How to Fill a Wall Crack Properly

A crack can be worrying, especially if they are big. The majority of cracks inside your home are surface cracks and can be repaired fairly easily with a few supplies and a bit of time. This article is intended for guidance only and if you are worried about cracks then get a professional to check this over.

There are many things that can cause a crack to appear. Things like having walls removed, felling or growing trees close to the house, badly fitted windows and doors, living on a hill, age of the plaster (which can pop) and not filling cracks properly can all cause this to happen.

• Decorators Multi-tool
• Dust Brush or Vaccum
• PVA Glue and Old Brush
• Mortar Mix or Sand and Cement
• Disposable Gloves or Filling Knife
• Polyfiller (I use Toupret)
• Continental Filling Blades
• Sandpaper

The first thing you need is some kind of digging tool. I use my decorators multi-tool which is my trusty friend and cost around £6. You need to remove all loose material from in and around the crack and open the crack up in a V shape. Don’t go crazy here, if you are having to use a lot of effort to do this then it is too much, you are aiming to just remove the loose stuff and open it up.

If you are breaking stuff off right to the block or brick, have a look to see if there is a crack in the block. If there is, have a look outside the house in the same area and check the brickwork. If there are no cracks in the brick you are generally ok, if there are and the external crack is around a window, door or corner of the house, do consider having a structural survey done.

In the image below, you can see that the top, which is just under a side window, the block mortar has also cracked – in this case it is because the previous owner of the house removed a wall between the kitchen and dining room and did a really bad job. However, the brickwork outside is sound so I decided to carry on – this is also another way to ‘test’. If after filling the crack properly it reappears within a year then you know you have movement and should get it checked out!

As this particular crack is very deep and large, it is wise to pack it with a mortar mix first – but only up to the hardwall plaster (sand colour) and not the smooth finishing plaster on top (white colour). If it’s a fairly small crack then this part is not needed and you can proceed to the top filling stage.

Brush or vaccum all the dust around and inside the crack before you start. Water down a bit of PVA glue in a ratio of 1:1 and brush it liberally inside the crack only and leave for around 10 mins to become tacky.

In an old tub put a small quantity of mortar mix or 4 parts sand to 1 part cement and stir together dry then add small quantities of water until you have a paste that will stick together if you squeezed a lump in your hand. Don’t worry too much about getting it perfect but it does need to stick so don’t make it on the dry side.

I always find using my hands to do the next bit easier but you can use a filling knife if you want – if you use your hands then pop on a pair of disposable gloves as cement is nasty stuff! Grab bits and press the mortar into the crack until you are up to the level of the hardwall plaster and no more will fit in then give it a gentle rub to smooth off the surface.

Leave the mortar to dry overnight or longer if the crack is particularly deep. You are now ready to put the smooth top fill on.

In a clean old pot mix the polyfiller of your choice – I use Toupret filler which is not the cheapest but it is superior in application, finish and longevity. Put the dry filler in the pot and add bits of water until you have a smooth paste similar to beaten butter – you want it soft and not runny or dry.

Using Continental filling blades smooth the filler over the surface so it is barely proud and a smooth finish – don’t leave it lumpy or with peaks as this will make it really hard to sand smooth.

Leave overnight to dry and sand smooth either by hand or with a small sanding machine with an 80 grit paper and paint your colour of choice – no need to spot prime.

I hope you found this guide useful, please do contact me if you have any questions and check back soon as I will be uploading more great guides for jobs you can easily manage yourself.