compound in modern strips.

Monoammonium phosphate. Activates above 180 degrees. Expands up to 40 times, the highest of the three. Forms a soft foam. Hygroscopic. Used in hinge pads more than perimeter strips.

The reason this matters: if you mix a graphite strip on one side of the door with a sodium silicate strip on the other, they activate at different temperatures and expand at different rates. The seal becomes uneven. The door fails its test conditions. Use the same brand and the same compound for all three sides of the perimeter (head and two stiles) plus any door edge strips. This catches people during repairs, when they buy a single replacement strip from a different supplier without checking what the existing strips were.

Sizes: the 10mm vs 15mm vs 20mm question

Three widths cover almost every FD30 and FD60 application. The depth is consistent at 4mm.

The rule for a domestic project is straightforward. New frame: buy 15mm. Retrofit to an old frame that's already grooved at 10mm: buy 10mm to match. FD60 (rare in domestic): 20mm. The Pyroplex part numbers (8500, 8700, 8600) and Lorient equivalents follow this same width convention, so identifying the product is easy once you've measured the existing groove.

The 4mm depth is not negotiable. The bearing-guided router cutters that fit these strips are pre-set to cut at exactly 4mm. The Trend C220X1/4TC for 15mm grooves and the C209X1/4TC for 10mm grooves both bottom out at 4mm regardless of how hard you push.

Frame groove or door edge: where it goes

BWF Fire Door Alliance and most fire-door manufacturers prefer the strip in a routed groove in the frame (lining), not the door edge. The reasoning is practical: if the strip is on the frame, the door edge can be shaved to adjust fit without touching the seal. If the strip is on the door, every adjustment risks damaging it.

Both positions are permitted by current standards. Pre-grooved FD30 linings sold at Howdens, Wickes, and trade merchants come with the groove already routed in the frame at 15mm width and 4mm depth. The strip pushes in by friction, no glue needed for most products, and sits flush with the lining face.

For retrofit, you have a problem. Routing a groove into an installed door frame is nearly impossible because the bearing-guided cutter can't reach the corners where the head meets the jambs. The router body fouls the architrave or the wall. The standard professional solution is to take the door off its hinges, lay it flat on trestles, and rout the groove into the door edge instead. Three sides only: the two stiles and the top. You don't groove the bottom edge.

Click to enlarge

How to fit a strip in a routed groove

The strip itself is a five-minute job once the groove exists. Here's the sequence for fitting strips to a new door set.

Mark and cut. The standard internal door is 1981mm tall, so the two jamb strips are cut to 1981mm and the head piece to fit between them. Most strips come in 2.1m lengths sized exactly for this. Cut with sharp scissors, tin snips, or a Stanley knife. Don't tear them or the foam core can dislodge.

Push them in. Start at the top of one jamb, align the strip with the groove, and push it in by hand. The plastic carrier grips the groove walls by friction. Work down the length. If it's stiff, the groove may have a slight burr from the routing; run a fingernail along the edge to clear it.

Fit the head piece. Trim to length so the ends butt tightly against the tops of the two jamb strips. A small gap (under 2mm) is acceptable; a visible cold-smoke gap is not. Some installers mitre the corners for a cleaner fit, but butt joints are standard and accepted by inspectors.

Hinge cutouts. The hinge mortices on the frame interrupt the strip groove. This is normal and accepted by every building control officer in the country. The intumescent strip on either side of the hinge does the sealing work; the hinge itself is bridged by intumescent hinge pads (a separate 1mm pad fitted behind each hinge leaf during hanging). Don't try to bridge the hinge gap with strip. Don't fit two short strips end-to-end through the hinge area. The hit-and-miss pattern through the hinges is the right pattern.

Painting: the rule that has nuance

Every fire safety guide says the same thing in capital letters: do not paint the strips. The reasoning is sound. Paint forms a film on the strip face that physically restricts expansion. Some paints react chemically with the intumescent compound. The strip can fail to activate, activate slowly, or activate but not expand fully.

The nuance most guides skip: smoke seals (the brush or fin part of a combination FD30S strip) must never be painted under any circumstances. Paint clogs the bristles or the rubber blade and stops the cold-smoke seal making contact with the door. This is non-negotiable.

Intumescent-only strips (without the smoke seal element) are sometimes painted by manufacturers in specific colours and explicitly marketed as paint-tolerant. Lorient and a few others publish guidance allowing single thin coats of standard decorative paint. But: this is brand-specific, depends on the paint type, and isn't a default. Treat painting as forbidden unless the strip's manufacturer documentation explicitly permits it for your specific paint system.

If you discover a strip has been painted (a common situation when buying an older property with existing fire doors), peel the paint off the foam face with a sharp craft knife. Test the strip by warming a section with a heat gun for ten seconds; it should expand visibly. If it doesn't, the strip has failed and needs replacing.

Routing the groove yourself

If you're retrofitting strips to existing doors and the lining isn't pre-grooved, the practical option is routing the groove into the door edge. You'll need:

A plunge router. Mid-range will do; the Trend T4EK or similar 1/4-inch collet model handles this easily. The job is light cutting in softwood or composite door material, not heavy work.

