Is my wall dot-and-dab dry lining or a stud wall?

Dot and dab is plasterboard stuck to masonry on blobs of gypsum-based adhesive rather than fixed to a timber frame. It became the standard way to line a masonry wall in Britain from the 1980s onwards because it is far quicker than wet plastering and it gives a flat surface on a wall that is not flat. The result feels exactly like a stud wall under your knuckles, and almost every cavity fixing sold for stud walls fails in it.

What you'll need

• A thin probe — a long bradawl, a piece of stiff wire or a straightened coat hanger
• A 4mm drill bit (optional)
• A magnet (optional)

Step by step

1. Tap slowly along the wall at waist height and mark every firm spot.Use a pencil. You are not listening for hollow versus solid so much as for the pattern of the firm places, because the pattern is the answer.
2. Read the pattern.Vertical firm lines at consistent centres, running floor to ceiling, are studs. Firm areas that are patchy, roughly in rows and columns but not evenly spaced, and that break up rather than run continuously, are adhesive dabs.
3. Check the perimeter.Dry lining almost always has a continuous ribbon of adhesive around the edges of each board and around openings, so the wall goes firm in a band next to every door frame, window reveal, ceiling line and corner. A stud wall is firm at the corners and at the head and sole plates, but the band is narrower and dead straight.
4. Drill a 4mm hole somewhere hidden and push a probe in.Behind a radiator, low down, or where a picture will hang. If the probe stops 20 to 40mm behind the back face of the board, it is dry lining on masonry. If it swings freely in a deep void, it is a stud bay.
5. Look at the dust from that hole.Board dust then a sudden change to red brick grit or grey block dust confirms dry lining. Board dust then nothing but air confirms a partition.
6. Try a magnet along the firm lines.A magnet that grabs strongly along a whole vertical line is a metal stud. A magnet that finds small individual points in a line is finding screw heads in a timber stud. A magnet that finds nothing while the wall is still firm suggests adhesive dabs.
7. Act on the answer when you fix into it.In a stud wall, aim for the timber and treat board fixings as a fallback for light loads. In dry lining, the load wants to reach the masonry, which means a plug and screw long enough to pass through the board and the dab void and still bed properly in brick or block.

Tips

• The void behind dry lining is not empty everywhere. Sockets and switches sit in dry-lining boxes clipped to the board, and cables run in that gap, so the same detection discipline applies as anywhere else.
• If you find dry lining and the room feels cold, the board may be insulated backed rather than plain — a thicker build-up with a foam or wool layer bonded to the back, which changes both the fixing length and how much of the void is actually void.
• Dabbed board sounds different from stud board when you tap it: slightly higher and shorter, because the air gap behind it is small. Once you have heard both in the same house it is easy to tell.

Common mistakes

• Using spring toggles or gravity toggles in dry lining — The wings need clear space behind the board to swing open. With masonry a couple of centimetres away, they jam half-open and hold almost nothing, and you cannot get them back out.
• Assuming the void is the same depth everywhere — Dabs are applied to a wall that is out of true, so the gap varies across the room. A plug length that works in one corner can leave the screw sitting in fresh air in another.
• Reading a dry-lined wall as a stud wall because a detector reported studs — Detectors respond to the adhesive dabs, to service runs and to the masonry itself. On dry lining they report a confusing mess, which is itself a clue.

Questions people ask