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How to fix cracking in hardwall plaster over brick?

I've got a section of hardwall plaster (on top of brick) where it looks like the plaster has 'swelled' and puffed out. In our garage, the previous owners left bags of hardwall plaster, so I'm thinking one way to fix this is to remove all of the swollen/cracked plaster and mix up new hardwall plaster and apply.

Is this the best approach and do you have any tips for applying it to brick and getting a smooth finish?

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MitchellMc

You’re on the right track with removing the damaged plaster @Simon_B, but there are a few things to keep in mind. If the plaster has already puffed out and cracked, it’s a strong sign that the surrounding plaster may also be poorly adhered, even if it looks intact. Start by carefully chipping away all the areas that are loose or proud of the wall, then check the plaster around that section by gently tapping or pulling it. Any areas that come away easily should be removed as well. Be prepared that you may need to redo a larger patch than just the visibly damaged section.

Once you’ve cleared the loose plaster and have a solid surface, you can apply your new hardwall plaster. First, brush off dust and debris from the brick, then wet the brick slightly to help adhesion. Apply the plaster in thin, even layers, pushing firmly into the brick to get a good bond. You can use a straight edge to level the plaster as you go, and then a trowel to smooth the surface. After it starts to set but is still workable, lightly float the surface to remove trowel marks and get a consistent finish. For the best result, finish with a clean, damp sponge or trowel to produce that smooth hardwall look.

If you do not have prior experience with hardwall plaster and if this turn out to be more than just patching that area, you should consider removing all the plaster and installing plasterboard instead.

A final note on safety: There’s a good chance it actually is hardwall, but unless you are 100% sure, and especially if your home was built before the mid-1990s, there is a possibility that asbestos could be present in the wall. You definitely do not want to disturb asbestos-containing material. If the property was built after the mid-1990s, that would rule it out, but until verified otherwise, assume there could be asbestos and take appropriate precautions.

Here's a helpful guide: How to hard plaster a brick wall.

Please let me know if you have any questions.

Mitchell

Simon_B

Thanks Mitchell! I'll be sure to follow that advice and will reach out if I run into any issues 🙂

Simon_B

An update on the project - I've removed the loose plaster and can see a small-medium crack in the wall. I'm thinking the next step might be to fill the crack, possibly just by repointing with mortar. Here I've read that I should use a lime mortar, to prevent damaging the old red bricks - is that correct and how do I blend up lime mortar?

I had a look at the guide for how to hard plaster a wall and noticed that the 1:1:6 ratio for the plaster (cement:lime:sand) is the same as for mortar. A bit of an ignorant question, but is plaster just mortar or is a different type of lime or sand being used here? I also found a follow up video that continued the above with a "white set". Is that something I should be doing too?

I've seen guides that talk about using undercoat plaster and then finish plaster, so it has just left me a bit confused about what I should do. Any help here would be great, thanks!

JacobZ

Hi @Simon_B,

I hate to be the bearer of bad news, but that is not a small/medium crack that can just be covered up and forgotten about. That is significant structural damage that you need to have assessed by a suitably licensed professional, such as an engineer, and remediated prior to any further work.

A crack like that indicates significant movement, which can destabilise the wall and, in turn, anything above it or tied into it.

I'd strongly suggest you stop work immediately and contact a professional for their assistance, as this is not something to take lightly.

Jacob

Simon_B

Ah man, no worries. It may look worse than it actually is as I'd removed the loose mortar which has made some cracks look bigger. The actual crack is about 5mm for 100 years of movement, but either way, I've taken your advice and have someone coming out tomorrow to look at it 🙂

I'm still keen to know the answers to my other questions just for my future reference, if possible

EricL

Hello @Simon_B

Once you've had your brick wall diagnosed, I suggest having a look at this discussion - Can you plaster over brick? by @kj2801. It gives an excellent description of how to plaster interior walls.

If you need further assistance, please let us know.

Eric

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