Learn how to repair cracks in plaster in three easy steps

Putty knife filling cracks in wall
(Image credit: Getty Images)

It's not difficult to repair cracks in plaster, but you really need to make sure that you are doing it the right way. Whether it's a hairline crack or a larger crack, the principle of repair is very much the same. You just need to know the right technique to get a good, long-lasting finish.

You can think about plastering walls, but this is an expensive alternative and if your cracks are only occasional and minimal a good filler applied properly will give you a finish that looks good and be strong enough to cope with nails and screws.

Here we show you how to quickly fix and repair all types of cracks in your walls and ceilings and tell you what filler you need to use. 

What is the best filler to repair cracks in plaster? 

Before you start dealing with any cracks you need to think about what filler you need for the job. Most cracks can be dealt with using patching plaster or a specialist wall/ceiling filler like Toupret interior filler powder which provides a good finish to paint over. 

If you have large cracks you may need to use a cement and sand mix to fill initially, but leave enough room – 4-5mm – to add a layer of filler or plaster when the cement has dried. 

How to repair cracks in plaster 

Here we take a look at the general process of repairing cracks in walls and ceilings around the home. 

1. Prep and score
Before you start, lay down sheets on the floor to catch any loose plaster and have a dustpan and brush and vacuum ready to get rid of the plaster.

Cracks will typically have loose edges, so these need to be removed to provide a secure base for the new plaster. Get a Stanley knife and run along the crack at an angle to remove the loose plaster. 

Repeat until you have widened the gap and you have a secure edge. Now gently brush the crack to remove plaster and finally vacuum to get rid of as much dust as possible.

2. Mix and add filler
Now get your filler and mix as instructed. For slightly deeper cracks – around 10-30mm – first work the mixture into the crack, making sure to secure the edges with the filler. Leave around 5mm to fill.  

3. Add tape
Now get a length of jointing tape, also known as drywall tape (such as the Eurobuild Everbuild tape from Amazon) and cut to cover the crack. You can also use plasterboard mesh tape (such as the 151 DIY plasterboard and drywall tape) to cover the crack. Add a little filler mixture to the crack, apply the tape and smooth out with your putty or filling knife.

Now add another layer of filler to cover the tape and smooth out with your putty knife and leave to dry as instructed. Once dry, sand down the filler until it is flat and blends in with the surrounding surface.  

You can now decorate or paint over the crack if needed.

How to fix hairline cracks 

If you have hairline cracks in your walls you simply need to mix up a suitable filler such as patching plaster or buy a small tub of ready mixed such as Everbuild All Purpose Ready Mixed Filler and apply. 

Get a putty knife and push the filler into the gap along the length of the crack then go back over with the putty knife to smooth it out. If the filler pulls up when you're trying to smooth it out, then add a fine spray of water or dip the putty knife in water.

Leave to dry and get a fine grade sandpaper – 180 is good – and combine with a sanding block. Personally I like to use a cork sanding block for walls. Gently rub down the plaster and blend with the surrounding surface.  

How to fill deep cracks in walls

Deep cracks might be a sign of structural issues that filling will not resolve. If this is the case, you need to get a surveyor in to identify the cause and then resolve the issue, before you start filling in the cracks.  

Once any issues have been resolved and you know you have a solid base to work with, start by filling with a sand and cement mix if putting straight onto brickwork. Use a 1 part cement and 4 parts soft sand mix. Leave around a 5mm gap to add your filler.

However, if you have a large crack in a wall that has been plasterboarded, you might be better off filling the holes in plasterboard by replacing the surrounding plasterboard and replastering.    

Can I just paint over hairline cracks on plaster? 

Yes you can especially if this is the first time they have appeared. However, if the crack is more than the width of a Stanley knife blade then you will be better off following the steps for a normal sized crack.

You can use a specialist flexible paint like Polycell’s Crack-Free Ceilings, which is slightly thicker than a lot of paints. Alternatively, apply an oil-based undercoat along the crack and leave to dry. This will act as a base and should fill the crack. If not, reapply a second coat. Now add a top coat to match the rest of the wall or ceiling to hide the crack.  

Can you caulk cracks in plaster? 

You can caulk over cracks in plaster but it's not the ideal solution if you want a smooth wall or ceiling. Typically the caulk will sink a small amount into a crack leaving an indent in the surface.  

However, if the crack is small enough and in a corner or near a window, door frame or skirting board, caulking is a quick and simple solution. But you won't be able to sand it down to get the perfect finish. 

If you need to paint over the caulk make sure that you know how to use decorators' caulk, sometimes known as painters' caulk, otherwise the paint won’t adhere properly. 

Why do plaster cracks keep coming back? 

There can be a number of reasons for this happening, but typically it's nothing to worry about. Walls and ceilings that have been plasterboarded onto a timber frame, typically a partition wall or stud wall, are subject to movement and cracks often appear where the edges of the plasterboard reside.

The same applies to most new homes, they are subject to what are known as settlement cracks, but once the foundations and structure settle these rarely get any bigger. Cracks can appear in areas where there is a lot of traffic, for example around stairs.

If you have cracks in plaster that are getting bigger, this may be a sign that there are other issues in the home. You can take off the plaster to see what lies behind to help resolve the problem. If there seems to be a bigger problem, you can get a builder to have a look and try and resolve the problem, or a surveyor to investigate and make proposals. 

Steve Jenkins

Steve is Homebuilding & Renovating's DIY content editor, and has been a writer and editor for two decades. He is an avid DIYer with over 20 years of experience in transforming and renovating homes. He specialises in painting and decorating, but has strong all-round building skills, having previously worked in the industry for 10 years.