SA-04Drill and fit the right fixing for your ceilingCables can run almost anywhere above a ceiling, including up to light fittings, so always check with a detector first and still treat the area as live until proven otherwise — detectors miss deep cables, cables in metal conduit, and anything behind metal-backed plasterboard. Find your stopcock, usually under the kitchen sink, and check it turns before you start; if there's any doubt at all, isolate the relevant circuit at the consumer unit before drilling. For plasterboard on timber joists, drill into the centre of the joist and use a 32mm wood screw. For plasterboard on metal furring channels or resilient bars, use self-drilling metal-stud fixings — never a wood screw or plastic plug, as they strip straight through 0.5mm steel. If you're fixing into plasterboard between joists rather than into a joist itself, use a light-duty plasterboard toggle rated for the alarm's weight — smoke alarms are light, so a basic toggle is enough. For a solid concrete or masonry soffit, drill a 6mm hole to 35mm depth, insert a wall plug, and use a 30mm screw. For lath and plaster, drill carefully through the plaster and into a lath or the joist behind it — plaster alone will crumble and won't hold a fixing.