Patch up small/medium holes in Lath and Plaster

[Marin]

Hi, I've just pulled off some horrible strips of wood that previous owners grip filled to the walls of my house (1910/20 terrace) and unfortunately some big ish chunks of plaster came off at the same time exposing the laths underneath (interesting to see animal hair binder in the original plaster!). The holes are above some windows - there's a gap behind the laths and then the bricks of the external wall. There's some staining on the ceiling so I think the plaster may have been weakened by water from a leaky roof at some point. There's a little bit of black mould that I'll clean off, but luckily the laths don't look rotten

Does anyone have any suggestion of a good (DIYer friendly) way to patch these holes up? From reading other blog posts on here I get the impression it's important to use lime plaster for the repair because it's an exterior wall for breathability and heat loss etc - is that correct? I've watched a few you tube videos on using lime and I must admit it seems pretty intimidating to someone who's used to using plasterboard. From looking at the layers I think the original plaster has been skimmed with a more modern style plaster so would that negate the benefits of lime breathability etc anyway and maybe I should not over think is for such a small area and patch up with plasterboard for ease?

Any ideas for the most simple method of repair much appreciated!

 

[paulc]

I have a large tub with a mix of lime putty & silver sand (1:2 putty & sand) that I've been using to patch up chunks of missing plaster. Press it well in to the void, and keep going back to it over the course of a few days, and press a thumb in to it to close up any cracks that may form. Once fairly dry, give it a thin skim of ground marble (or chalk) mixed 2:1 with lime putty.
But... It looks like your walls have had a skim of Multifinish in the past.

 

[housewinnie]

Have a go yourself with lime plaster. You have edges to key to so should take quite well

 

[fernicarry]

For repairs like this I start of with haired coarse lime plaster to build up the depth. The darker brown layer that you can see in your pictures will be made of that. You can get ready mixed buckets from Wormerlsey's online although probably far more than you'll need for this job, but it does keep for quite a few months in case you find anything else that needs doing.

Push a good dollop in so that it squeezes out the back of the laths and get it in good contact with the edges of the existing plaster. Build it up in 2 or 3 passes. Try to keep it all below the existing level as you don't want the coarse stuff showing through at the finished surface. It'll take a few days between layers depending on conditions. if it shrinks and cracks give it a bit of a squeeze, or even a light bash with a brush.

Once you are up to the level of the lighter layer you can finish off with something finer that you can sand to a good finish. I generally use Toupret since I find it easier to work with than the fine putty based plasters. I really should teach myself to use the putty though... Losing a bit of breathability over a small area won't be the end of the world. The main advantage of the haired lime for a repair is that it gets a fantastic grip onto the existing materials.

Remember to give the laths and the edge of the existing plaster a good soaking a few hours before you start. I use a garden sprayer. And vacuum out the loose chunks and sand first. Use appropriate PPE, its quite caustic.

You could cut back to the studs and use plasterboard, but you'll still need to skim over to blend it in and you'll risk it cracking at the edges of the board over time. The lime isn't as daunting as you might imagine once you get stuck in and you'll almost certainly find other places in your property that you need the same skills. The biggest problem with this area, and this will be the same no matter what material you use, is that it'll be really awkward to get in between the window casing and the cornice.

 

[Marin]

Thank you so much for all the replies and advice/encouragement. Very helpful to know where I can order pre-aged/mixed lime and good point about matching materials to get the best bond with what’s already there. I’m going to order myself some lime and yes I’m sure other things will come up to use up the rest of the batch.

Shame about the multi finish skim over the walls. I guess that probably contributed to the mould on the laths as the moisture from the roof leak couldn’t get out so easily? Don’t think there’s much that can be done about that now though without chipping off the plaster (which I would definitely rather not do!) so I’ll have to hope that doesn’t become a problem

Thanks so much for all your help, big learning curve!

 

[paulc]

Depending on whether you want to pay carriage charges (which can get expensive), or want to collect.... Mike Wye or Ty Mawr would probably be best for mail order. Anglia Lime in Sudbury, likely to be the nearest. Anglia Lime do a premixed chalk plaster with synthetic fibres added. Very good for your base coats, and can be built up in fairly thick layers (I think they recommend 20mm max).
You could try ringing a few local builders merchants to see if they stock lime products other than hydrated lime - Up here in Norfolk, CT Baker is my go-to for putty. Not the cheapest, and brand depends on what they can get in the cheapest, but they are only a short drive from *here.

 

[fernicarry]

I guess that probably contributed to the mould on the laths as the moisture from the roof leak couldn’t get out so easily?

It looks like its been painted too so that could be a contributor if its a vinyl paint. Ideally you want to clean that off the surviving plaster. A steamer will soften it up but go easy in case any plaster or random fillers underneath soften up too.

Have to admit I've never seen mould on laths and I've pulled out plenty that have obviously been very wet at various points in their lives.

 

[Mummy will fix it]

Have a look into Patchcote.