How thick is the plaster on my wall, and why does it matter?

Plaster is not one material applied once. A backing coat builds out the wall, straightens it and takes up the difference between a rough background and a flat surface; a thin finish coat on top gives the smooth face you paint. Knowing how deep that stack goes on your wall is what tells you how long a plug has to be, how deep a patch has to be built up, and whether the crack you are looking at is in the surface or in the wall.

Step by step

1. Take the faceplate off a socket or switch, with the circuit isolated, and look at the cut edge.The cutout gives you a clean cross-section of the whole build-up for free. You can see the finish coat, the backing coat and the material behind it in one look.
2. Alternatively, measure at an existing hole.An old wall-light position, a removed shelf bracket, a loft hatch edge or a hole left by a radiator bracket. Push a nail in and mark where the surface is.
3. Identify the layers rather than the total.A pale, dense, very smooth layer 2 to 3mm thick on the face is the finish coat. A coarser, greyer, sandier layer under it is the backing. A paper-faced board under that is plasterboard. Coarse grey material with hair and no board is old lime plaster.
4. Note the total depth and where the sound material starts.This is the number that matters for fixings. A plug has to sit entirely in solid material; if 15mm of it is in plaster it is essentially unsupported, which is why long plugs exist and why short ones pull out of thickly plastered walls.
5. Check it in more than one place.Plaster thickness is not constant. It is thickest where the wall was out of true, which is often near corners, at the top of a wall and around openings. A patch that matches in the middle of a wall can be 5mm out at the corner.
6. Use the figure when you plan a repair.A hole deeper than about 5mm is not a filling job in one pass — it needs building up in coats or filling with a backing material first, or it will sink and crack as it dries.

Tips

• Skim on plasterboard is thin enough that a heavy sanding can go through it to the paper. If your ceiling shows brown fluffy patches where you have sanded, that is exactly what has happened.
• If your drill reaches 25mm before touching anything hard on a masonry wall, budget for longer fixings throughout the room rather than discovering it one bracket at a time.
• Where new plaster meets old at a patch, the step is almost always a thickness difference rather than a filling failure, and it is solved by feathering the repair much further out than feels necessary.

Common mistakes

• Buying 30mm plugs for a wall carrying 20mm of plaster — The plug expands mostly in the plaster, which crumbles, and the fixing works loose within weeks. Depth of embedment in the masonry itself is what carries the load.
• Filling a deep patch in one go — Thick filler and thick plaster both shrink and crack as they dry. Depth is built in coats, letting each firm up first.

Questions people ask