Replace a hard-wired smoke alarm like-for-like

Swap a hardwired smoke alarm safely, working out your ceiling type before fixing the new base and reconnecting the wiring.

SA-01Buy the right replacement alarmTake the make and model number off the old alarm, or unclip it and check the base underneath. Buy the same brand and interconnect type where you can — mixing brands on an interlinked system often stops them talking to each other. Lay a dust sheet under the work area before you start.

1 each · Replacement hardwired smoke alarm (matching type/brand)

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1 each · Dust sheet

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SA-02Turn off the power and know your stopcockSwitch off the circuit feeding the alarm at the consumer unit — it's usually on the lighting circuit — and prove it's dead with your voltage tester before touching any wires. Even though this job doesn't involve plumbing, find your stopcock (usually under the kitchen sink) and check it turns, in case you ever hit a pipe while drilling into a ceiling. If in doubt at any point, leave the circuit isolated at the consumer unit until you're finished.

SA-03Work out what's above the ceilingRun a cable and pipe detector across the ceiling before you drill anything — but still expect a cable to be there and prove otherwise, since detectors miss cables that are deep, in conduit, or behind metal-backed board. To find out what's holding the plasterboard up, tap along the ceiling for a dull sound over a joist versus a hollow sound between them, or drill a small pilot hole: timber dust means a timber joist, fine grey swarf means a metal furring channel. Metal ceiling frames are common in flats and anywhere refitted since the 1990s.

1 each · Cable and pipe detector

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SA-04Remove the old alarm and label the wiringTwist the alarm head anticlockwise to unclip it from the base. Take a photo of the connector block before you touch it, noting which coloured core goes into which terminal — usually permanent live, neutral, earth, and an interconnect core. Loosen the connector block screws and free the old alarm head completely.

SA-05Check whether the base plate needs replacingIf the existing base is sound, clean, and matches the new alarm's fitting, you can reuse it and skip straight to wiring in. Replace the base if it's cracked, yellowed, or a different fitting to your new alarm. If replacing it, unscrew the old base and set it aside.

SA-06Fix a new base into a timber joist or metal ceiling frameFor a timber joist, drive a wood screw into the centre of the joist through the base's fixing hole — never at the edge, where timber can split. For a metal furring channel, use a self-drilling metal-stud screw made for thin steel; a plain wood screw or a plastic wall plug will strip straight through and won't hold. Check the screw grips firmly before letting go of the base.

2 each · Wood screws 4mm x 30mm

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2 each · Self-drilling metal-stud screws

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SA-07Fix a new base into plasterboard, a solid ceiling, or lath and plasterIf there's no joist under the fixing hole, use a spring or hollow-wall toggle fixing rated for a light load — a smoke alarm base is light, so a small plasterboard toggle is enough. For a solid concrete or masonry ceiling, drill with a 6mm masonry bit to a depth of 35mm, fit a wall plug, and screw in. For lath and plaster, don't rely on the laths themselves — find the joist behind them with your detector and screw into that, since old lime plaster and thin laths crumble under a screw. Dot-and-dab doesn't apply here; it's a wall system, not a ceiling one.

2 each · Plasterboard toggle fixings

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2 each · Wall plugs (masonry)

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2 each · Screws 4mm x 40mm

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SA-08Wire in the new base and reconnect the circuitMatch each core into the connector block exactly as it was on the old alarm, using your photo as a reference. Make sure every terminal is tight and no bare copper is showing outside the block. Like-for-like replacement on the existing cable and base isn't usually notifiable work, but if you change any of the wiring or add a new circuit, that's different.

RULES

1 each · Connector block (choc block)

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KNOW THE RULES

Part P of the Building Regulations covers electrical work in dwellings in England. A like-for-like alarm swap on its existing cable and base is not usually notifiable, but altering the wiring or adding a circuit is — check current Part P rules or use a qualified electrician if unsure.

SA-09Fit the alarm head and restore powerClip the new alarm head onto the base and twist clockwise until it locks into place. Go back to the consumer unit and switch the circuit back on. If the alarm has a standby battery, fit it now.

1 each · Backup battery (if required by alarm)

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SA-10Test the alarm and check any interlinked unitsPress the test button on the new alarm and check the LED lights and it sounds correctly. If it's interlinked with other alarms in the house, confirm they all sound together — if not, check the interconnect wiring on the new base against your photo. Wait a minute for the standby indicator to settle before you leave it.

SA-11Tidy up and dispose of the old alarmRemove and separate any battery from the old alarm before disposal. Take the old alarm to a WEEE recycling point rather than the general bin — most councils and larger DIY stores take them. Fold away the dust sheet and put your tools away.

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