AB-02Work out what your ceiling is actually made ofFlats with a concrete floor above, and many purpose-built blocks, have solid concrete ceilings that sound dense and give the same dull thud wherever you tap them. In older houses, and under most upper rooms, the ceiling is plasterboard or lath and plaster fixed to timber joists, so tapping sounds hollow between joists and firmer over each one, roughly every 400mm or 600mm. Run your joist detector along the ceiling to find joist centres, or — after doing the safety checks in the next step — drill one small test hole: solid grey dust and hard, even resistance means concrete, pale wood dust means a timber joist, and fine grey metal swarf with a sudden stop means a metal furring channel. Lath and plaster ceilings, common in homes built before about 1930, feel slightly springy when tapped and the plaster can crack if you drill without finding the lath or joist behind it.