Fire Doors and the Escape Route for a Loft Conversion

The walls nobody ever tested

Open a stairwell wall in a house built before the war and you find lath and plaster on 100 mm studs, or a 1970s refit in 9.5 mm board with a skim over it, or both within the same run because a chimney breast was taken out in between. Lath and plaster in genuinely sound condition is not a poor performer, but no supplier will hand you a certificate for it, and a certificate is what the enclosure now needs. The practical choice is to overboard the stair side with fire-rated board and skim, or to strip back to the studs and reline — and that choice is made on the state of what is behind the plaster, not on preference.

Overboarding costs depth, and depth costs joinery. Add 12.5 or 15 mm to each face of a stairwell and every architrave, skirting, door lining, string, newel and light switch on that wall moves out with it. In a narrow terrace hall the clear width of the landing is measured after that board is on, so agree the lining thickness, then check the widths, then order the doors — a lining that pushes a leaf into a newel is discovered at hanging.

The layer count is not a judgement call. It comes from a tested system in a manufacturer's system literature — the British Gypsum White Book, the equivalent Knauf and Siniat system documents — matched to the stud, the board type and the fixing centres actually being used, with the fire resistance evidenced against BS 476-21 and BS 476-22 or the European tests such as BS EN 1364-1 for non-loadbearing walls. Read the layer count off that system and carry it into the take-off. A rating on its own tells you nothing about how many boards are on the wall.

Quantify the enclosure as elevations rather than as rooms. A stairwell is a tall, narrow, awkward set of faces: two or three walls per storey, a raking soffit, the return above the landing, a spandrel under the flight. Measure each face on the plan, add them, and where the tested system puts board on both sides of a partition — a stud wall between the stairwell and a bedroom, boarded to the system on both faces — the sheet count doubles with it.

One face, one order

Several of the largest surfaces on this job are only ever boarded once, and reaching for the both-sides arithmetic on them buys twice the board. The underside of the new flight is one: it forms part of the enclosure and is lined from below, while its upper face carries treads. A masonry party wall dry-lined on the stairwell side is another — there is no far face to board, because the far face is in the neighbour's house. The back of an eaves cupboard partition, the spandrel filling the triangle under the flight, and the stair-side face of any wall whose other side sits outside the protected route all behave the same way.

The over-order is not harmless. Fire-rated board comes in longer sheets than standard, it is heavier than standard, and on a terraced conversion it arrives on a pavement and goes up two flights by hand. Twenty extra boards is a day of labour and a hallway nobody can walk through.

Measure raking soffits on the slope, not on plan. The underside of a flight rising 2.6 m over a 3.4 m run is a shade over 4.2 m long on the rake before the winders are counted, most of a metre more than the run it sits above, and a soffit measured off the floor plan comes up short every time. Take the flight length from the geometry rather than from the drawing scale, deduct only openings large enough to leave a reusable offcut, and order the layers the tested system asks for on that single face.

The floor that quietly became a fire barrier

The new loft floor is not only structure. From the moment the loft is habitable it separates one storey from another, and it has to hold a fire from below for long enough that anybody coming down the stair gets past it. Approved Document B sets out a modified standard for this floor in defined loft-conversion circumstances, with the periods for loadbearing capacity, integrity and insulation given in its fire resistance tables and specific conditions attached to when the relaxation applies. Read those conditions in the edition in force rather than repeating a figure heard on another job; this is one of the provisions that has moved between editions.

The ceiling below is the fire side, which decides the sequence. Retaining a sound lath-and-plaster ceiling and adding a layer of fire-rated board beneath it keeps a lot of history intact and loses 15 mm of head height in a bedroom; stripping it means a skip, a dust screen and a room out of use, and gives you a known construction with a system reference behind it. Working from above through the new floor deck feels cleaner and does nothing for the surface that matters. Whichever way it goes, the boards below and the joints between them are the barrier, and the deck above is not.

Then come the holes. Downlights cut straight through the barrier unless the fitting is one tested in that ceiling construction or is covered by a fire-rated hood that has been; extract fans and their ducts need the same thought, and so does a soil pipe boxed through the corner of the bedroom. A loft hatch that opens onto the protected landing is part of the enclosure and needs a fire-resisting hatch and frame, not an insulated one. All of them are cheaper to place before boarding than to cut in later, which is how a barrier ends up perforated by a trade that never saw the fire strategy.