A bearing-guided intumescent cutter. The Trend C220X1/4TC is the 15mm cutter, sized to the standard FD30 strip and pre-set to a 4mm depth via the bearing. £36 from Protrade or Trend Direct. The 10mm version (C209X1/4TC) is slightly cheaper. Both are 1/4-inch shank.

A pair of trestles or saw horses. The door has to come off and lie flat.

The process: take the door off, remove all ironmongery (hinges, latch, kick plate if any), lay it flat on trestles, set the cutter in the router, run the bearing along the door edge to cut the groove. One pass per side. Three sides, not the bottom. The bearing keeps the cut at exactly 4mm depth and the cutter width sets the groove at 15mm. Total time per door: under fifteen minutes once you've set up.

For the second door, the cutter is already set up. Spread the cutter cost across five new doors and it pays for itself comfortably on the project. For a single door, the router and cutter cost more than just buying a pre-grooved lining and a new door. Do the maths.

Cost and where to buy

Pricing varies more by purchase format than by retailer. Buying door packs (pre-cut lengths sized for one door) is convenient but pays a premium per metre. Buying full 2.1m lengths in 5-packs or 10-packs gives you the lowest per-door cost.

The going rate per FD30 door set lands in the £23 – £23 per-door range when you buy in 10-packs, or the £9 – £11 range for single-door pre-cut packs. Pay the higher end if you only need one or two doors and want the convenience of a pre-cut pack. Pay the lower end if you're doing a loft conversion with eight or nine doors and the bulk pack pays for itself twice over.

Howdens stocks Lorient brand strips at trade pricing, but only available through trade accounts. If you have a kitchen fitter or carpenter with a Howdens account, the Lorient pricing is competitive with Toolstation. Otherwise, Screwfix and Toolstation are the practical homeowner options.

For FD60 (which you almost certainly don't need in a domestic build), Fire Seals Direct and Safelincs are the specialist suppliers. Don't expect to find 20mm strips at Wickes or B&Q.

The doorset trap (and how to avoid it)

This is the one mistake that costs more than all the others combined. You buy a Certifire-listed FD30 door from Wickes or Howdens, you fit it into the existing door frame in your loft conversion, and the inspector fails it. Every time.

The certified door leaf carries a fire rating only when installed in a frame that was part of the same test assembly or a recognised assessment. A 2015 internal door lining from your existing house is not part of any FD30 test certificate. The hinges aren't right (FD30 needs Grade 13 minimum, three per door). The latch isn't certified. The frame depth might be wrong. There's no intumescent strip groove. The whole thing fails as a system, even if the door itself is genuinely fire-rated.

The fix is to buy the complete doorset: door, lining, ironmongery, seals, all from one tested assembly. Howdens sells these as a package. Wickes sells FD30 lining kits separately at £40 – £65 per kit, designed to match their FD30 doors. Trade specialist suppliers (Doors Direct, Vicaima, JELD-WEN) sell complete certified assemblies.

Cheapest route for a homeowner: a Howdens or Wickes FD30 door plus their matching FD30 lining kit, both bought together. Mid-range route: a Vicaima or JELD-WEN doorset with assessment evidence covering the assembly. Premium route: a fully certified Lorient-strip and Certifire-stamped commercial doorset, which you'll only see in HMO conversions and high-spec builds.

Common mistakes

Painting over the strips during decoration. The single most common reason FD30 doors fail at final inspection. Strip after decoration, not before. If a painter has already painted them, peel the paint off the foam with a sharp knife and test with a heat gun.

Mixing intumescent compound types around the same door. Sodium silicate on one side, graphite on the other, uneven activation, failed test conditions. Buy all three perimeter strips from the same brand and same product line.

Buying a fire door without checking the frame. A certified door in a non-certified frame fails. Buy the doorset, not the door.

Using the pre-2022 "FD30 without strips is FD20" rule. TGN9 was revoked in January 2022. Any source quoting that concession is out of date.

Forgetting the head strip. Some installers fit strips on the two stiles and forget the head. Three sides minimum. Without the head strip, hot gases and smoke push through the top gap unimpeded.

Routing the groove into the bottom edge. Don't. The bottom of the door has a different gap profile (up to 22mm to clear flooring) and isn't part of the strip's intended function. Three sides only: head and two stiles.

Cutting the strip too short. Each piece must run continuously along its full edge with no breaks except at hinge mortices. Two short strips end-to-end with a gap in the middle is a fail.

Using a smoke seal in place of an intumescent strip. They're different products with different jobs. Smoke seals stop cold smoke. Intumescent strips stop heat and flame. FD30S means both. Substituting one for the other fails inspection on the missing function.

Where you'll need this

• Building Control Final Inspection - inspectors check intumescent strips at sign-off on any FD30 door in the build
• Building Control Inspection Stages - what to expect when the inspector arrives at completion stage
• Decoration - fit strips after decoration finishes, never before, to avoid paint contamination
• Snagging Checklist - check every fire door for continuous strip runs, undamaged seals, and no paint on foam faces

Intumescent strips appear at one specific stage of any extension or renovation project that involves an FD30 fire door: late second-fix, after decoration, before final building control sign-off. They're cheap, the rules around them are tight, and getting them wrong costs you a re-inspection. Buy the right size for the groove, fit them in the right sequence, keep paint off the foam, and don't mix brands. Those four things will get you past inspection without drama.