WM-04Work out exactly what's behind the plasterLath and plaster is thin timber laths nailed across timber studs or joists, spaced roughly 400-600mm apart, covered in 15-20mm of lime plaster. A pilot hole through true lath and plaster feels soft and crumbly at first, then hits solid timber. If the wall feels flat and even with a dimpled screw pattern instead, you likely have plasterboard on timber studs, not lath and plaster — a magnet won't stick, and a pilot hole brings out fine timber dust. Plasterboard on metal studs has the same screw-line pattern but a magnet barely grips, and a pilot hole brings out fine grey metal swarf instead of dust — you will not find this often behind original lath and plaster, but check if the wall has clearly been re-boarded. Dot-and-dab feels hollow with sudden solid patches roughly every 300mm; solid brick or block feels solid everywhere and a pilot hole brings out brick-red dust.