For the quantity, take the ceiling area under the whole loft footprint, then add the ceiling over the landing and the stairwell, which gets left off because it is small, awkward and reached off a ladder. Ceilings carry more cut waste than walls.

Where the new flight is allowed to begin

The new flight has to rise from inside the protected enclosure. That single sentence reorganises more loft conversions than the doors do, because the obvious place to put a loft stair on a three-bedroom terrace is over the existing flight, in the space currently occupied by the smallest bedroom, and the obvious way in is through that bedroom. A flight reached through a habitable room means the route out of the loft passes through a room where the fire may have started, and no amount of boarding fixes it. Either the landing is extended to reach the foot of the new flight, or a partition and a door make the approach part of the enclosure — and that partition is more board on the take-off.

The geometry itself is Approved Document K's territory. A private stair is worked to a maximum rise and a minimum going, with a maximum pitch, and the risers in a flight are to be equal; BS 5395-1 sits behind it as the code of practice. Loft conversions get two specific concessions worth knowing before the plan is drawn: reduced headroom over the flight, measured lower at the side of the stair than at its centre, and alternating tread stairs, which are permitted only where there is genuinely not the space for a conventional flight and only where they serve a limited amount of accommodation. Neither is a design preference. Both are last resorts with conditions, and an officer who sees an alternating tread stair on a plan that had room for a conventional one will ask why.

Settle the rise, the riser count and the going before the enclosure is measured, because the flight is what generates most of the new surface area. The number of risers fixes the flight length, the flight length fixes the raking soffit, the soffit and the wall alongside it are the surfaces being lined, and the balustrade and handrail follow the same line. Change the riser count late and every one of those quantities changes with it.

The alarm goes in before the boards do

Approved Document B does not design the alarm system; it points at BS 5839-6, the code of practice for fire detection and fire alarm systems in domestic premises, and that standard describes systems by grade and by category. Grade covers what the equipment is and how it is powered — mains supply with a standby, in the case a house conversion normally lands on — and category covers how much of the house is covered, from detection in the escape route alone through to detection in the rooms as well. Which grade and category apply to your job is a question for the officer and the standard together, and Scotland works to a broader requirement of its own that the Building Standards Technical Handbook sets out. The one constant is interlinking: one alarm sounding has to sound them all, or somebody asleep on the second floor learns about a hall fire too late for the route to matter.

The site consequence is about sequence, not about devices. Interlink cabling runs in the ceilings and along the stairwell, which is to say through the barrier that is about to be built. First-fix it before the fire-rated board goes up. Chasing a cable into a finished, taped and skimmed fire-resisting ceiling means cutting the thing you just paid to install, and a repaired penetration is only as good as the person who repaired it. Radio-interlinked alarms exist and have their place on retrofits, and they still want a mains supply at each head.

Treat any per-room arithmetic as a first pass for ordering rather than as a design. The standard positions heads by circulation space, by distance from doors and by what is above the stair, and a kitchen takes a heat alarm rather than a smoke alarm because a smoke alarm over a hob is an alarm that gets taken down. Count the heads to get a number on the order, then have the installer position them to the standard and issue the certificate, which is what gets handed over at completion.

Half an hour leaks at the joints

A fire lining works as a continuous membrane, and the tested construction includes how the boards are joined: joints taped and filled, board edges closed at the perimeter, and the junction between the new lining and whatever it meets — masonry, the underside of the old ceiling, the string of the flight — all sealed. This is not the decorator finishing a wall. It is the last few millimetres of a barrier, and it is the part most likely to be rushed because it happens on the same day as the skim. Service penetrations get the correct product with it: fire-rated sealant, collars or batts to the manufacturer's tested detail, with the ASFP guidance on fire stopping and penetration seals describing what a competent seal looks like — not decorator's caulk, which shrinks, and not expanding foam trimmed flush, which is the most-photographed defect in the trade.

For the take-off, the jointing area equals the boarded area. Every board is jointed on all four edges, so the compound quantity follows the same square metres as the sheets rather than a length of tape, and a two-layer lining is jointed twice: the base layer filled, the face layer taped and finished.