Repairing cracks in plaster: patch or replaster?

Patch or replaster: how do you choose?

The choice does not depend on how ugly the crack looks, but on two questions. Is the wall still moving? And is the plaster next to the crack still firmly attached? Only when you know both do you know whether a repair will last.

Knock along the crack with your knuckles. If it sounds hollow, the layer has come away from the wall there. Also press gently on the edges: if the plaster gives way or crumbles, patching is pointless.

If it is a new-build home, first check whether these are ordinary shrinkage cracks in a new home. There are also warning signs where you are better off asking a structural engineer or architect to take a look.

Why does a filled-in crack often come back?

This is the biggest misunderstanding about cracks in plaster: that a crack is gone once it is filled. Filler and gypsum are hard. If the wall then moves even a fraction, the repair splits open again in exactly the same place, often within a few months.

Gypsum manufacturer Knauf takes this into account in its own instructions. The technical data sheet for MP 75 gypsum plaster states that where there are cracks in the substrate and at transitions between different substrates, a glass-fibre reinforcement is embedded in the fresh plaster to reduce the risk of new cracks. A thin fill without reinforcement does not do that.

Sealant is not a real solution either for a plastered wall that will be painted. Sealant stays flexible, but has a different surface from plaster. Under raking light, for example from a window, you often see the repair as a streak in the paint.

A crack that keeps moving cannot be solved in plaster. If wallpaper is going on the wall, renovation fleece is a better choice, because it bridges small cracks rather than following them. Not every crack disappears beneath it, so have it checked beforehand which cracks under renovation fleece disappear and which do not.

How is a crack in plaster repaired properly?

A repair that lasts is mostly preparation. The filling itself is the smallest part of the job. Our plasterers work in this order.

1. Open it up. The crack is widened slightly so the new plaster has something to grip and does not merely sit as a thin film on the edge.
2. Remove loose parts. Anything that gives way or sounds hollow comes out until only firm plaster remains.
3. Dust off and pre-treat. An absorbent substrate first gets a primer, so the plaster does not dry out too quickly.
4. Fill, with reinforcement where needed. For a crack along a joint or transition, reinforcement goes into the fresh layer.
5. Level over a wider area. The repair is feathered out wider than the crack itself, so no bump or edge appears.
6. Leave to dry and sand. Only then do the primer and paint follow.

In its technical data sheet, Knauf describes its thin-coat plaster Multi-Finish as suitable, among other things, for filling holes and cracks, and specifies that with several layers the layer beneath must be dry first. That is why we fill a deep crack in stages, not in one thick blob.

Painting also waits for drying. According to the product data sheet for Sigmafix Universal primer, a gypsum-based substrate may contain at most 1% moisture before the primer goes on. So allow for the drying times of plaster and skim coats before the roller comes out.

What do you do about cracked plaster after removing wallpaper?

You often only discover cracks once the old wallpaper is off, especially if you are renovating a 1930s house. They have often been there for years, and the wallpaper was chosen precisely to hide them. What you do then depends on what is going back on the wall.

If wallpaper is going back on, filling the open cracks locally is usually enough. If the wall is going to be painted, every repair shows through the paint. With more than a few cracks, a new layer over the whole wall is then the calmest solution.

Remove all paste residue first before anything is repaired, because plaster adheres poorly to old wallpaper paste. So first take off the wallpaper paste residue, then fill. And cracks are not the only thing that appears once wallpaper has been removed.

What does it cost to have cracks in plaster repaired?

Patching a few cracks is not work priced per m², so it is not a fixed line in the price list. The price is set out in the final quote after the on-site survey. If a wall has to be redone, we use the fixed rates below.

Prices exclude VAT. For a home over 10 years old, 6% VAT usually applies; otherwise 21%. The VAT rate is stated in the quote.

An example: a bedroom wall of 4 by 2.6 metres with a web of fine cracks is 10.4 m². Replastering that wall ready for painting costs 10.4 × €22.99 = €239.10. If there is a loose layer on it, removal is added.

The price list also includes a small-job surcharge of €249.99 and a one-off call-out charge of €99.99. Which items apply to your job is shown in the quote. Not sure whether a loose layer really has to come off?

That depends on when it makes sense to take an old layer off the wall. Our price overview lists all the other fixed rates.

How do we repair cracks in your plaster?

The work is done by one of our 32 in-house plasterers, such as Bram, who has worked with us for 7 years. They are part of a team of 97 tradespeople, together with 40 wallpaper hangers and 25 painters. If the wall is to be painted after the repair, we only schedule the painter once the repair is dry.

Before you apply, check whether the crack has widened in recent weeks, and knock along its edge. Mention that, and whether the wall will afterwards be painted or wallpapered. You will then receive a free quote within 24 hours that matches.

During the on-site survey we knock the wall ourselves, check whether the cracks have stopped moving and decide whether patching is enough.

If a whole wall or room has to be redone, that goes quickly: a plasterer plasters on average 100 m² a day. If we see a crack that points to more than shrinkage, we say so and first recommend a structural engineer or architect. Cracks in a facade fall under our facade work, and exterior rendering is always on request.

If your walls need more than a repair, we will choose together from all the types of wall finish our tradespeople apply. And if you simply want to hire one of our tradespeople for your job, that is possible too.