Fit a carbon monoxide alarm to a lath and plaster wall

Fit a battery carbon monoxide alarm to a lath and plaster wall, with safe drilling and the right fixing for your actual wall type.

CO-01Choose the right spot for the alarmFit the alarm 1-3m from the boiler, gas fire or wood burner it's protecting, at head height on the wall (150-300mm below the ceiling) or on the ceiling itself. Don't fit it right above a door, in a corner, or behind curtains - these are dead air pockets where CO can sit undetected. Check the box for the exact placement your model needs, as this varies slightly between makes.

RULES

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

Buy:ScrewfixB&QToolstationAmazon

KNOW THE RULES

If you're a landlord in England, the Smoke and Carbon Monoxide Alarm (England) Regulations 2022 require a CO alarm in any room with a fixed combustion appliance (not gas cookers). Rules differ in Scotland and Wales - check local requirements if you're letting the property.

CO-02Gather your toolsYou'll need a cable, pipe and stud detector, a drill with a small wood bit and a masonry bit, a bradawl, a pencil, a spirit level and a dust sheet. Lay the dust sheet under your work area before you start - lath and plaster sheds a surprising amount of dust and debris when drilled.

1 each · Cable, pipe and stud detector

Buy:ScrewfixB&QToolstationAmazon

1 each · Bradawl

Buy:ScrewfixB&QToolstationAmazon

1 each · Dust sheet

Buy:ScrewfixB&QToolstationAmazon

CO-03Find your stopcock and consumer unit before you drillCables and pipes can run almost anywhere in a wall, not just in the 'safe zones' - a detector helps but can miss cables that are deep, in metal conduit, or behind old metal lath. Before drilling, 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 about what's behind the wall, isolate the relevant circuit at the consumer unit first.

CO-04Confirm your wall typeTap the wall - lath and plaster sounds like a soft drum over solid timber strips, and the wall is usually only 20-30mm thick, common in houses built before the 1930s. If you're not sure, drill a small 2mm test hole where it'll be hidden by the alarm base: fine white plaster dust with tiny wood splinters means lath and plaster; solid brick dust means masonry; a hollow sound followed by hard resistance a bit further in means dot-and-dab plasterboard on masonry.

CO-05Drill and fix into lath and plasterAim your fixing at a lath (the horizontal timber strips behind the plaster), not the gaps between them, or the plaster key will crack. Drill a 2mm pilot hole and check for timber dust; if you hit only plaster dust, shift 15-20mm sideways and try again. Once you're into a lath, use a short self-tapping screw or a fine wall plug rated for lightweight fixings - lath and plaster is brittle, so drill slowly and don't hammer-drill.

1 each · Fine wood drill bit, 2mm

Buy:ScrewfixB&QToolstationAmazon

2 each · Self-tapping screws, 25mm

Buy:ScrewfixB&QToolstationAmazon

CO-06If your wall turns out to be a different typeSolid brick or block: drill with a 6mm masonry bit to 35-40mm depth, fit a wall plug, then screw in. Plasterboard on timber studs: find the stud with your detector or by tracking a line of screw heads, drill a 2mm pilot hole (timber dust confirms it), and use a 35mm wood screw straight into the stud centre. Plasterboard on metal studs: a magnet or detector picks up the steel track; a small pilot hole brings out fine metal swarf instead of dust - never use a plain plug or wood screw here, as it strips straight through 0.5mm steel, so use a self-drilling metal-stud fixing or a toggle-type hollow-wall anchor instead. Dot-and-dab plasterboard: a lightweight CO alarm is usually fine on a plasterboard toggle fixing between the dabs, but if you want it fixed through to the masonry behind, you'll need a fixing long enough to cross the 10-40mm air gap.

1 pack · Wall plugs, 6mm (pack of 10)

Buy:ScrewfixB&QToolstationAmazon

1 pack · Wood screws, 35mm (pack of 10)

Buy:ScrewfixB&QToolstationAmazon

1 pack · Self-drilling metal-stud fixings (pack of 4)

Buy:ScrewfixB&QToolstationAmazon

1 pack · Plasterboard toggle fixings (pack of 4)

Buy:ScrewfixB&QToolstationAmazon

CO-07Mount the bracket and click on the alarmHold the alarm's mounting bracket level against the wall, mark the fixing points with a pencil, and screw the bracket on through the holes you've already drilled and fitted. Twist or click the alarm body onto the bracket according to the manufacturer's instructions - most models lock on with a quarter-turn.

CO-08Test the alarmPress and hold the test button until you hear the alarm sound - most units give a short beep pattern to confirm the sensor and battery are both working. Don't use lighters, matches or any other combustion source to test it; the test button checks the full circuit safely.

CO-09Tidy up and log the fitting dateVacuum up plaster or brick dust from the dust sheet and wipe the wall below the alarm. Write the fitting date on the alarm body or a nearby label if there's space, and note the replacement date - most sealed 10-year alarms need replacing as a whole unit, not just a battery change.

Some links may earn us a small commission at no extra cost to you.

Save this as a project

Back to all guides