BS-04Work out what's behind the tileTap the wall lightly across your marks. A dull, hollow sound with a slight give usually means dot-and-dab plasterboard over masonry. A regular hollow note every 400mm to 600mm, with screw heads showing under a detector, means a timber or metal stud wall. Run a magnet over the marks - it grips at regular intervals over metal studs but not timber - and if still unsure, drill a 3mm pilot hole: pale dust means timber, grey swarf means steel, and solid resistance all the way through means masonry.