RB-03Work out what's above you before you drillRun a stud detector or magnet along each marked line to find the joists or metal channels above the plaster or boards. A hollow sound between hits usually means you're between joists; a dull, solid sound means old lath and plaster, which is much thinner and softer than modern plasterboard. Drill a small pilot hole at one mark and check what comes out: pale timber dust means a joist, fine grey powder means old lime plaster with no joist behind it, and bright metal swarf means a metal furring channel. Dot-and-dab is a wall-only technique, so you won't find it on a ceiling — skip that check here. Before drilling anywhere, remember cables can run across a ceiling void almost anywhere, especially near light fittings, and bathroom pipework can run above a ground-floor ceiling too. A detector can miss cables in metal conduit or behind metal-backed board, so find your stopcock (usually under the kitchen sink) and check it turns before you start, and if you have any doubt at all, isolate the relevant circuit at the consumer unit before drilling.