Plastering advice

[GraemeR]

I was recently stripping wallpaper and some of the old plaster came crumbling off with it. I'm not sure if the plaster is lime or gypsum but there is animal hair in the mix, so I suspect it is original. It is an internal structural (brick) wall and the crumbly patches are close to the front window - it seems more solid further along the wall. My questions are:
- What needs to be done to prepare for replastering? Should it be chipped back to the brick? If so, can this be restricted to the crumbling part?
- Would gypsum plaster be acceptable, noting that it is an internal wall, although joining with an exterior wall.

Thanks in advance.

 

[NecessaryWing]

Yes looks like lime plaster to me. It is very delicate even when it is new. I had an external facing wall done last year professionally and a chunk still fell off when I was drilling holes (carefully) for a new curtain pole. If you can get away with it then patch repair the bit that is crumbling if it is possible. If it needs fully replastering there is no need to plaster in lime if it is an internal wall.

 

[GraemeR]

Yes looks like lime plaster to me. It is very delicate even when it is new. I had an external facing wall done last year professionally and a chunk still fell off when I was drilling holes (carefully) for a new curtain pole. If you can get away with it then patch repair the bit that is crumbling if it is possible. If it needs fully replastering there is no need to plaster in lime if it is an internal wall.

Brilliant, thanks for that.

 

[aockenden]

Yes looks like lime plaster to me. It is very delicate even when it is new. I had an external facing wall done last year professionally and a chunk still fell off when I was drilling holes (carefully) for a new curtain pole.

Sry man, it sounds like the professional you paid isn’t very good. There is nothing inherent to lime that should make it weak and easily falling off the wall. It’s true it does have a lower COMPRESSIVE strength than cementitious products, but that’s not super relevant when considering plaster (as opposed to bedding or pointing mortar).

If the lime is crumbling into bits within itself then it could be that the person used some stale lime powder that had partially or fully carbonated over a long period sitting in the bag, or that the lime dried out on the wall before it had time to carbonate fully. Or maybe there wasn’t enough lime used for the amount of sand.

If the lime is strong within itself but is delaminating off the substrate and falling off in sheets, then it’s probably an issue with surface preparation or the original consistency of the plaster mix. Maybe the substrate had no mechanical key or the plaster mix was too dry to begin with or the substrate surface was super dusty and they didn’t clean it off or damp it down before applying.

 

[NecessaryWing]

@aockenden are you sure about that? You think lime should be easy to drill into without causing an unecessary damage? I must say that was the first time I have had lime plaster done professionally so have no experience of drilling into new lime plaster; only old lime plaster. So when I say carefully drill what I did was use a slow speed settnig and not using the hammer action. When I carefully drilled through the lime plaster I then used the hammer action for the brick behind. Here is what happened to all four holes I drilled into that plaster (picture only shows two holes):




I would appreciate any input from others drilling into new plaster. This was put down August 2022 by a very reputable local lime plasterer (4th generation). He used Ty-Mawr lime with traditional three coat system. Any advice is much appreciated, thank you.

 

[aockenden]

@aockenden are you sure about that? You think lime should be easy to drill into without causing an unecessary damage? I must say that was the first time I have had lime plaster done professionally so have no experience of drilling into new lime plaster; only old lime plaster. So when I say carefully drill what I did was use a slow speed settnig and not using the hammer action. When I carefully drilled through the lime plaster I then used the hammer action for the brick behind. Here is what happened to all four holes I drilled into that plaster (picture only shows two holes):

View attachment 12490


I would appreciate any input from others drilling into new plaster. This was put down August 2022 by a very reputable local lime plasterer (4th generation). He used Ty-Mawr lime with traditional three coat system. Any advice is much appreciated, thank you.

Eh, well if he used premixed stuff from Ty-Mawr it should be a good quality product, and to be fair that damage in the pic is much more minimal than I had imagined from your original post.

 

[NecessaryWing]

Eh, well if he used premixed stuff from Ty-Mawr it should be a good quality product, and to be fair that damage in the pic is much more minimal than I had imagined from your original post.

Ah ok nice, that makes me feel a bit better. Also, looking back at the photos of when it was being done one of the holes on either side of the window is on this top batten as there was no brick behind it when I drilled the lower hole and that's exactly where it looks to be:



He did use mesh because of those battens in the wall so maybe its a bit weaker there.

 

[aockenden]

Those are pieces of timber embedded into the brickwork? And that’s where you drilled your holes? Yes those areas could have poorer adherence to the substrate (or even hollowness behind the lime) and might result in more damage when drilling into it.