PH-03Work out what's above the plasterboardUse your detector or a magnet to trace the line of screw or nail heads across the ceiling — these mark where the board is fixed to a joist or metal channel, usually every 400mm or 600mm. Tap along the ceiling: a solid, dull sound means you're over a fixing point; a hollow, drum-like sound means you're between them. Drill a small 2mm test hole at your marked point — pale timber dust means a wood joist, fine grey swarf means a metal channel, and no dust with the board flexing means you're between fixings. If the ceiling is old, uneven, and made of narrow wooden strips under the plaster, you likely have lath and plaster, so find the joist behind it rather than relying on the lath alone.