New plaster carries a lot of water. As that water leaves, the plaster shrinks very slightly and the background behind it moves too, especially where a new extension or a loft conversion is still settling on fresh foundations or new joists. Knowing which of those two things you are looking at saves an argument and a lot of unnecessary work.
The cracks that are normal
Fine map cracking across a skimmed wall, a straight line following the joint between two plasterboards, or a crack in the corner where a ceiling meets a wall are all drying and movement of the board itself. They are usually under half a millimetre wide, they stop growing once the building has been through a heating season, and they are filled with a flexible caulk or a fine filler before the second coat of paint.
Cracks at the junction between different materials are also expected. Where new plasterboard meets old lime plaster, or where a stud wall meets masonry, the two sides move at different rates. A scrim tape joint reduces it but never removes it entirely.
- Hairline cracks along plasterboard joints
- Fine corner cracks between wall and ceiling
- Map or crazing patterns on a skim coat that dried too fast
- Cracks at the meeting of new plaster and older lime work
The cracks that need looking at
A diagonal crack running from the corner of a door or window opening, wider at one end than the other, is a movement crack. So is a horizontal crack that follows a line of blockwork, or any crack you can push a coin into. Cracks that reappear in the same place within weeks of being filled are telling you the background is still moving.
In a new house extension, a crack where the new work meets the original wall is common and usually cosmetic, because the two structures have different foundations and settle at different rates. That is different from a crack in the middle of a new wall, which should not happen and points to the blockwork, the lintel bearing or the foundation rather than the plaster.
Why plaster cracks in the first place
Drying too fast is the most common cause of crazing. Plaster that is heated hard, or worked in a room with the windows open and a draught through it, loses water from the surface before the body of the coat has set. Overworking the trowel and adding water at the polishing stage does the same thing.
Thickness matters as well. A skim coat put on too thick, or a bonding coat that has not been allowed to firm up, will shrink more than the specification allows. On a background that was not primed or dampened correctly, suction pulls water out unevenly and you get cracking in patches rather than across the whole wall.
Putting it right properly
Hairline shrinkage is raked out lightly, filled with a fine surface filler or a flexible decorator's caulk in the angles, sanded and painted. Do not try to fill a hairline crack with the same plaster, because it will not bond in a narrow gap and will fall out.
Wider or repeating cracks are cut out to a V, taped with a scrim or a fibre tape and refilled in two passes. Where the crack is structural, the fix is at the structure, not the finish. That might mean a new lintel bearing, a stitched repair across blockwork, or simply waiting a full heating cycle before making good so it does not have to be done twice.
With over forty years on the tools between us, we would rather tell you a crack is normal drying and to leave it alone than sell you a repair that does nothing.
Common questions
How long after plastering do cracks usually appear?
Most shrinkage cracks show up between two weeks and three months, and often become visible only once the first coat of paint goes on. Cracks that appear for the first time a year later are more likely to be movement than drying.
Should I fill cracks before or after the first coat of paint?
Mist coat first, then fill. The mist coat makes fine cracks visible and stops filler soaking into raw plaster and drying out before it cures.
Is a crack where my new extension meets the house a defect?
Usually not. Two structures on different foundations move independently and that joint is the first place it shows. It is normally finished with a flexible bead or caulked joint rather than rigid filler.
How wide does a crack need to be before I get advice?
As a rule of thumb, anything you can fit a two pence coin into, anything diagonal from an opening, or anything that is measurably wider than it was a month ago is worth having looked at.
Can you tell what caused a crack from a photograph?
We can often narrow it down from a clear photo with something for scale, but the useful information is behind the plaster, so a short site visit is what settles it. Those visits are free.