How do I fix into dot and dab plasterboard?
Why plasterboard fixings and toggles both fail on a dabbed wall, and the long-plug method that reaches the blockwork behind.
- Difficulty
- intermediate
- Time
- 45 min
- Read
- 4 min
Short answer
There is a 10–40mm void behind the board with masonry behind that, so toggles cannot open and plasterboard plugs hold on a sheet with nothing behind it. Use a long screw and plug that passes through the board and the void and anchors at least 50mm into the block.
Dot and dab looks identical to a stud wall from the outside and behaves completely differently. Most fixings that pull out of a modern wall were fitted on the assumption of a stud partition.
What you'll need
- A masonry drill bit long enough for board, void and block
- Long frame fixings or screws with nylon plugs
- A detector
- Plastic packers or a spacer sleeve
- A vacuum
- Masking tape
Step by step
- Identify the wall type.Tap it. A dabbed wall sounds hollow in places and solid over the dabs, in a patchy pattern. A stud wall sounds uniformly hollow between regularly spaced solid lines.
- Confirm with a small drill.A 3mm bit. Board, then air, then masonry within about 50mm total is dot and dab. Board then air then more air is a stud partition.
- Scan for cables and pipes.Services in a dabbed wall are usually chased into the block behind or run in the void, so a detector matters as much here as anywhere.
- Choose a fixing that reaches the block.The whole principle is that the load goes into the masonry, not the board. Add the board thickness, the void depth and 50mm of anchorage to get the length you need.
- Measure the void.Drill a small hole, push a bent wire in and mark it. Voids of 10–40mm are typical and it varies across one wall.
- Drill through with a masonry bit.Tape over the surface to stop the board face tearing. Rotary through the board, then hammer for the block.
- Use a plug long enough to sit entirely in the masonry.A plug that starts in the void and only partly enters the block expands into fresh air. It has to be fully in the masonry to do anything.
- Do not overtighten.This is the key failure mode. Tightening a screw pulls the fitting hard against the board, and the board crushes into the void, so the fixing goes slack and the plaster cracks.
- Use a spacer or frame fixing.A plastic spacer sleeve, a frame fixing designed for standoff, or a nut and washer arrangement transfers the clamping load into the masonry rather than crushing the board.
- Consider a proprietary dot and dab anchor.Several manufacturers make fixings specifically for this situation, which brace against both the board and the block. They are worth using for anything heavy.
- Spread the load for heavy items.A timber batten fixed at multiple points into the block, then the item fixed to the batten. It is the standard approach for TVs, boilers and wall cabinets.
- Vacuum as you drill.Block dust falls into the void and comes back out of the hole for weeks otherwise.
Tips
- The tell is that the fixing goes tight then suddenly slackens as you keep turning. That is the board crushing, and it means the load path is wrong.
- For a wall-mounted TV on dot and dab, a batten across several fixings into the block is the professional answer and takes twenty minutes.
- Cavity closers and insulation can sit behind the board near openings, so a fixing near a window reveal may not find block where you expect it.
Common mistakes
- Using a plasterboard fixing on a dabbed wall — Plasterboard anchors are designed to spread load into a board that is supported by a frame. A dabbed board is glued to blockwork at intervals with nothing in between, so the anchor is holding a sheet of gypsum over a void with no structure at all behind it.
- Overtightening a long fixing — The screw pulls the fitting towards the block and the board is the only thing in between. It crushes, the fixing goes loose without warning, and the plaster face cracks in a ring around the fixing.
If it doesn't work
Fixing tightens then goes slack
Cause: Board crushing into the void — Fix: Back off, fit a spacer or frame fixing so the load bears on the masonry.
Plug spins in the hole
Cause: Plug partly in the void rather than fully in the block — Fix: Use a longer plug that sits entirely within the masonry.
Ring of cracked plaster around the fixing
Cause: Overtightened — Fix: Remove, fill, and refix with a spacer and less torque.
Drill finds no masonry at all
Cause: It is a stud partition, or a cavity closer at a reveal — Fix: Locate studs and fix into them instead.
Heavy item still moves slightly
Cause: Single fixing on a compressible build-up — Fix: Fit a batten with multiple fixings into the block.
Questions people ask
How do I tell dot and dab from a stud wall?
Drill a 3mm test hole. Board, then a small void, then something hard within 50mm is dot and dab. Board then a deep void with occasional timber is a stud partition. Tapping is a good first indication but the test hole is definitive.
Can I use plasterboard plugs at all?
For very light items — a small picture, a hook — they are acceptable. For anything with real weight or leverage, the fixing has to reach the blockwork.