How to Tell If a Wall Is Hollow Before Drilling

One question I'm asked a lot before any drilling: "is this wall hollow or solid?" Getting that answer wrong means either the screw spins forever in plasterboard with no anchor, or you destroy a masonry bit on a brick wall you didn't know was there. Here's how to tell, every time.

The simplest test: the knuckle tap

Lightly tap the wall with your knuckle. Move along, tapping every 10cm.

• Hollow drum-like sound — stud wall (plasterboard on a timber or metal frame with air behind).
• Dull thud, no resonance — solid wall (brick, block or concrete behind the plaster).
• Slightly less hollow than the rest — you've found a stud behind plasterboard.

What's typical in London homes

• Victorian and Edwardian: external walls are solid brick (single or double skin). Internal walls between rooms are usually stud (lath-and-plaster originally, often re-boarded). Chimney breasts are solid brick.
• 1930s-1960s: external walls are usually cavity (two skins of brick or block with a gap). Internal partitions are often stud.
• New-build (post-2000): internal walls almost always plasterboard on metal or timber studs. External walls block or brick.
• Conversions and refurbs: can be a complete mix — even within a single wall.

The stud detector test

A multi-detector picks up:

• Studs — either the wood itself (with a density detector) or the drywall screws on the side facing you (with a magnetic detector). Magnetic ones are cheap and reliable.
• Live cables — an essential check before any drilling.
• Metal pipes — both copper and steel.

Calibrate first: hold the detector against the wall and press the calibrate button. Then sweep horizontally. Studs typically appear at 400mm or 600mm centres.

The visual check: skirting and ceiling

Look at the skirting board where it meets the wall, and the ceiling line:

• If there's a thin gap between skirting and wall (3-5mm), it's often a stud wall — plasterboard sits slightly off the floor.
• If there's a tiny rib or transition near the ceiling, the wall may stop short of the joist — sometimes seen in modern conversions.
• Sockets and switches: open the faceplate (with the power off!) and look at the back box. A metal box screwed into brick = solid wall. A plastic dryliner box = stud wall.

The pilot hole test (last resort)

If you've checked everything and still aren't sure, drill a small pilot hole (1.5-2mm) at the planned fixing point.

• No resistance, just powder — pure plasterboard with cavity behind (stud wall).
• Resistance, red powder — brick.
• Resistance, grey powder — concrete or breeze block.
• Resistance, wood shavings — you hit a stud. Bonus — safest fixing.
• Resistance, then sudden give-through — you've gone through plasterboard into a cavity behind. Could be a stud wall with a stud just past, or dot-and-dab plasterboard with brick behind. Stop and check carefully before going further.

Dot-and-dab: the trap most people miss

In many post-1970s homes, plasterboard is "dot-and-dabbed" to brick — glued in spots with a 10-25mm air gap behind. Tapping sounds slightly hollow, but a solid brick wall is right there. Your fixings need to be long enough to reach the brick. A 40mm screw won't get there; a 75mm masonry screw will.

Why this matters for your fixing choice

• True stud wall — use plasterboard anchors (Geefix, toggle, self-drive metal) or screw into a stud.
• Solid brick / block — masonry bit, brown plug + screw, or masonry screw.
• Concrete — SDS hammer drill, concrete screws (e.g. Tapcon) or high-spec plug + screw.
• Dot-and-dab — long fixings that reach the brick, or specialised dot-and-dab anchors.

When to ask a handyman

If you've got a wall you can't figure out, or your house mixes wall types and you don't know which is which — a 20-minute scoping visit (or even a few photos and a description on WhatsApp) usually gets a clear answer. Better than a damaged wall and the wrong fixings.