Why did my plaster crack while it was drying?

Plaster does not dry, it sets — a chemical reaction that needs its water. Anything that takes the water away too early, or asks a coat to be thicker than it should be, produces cracks as it hardens. The pattern of the cracking tells you which mistake was made, and the repair differs completely between them.

What you'll need

• a torch for raking light
• scrim or paper tape (optional)
• filler or fresh finish plaster
• a spray bottle of water (optional)

Step by step

1. Look at the pattern of the cracks in raking light.Fine hairlines in a random web across a whole area, wider cracks forming irregular islands, or one or two long straight cracks. These are three different faults.
2. Fine crazing means it dried too fast.A hot room, a heater aimed at the wall, direct sun, a draught through an open window, or a background with very high suction pulling the water out of the back of the coat while air pulled it out of the front.
3. Map cracking means too thick or too wet.A coat applied beyond the thickness the product allows shrinks more as it sets. Over-watered mix does the same. Both produce wider, deeper cracks in an irregular pattern.
4. Straight cracks mean movement or a joint below.Follow the line. If it runs vertically or horizontally in a dead straight line, there is a board joint, a background change or a structural junction under it that was not taped.
5. Check whether the cracks are through the coat or just the surface.Run a knife point along one. Surface crazing has no depth; a crack that opens is through the coat and needs cutting out and filling rather than skimming over.
6. Repair crazing by filling and sanding, or by re-skimming.Fine crazing under paint will show, so a thin skim coat or a fine surface filler applied over the whole area is the usual answer.
7. Repair map cracking by cutting out and doing it in proper thicknesses.If the coat is too thick, it will keep moving. Take it off and build up in the coats the product specifies — a backing coat and then a thin finish.
8. Repair a straight crack by bridging it.Rake it out, tape across it with scrim or paper tape bedded in filler or jointing compound, and finish over the tape in thin feathered coats. Filling without tape reproduces the crack.
9. Control conditions for the repair.Moderate temperature, no heaters aimed at the wall, no draught, no direct sun. Plaster wants to set slowly and evenly.

Tips

• If a wall has very high suction and you cannot control it, dampening the background down with clean water before plastering slows the loss and is free.
• Do not be tempted to spray water on plaster that has already started to crack — it is a hydration problem in the body of the coat, not on its surface.
• Crazing that appears only near a radiator or a window is a conditions problem, and it tells you exactly where the heat or draught was.
• New plaster in a cold, unheated room is at risk of the opposite problem: it stays wet for a very long time and can be damaged by frost.

Common mistakes

• Putting a heater on the new plaster to speed it up — It dries the surface far faster than the body, so the surface shrinks against a still-wet layer underneath and crazes. It also stops the setting reaction completing properly, which leaves the surface soft and dusty.
• Applying finish plaster too thick to save doing a backing coat — Finish plaster is designed to go on in a couple of millimetres. Beyond that it shrinks unevenly, cracks, and often does not gain strength through its full thickness. Depth is built up with the backing coat, which is formulated for it.
• Filling a straight crack without tape — The crack is reporting movement in a joint below. Filler is rigid and has no ability to bridge a moving joint, so the same crack reappears through the filler, usually within one heating season.

If it doesn't work

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