AB-07Fix quadrant beading along the wall stringOn a closed-string staircase, the expansion gap against the wall needs a strip of quadrant or scotia beading. Work out what's behind the wall first: run a stud detector along it, look for a line of screw heads showing spacing, and listen for a hollow sound between solid points, which suggests plasterboard on studs; an even solid sound all along suggests masonry. Drill a small pilot hole in an unobtrusive spot to check - timber dust means a timber stud, fine grey swarf means a metal stud, and brick dust means solid masonry or dot-and-dab. Fix into solid brick or block with a wall plug and screw in a 6mm hole 35mm deep. Fix into a timber stud with a wood screw driven into the centre of the stud. Fix into a metal stud with a self-drilling metal-stud fixing or a small hollow-wall toggle rated for the beading's weight - never a plain wall plug or an ordinary wood screw, as these strip straight through 0.5mm steel. For dot-and-dab plasterboard, use a fixing long enough to pass through the board and adhesive dabs into the masonry behind, typically 60-75mm. For lath and plaster, locate a solid timber batten or joist behind the laths and fix into that, since the laths alone will crack and won't hold a screw. Before you drill anywhere, remember cables and pipes can run almost anywhere in a wall - a detector can miss cables that are deep, in metal conduit, or behind metal-backed plasterboard. Find your stopcock (usually under the kitchen sink) and check it turns before you start, and isolate the relevant circuit at the consumer unit if you're at all unsure.