Fit a carbon monoxide alarm to a dot-and-dab plasterboard wall

Fit a battery CO alarm to any wall type, using the right drill size and fixing for what's actually behind the plasterboard.

COA-01Buy the right alarm and check how it's poweredChoose a CO alarm marked BS EN 50291. This job covers a battery-powered alarm on a wall bracket. If you're fitting a mains-wired, interlinked alarm instead, that's different work.

RULES

1 each · CO alarm (BS EN 50291, sealed 10-year battery)

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KNOW THE RULES

A mains-wired alarm connected into your fixed wiring is notifiable electrical work under Part P — a registered electrician must do it, or it must be self-certified by a competent person. A battery alarm on a wall bracket is not notifiable.

COA-02Pick the best spot on the wallFit the alarm in the room with the fuel-burning appliance (boiler, gas fire, wood burner) and consider one near where people sleep too. Position it 1-3m from the appliance and 150-300mm down from the ceiling, away from corners, windows, doors and vents. Never fit it directly above a sink, hob, or anywhere it will get steam or grease.

COA-03Gather your toolsYou'll need a cordless drill with wood and masonry bits, a pozidriv screwdriver, a cable and pipe detector, a spirit level, a pencil, a dust sheet, and a vacuum for the mess afterwards.

1 each · Cable, pipe and stud detector

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1 each · Cordless drill and bit set

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1 each · Dust sheet

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COA-04Check your stopcock and think about safety before you drillCables and pipes can run almost anywhere in a wall, not just in the usual safe zones around sockets and switches. Run your detector over the spot first, but still treat the wall as live until you've had a good look — detectors miss deep cables, cables in metal conduit, anything behind metal-backed plasterboard, and they get confused by metal stud frames. Before you drill, find your stopcock (usually under the kitchen sink) and check it turns freely, so you can shut the water off fast if you hit a pipe. If you have any doubt at all, isolate the circuit at the consumer unit before drilling, and turn off the water at the stopcock before cutting into anything that could carry water.

COA-05Work out what's behind the wallSolid brick or block feels solid all the way when you tap it, with no bounce, and a pilot hole brings out brick dust. Dot-and-dab sounds hollow in most places but denser every 300-450mm where the adhesive dabs sit behind the plasterboard. A stud wall sounds hollow between the uprights — track along it with your detector or a magnet to find a vertical line of screw heads, then drill a small pilot hole: timber dust means a timber stud, fine grey metal swarf means a metal stud (common in flats and anything built or refitted since the 1990s).

COA-06Mark and drill the fixing holes for your wall typeSolid brick or block: drill a 6mm hole 40mm deep, fit a plastic wall plug, and drive in a 4x40mm screw. Dot-and-dab: drill through the plasterboard and the adhesive dab into the masonry behind, about 50-60mm deep, then plug and screw at that depth for a fixing that reaches solid wall rather than just board; if you land in the hollow void between dabs, use a plasterboard fixing rated for at least 1kg, which is plenty for an alarm this light. Timber stud: find the centre of the stud and drive a 4x30mm wood screw straight in, no plug needed. Metal stud: never use a plain wall plug or an ordinary wood screw here — it strips straight through 0.5mm steel — use a self-drilling metal-stud fixing driven into the steel track, or a small toggle-type hollow-wall anchor if you land on the board between studs. Lath and plaster: find a lath behind the old lime plaster and drive a short 3x25mm screw in gently by hand to avoid cracking the plaster, or use a light-duty plasterboard-style fixing if you can't locate solid lath. A CO alarm weighs under 500g, so none of this needs a heavy toggle or a timber noggin — those are only for things that pull away from the wall, like a TV arm or grab rail.

4 pack · Plastic wall plugs (assorted)

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4 pack · Wood screws 4x30mm

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2 pack · Metal-stud self-drilling fixings

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2 pack · Hollow-wall toggle fixings (light duty)

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COA-07Fit the bracket, mount the alarm and test itScrew the mounting bracket to the wall, then twist or clip the alarm onto it following the manufacturer's fitting method. Press the test button and check the alarm sounds and the indicator light behaves as the manual describes. Give it a wipe and make sure it's sitting level.

COA-08Tidy up and set a reminderVacuum any dust from drilling and fold away your dust sheet. Note the fitting date and the alarm's replacement date (most sealed alarms last 7-10 years) somewhere you'll actually see it, such as a phone reminder. Put spare fixings and tools away.

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