Blockwork walls explained

Learn to recognise a blockwork wall by sound and feel, and drill into one safely.

BW-01Know what blockwork actually isBlockwork walls are built from concrete blocks (bigger than bricks, usually grey) stacked in rows with mortar between them, then finished with a thin skim of plaster. You'll find blockwork in most modern internal walls and in cavity external walls. It's different from a stud wall, which is a timber or metal frame with plasterboard screwed to it, and different from solid brick, which uses smaller, redder units.

BW-02Tap the wall to test itMake a fist and knock firmly on the wall every 300mm as you move along it. Blockwork gives a solid, dull thud with almost no change as you move across it. A stud wall sounds hollow between the studs, then suddenly solid where you hit a timber or metal upright. The mistake beginners make is tapping once, hitting a stud, and wrongly deciding the whole wall is solid blockwork.

BW-03Check a door or window revealLook at the exposed edge where a door or window frame meets the wall. Blockwork reveals usually look thick, grey and slightly rough under the plaster skim, often 100mm or more deep. A stud wall reveal is much thinner and you may see the edge of the plasterboard sheet. This gives you a second check to back up the tap test.

BW-04Assume cables and pipes are hidden insideElectricians and plumbers often cut narrow grooves into blockwork, called chases, to bury cables and pipes before plastering over them. There are conventional 'safe zones' near sockets, switches and corners where cables are supposed to run, but blockwork walls, especially in older or amateur-wired houses, don't always follow them. Never assume a wall is empty just because it's blockwork rather than plasterboard.

BW-05Use a detector before drilling, then stay cautious anywayBefore you drill or chase into a blockwork wall, run a cable and pipe detector over the area in both directions. Remember a detector reduces risk but does not remove it: it can miss cables that are deep, run in metal conduit, sit behind metal-backed plasterboard, or lie outside the usual safe zones. Treat every wall as if something is there until you've proven otherwise. If there's any doubt at all, isolate the circuit at the consumer unit before drilling, and turn the water off at the stopcock before cutting into anything that could carry water.

BW-06Confirm you've drilled into real blockworkOnce you start drilling, blockwork produces fine grey dust and steady, even resistance all the way through. Plasterboard gives soft white dust and almost no resistance, and solid brick gives a reddish or orange dust. If the drill suddenly feels much easier partway through, or the dust changes colour, stop and work out what you've actually hit before carrying on.

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