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How to patch limestone blockwork cracks?

Hello! We've got some cracks in our limestone blockwork walls which need patching. As you can see from the photos some cracks are fairly mild but in other places the grout has fallen away and the cracks are quite deep. The house was built in a very wet year and so when it dried the walls cracked. The walls have also been rendered and then bagged with some kind of water based paint and so I'm also wondering if it's possible to repaint these with standard house paint or because of the porous nature of limestone do we need to use special paint?

Thank you,

Eliza

Image (4).jpg Image (3).jpg Image (2).jpg Image (1).jpg

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MitchellMc

Welcome to the Bunnings Workshop community @_Lize_. It's wonderful to have you join us, and many thanks for your question about repairing cracks.

From what you’ve shown, the cracking you’re seeing is more than a cosmetic category. Hairline cracking is what we’d usually associate with minor render shrinkage, but even in the first image the cracks appear to run through the full thickness of the render to the limestone beneath. Images two and three in particular show medium to severe cracking, and image two is a clear case of render failure rather than just surface cracking. That corner section looks to have fully delaminated from the blockwork and is likely only being held together by its own shape, not actually bonded to the wall behind. In that area, the loose material really should be removed completely so you can assess how far the delamination extends. You’ll probably find that a fair portion of that corner sounds hollow and will come away once you start opening it up, and it’s best to remove everything that isn’t properly attached before attempting any repair.

In images three and four, where the cracks largely follow mortar lines, the situation is a little different. If the blockwork itself feels solid and there’s no noticeable movement, these areas can often be patched with new render once the cracks are opened up and cleaned out. That said, cracks that track along mortar joints can sometimes point to underlying movement, so it may be worth having the foundations checked, especially given the history of very wet conditions during construction. While moisture during the original build could certainly have contributed to poor render adhesion, structural movement is something you’d want ruled out before spending time and money on cosmetic repairs.

The last image looks more like delaminated paint or a very thin skim of render rather than deep structural cracking. In that case, it’s simply a matter of scraping or chipping back to a sound surface and recoating once you’re confident the substrate is stable.

In practical terms, repairs should start with removing anything loose or hollow-sounding, chasing out cracks so you’re not just smearing over the top, and then rebuilding those areas with a compatible render. If, while opening things up, you uncover larger areas of debonded material, like in image two, those sections need to be fully removed and re-rendered rather than patched.

As for repainting, limestone is porous, so if standard water-based paint was used, you can re-paint with it again. Normal wall paint is water-based. Sometimes limestone is painted with a breathable mineral paint, but that's different from standard water-based paints.

Overall, a lot of this does look like original construction-related issues rather than ongoing deterioration, but image two in particular does warrant further investigation before patching. Once everything unsound is removed and the surface rebuilt properly, you can repaint and expect a much more durable result.

When removing render or chasing cracks, make sure you wear appropriate PPE. This includes eye protection, gloves, and a suitable dust mask or respirator, as limestone and render dust can be harmful if inhaled. Long sleeves are also a good idea, as the dust can be irritating to skin.

Please let me know if you have any questions.

Mitchell

_Lize_

https://www.workshop.bunnings.com.au/discussion/comment/61688#Comment_61688

Hi Mitchell, thanks for taking the time to wlak me through this, this is really helpful!

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