Lath and plaster interior wall advice

[irina]

Hi,

I'm trying to fill a gap approx 2m high by 60cm wide in an interior wall. The original wall is of lath and plaster construction. I know there is a bewildering array of lime-related Q&A on this forum, but that's just the problem, too many topics to sift through. So could the learned folk on this site please indulge me and answer these queries:

1. Does the caveat re working with lime late in the year (i.e. after October or so) apply to interior work? I'm guessing not, but can you confirm.

2. Once I've built my frame of timber struts and laths, what lime do I use? I realise there's hydraulic and non-hydraulic lime, which one do I go for? Whichever one, I'd prefer to use the putty form, as I've tried mixing the dried stuff and nearly choked on the dust.

3. I'm assuming the main mix is something like 1 part lime to 3 parts sand, with some hair thrown in just before getting ready to apply (I've downloaded the article by Bennett from buildingconservation.com, which doesn't however tell me whether to use hydraulic or non-hydraulic lime). But that will be quite heavy. The original plaster seems really lightweight and powdery, with lots of coal flecks in it. Should I reduce the amount of sand and substitute some lighter-weight material? Any suggestions with what material?

4. And what about this 'pozzolanic' I keep reading about? Presumably the coal was the pozzolanic in the original plaster. What do people use these days, if anything?

Many thanks in advance for your advice.

 

[Andrew Bodenham]

irina,

Pragmatically, use plasterboard and skim with gypsum board finish.

However, if you have time to spare and like the idea of replicating the original construction then lath and lime is the obvious way forward.

To answer your questions:

1. Inside work fine - helps if room is not heated, spray work frequently with water to prevent rapid drying.

2. Non-hydraulic. Mix your own putty, outside, wearing mask, gloves. Not too bad if you don't throw the powder around, helps to use an excess of water. Use nice fresh hydrate, not a bag that's been lying around open for months.

3. Mix 1:3 or 1:2-ish lime to sharp sand for initial coats. It's not that critical. A thin skim of 1:2 lime & fine sand to finish, if that's what the original looks like. Plaster is heavy - not a problem.

4. Pozzolanic additives - not needed.

Is this your first time? Let us know how you get on.

 

[Feltwell]

If it's an alteration, i.e. there was nothing there originally, then many would argue that a modern stud partition (timber frame clad in plasterboard) is a more honest approach - as well as being quicker and cheaper. I presume the building is not listed?

Even if it is it might be allowed. I used stud partitions to block some archways off in a listed town hall - CO required that they were installed such that it was 100% reversible in future, all mouldings left intact etc, and that they were recessed into the arches so that their outline could still be seen.

 

[irina]

Thanks for your replies. I'll try mixing non-hydraulic lime outside whilst holding my breath as much as I can, and on a non-windy day (almost impossible to get in these parts...).

As to why I'm going down this route:

No, the building is not listed, but if I use plasterboard and skim then the surface will look too flat and out of place compared to the existing lumpy and bumpy lath & plaster walls. There will be more work going on in the same room, which will probably cause cracks in any nice new plasterboard and skim, so I want something that can flex a bit more. Also plasterboard is really thin, and doesn't exactly insulate well against sound. And if you try and use insulated plasterboard, then that insulates thermally but not necessarily acoustically, plus it uses all that environmentally nasty foam stuff. Stuffing the cavity inside the stud wall is apparently only effective if you use something pretty dense and leave an air gap between layers, since you need something relatively dense to insulate noise effectively (laths with thick "squidges" of plaster make for a pretty dense mass I would say).

Yes, this is the first time I've attempted lath & plaster, although I've patched up plenty with polyfilla (purists pardon me please). I want to practise my skills on this unobtrusive bit of wall first, as it's only 60cm wide and the surrounding wall area is a little uneven anyway, before I tackle fixing the ceiling in the same materials....

 

[MdB]

Thanks for your replies. I'll try mixing non-hydraulic lime outside whilst holding my breath as much as I can, and on a non-windy day (almost impossible to get in these parts...).

Just a quick note on terms - NHL = naturally hydraulic lime and isn't the same as non-hydraulic lime so you don't want to go buying that by mistake. Non-hydraulic lime is normally called lime hydrate and is available at most builders merchants for around £6 for a bag. I'm sure you know this but it is worth pointing out as it is confusing.

I make my own putty from hydrate outside using a mask and goggles. It is easy enough to do and will keep forever.

 

[skier-hughes]

YOu may find to save time and energy mixing you could just buy ready mixed haired lime plaster scratch coat and use that, with some ready mixed lime finish coat for the top.
We had two coats of haired one coat of top.
Takes all the guess work out of what proportions to use, costs more, but saves time.
Once you get the hang of using what you are supposed to be using, then mixing it up for the next job will be so much easier as you know what consistency etc you should be aiming for.

 

[Penners]

if I use plasterboard and skim then the surface will look too flat

Clearly you've never seen my gypsum plastering. :wink:

 

[Feltwell]

Yes, this is the first time I've attempted lath & plaster..

Well, good for you for being brave enough to give it a go! Let us know how you get on.

 

[irina]

Well, thanks to all the hints and tips. I have now braved the plastering, and got the first coat on each side of the lath wall (the structural timber work and nailing the laths was the easy bit...). Tomorrow will tell if it is still holding up.

There is one builder's merchant in these parts who does all things lime related, and they supply ready mixes too. So I went for the ready-mixed coarse stuff with hair, and have some top coat, also ready mixed, waiting. So far I've discovered that coarse lime plaster seems a bit like cow dung, and the best method for me to get the stuff not just onto the laths but between them as well was to 'squidge' it on with the palm of my hands. I tried trowels and that flat wide trowel (don't know the name) plasterers use, but it didn't work as well as hands. Also trowelling seemed to draw water to the surface, something I read should be avoided at least for the scratch coat. The 'cowpat' method seems to have worked in any case. Being pretty unprepared I didn't have half the professional tools, but for some bizarre reason a load of kitchen implements came in handy. So an old washing up bowl was used to 'stir' the plaster (again, no idea of technical term, but I used a spade to 'slice' the ready mixed stuff til it got more pliable), my marigold gloves to apply plaster, and the fork from a salad bowl set made a great scratch tool ... Wall is now covered with dust sheet overnight, as I read about the problems of drying too rapidly.

If the experiment works, I'll try my hand next at reinstating a smallish patch of lath and plaster ceiling.

 

[Flyfisher]

Wonderful stuff. Any chance of some photos?

 

[skier-hughes]

Keep it dampish, it it starts to go a bit hard, -can't indent with nail, but you scratch it - give it a fine mist with water, for a few days.

That was my job over the weeks we had our plastering done......

Only thing I was trusted with

 

[DaveBrigg]

A bit late to tell you now, but some of our lath and plaster walls have hay in the void to provide insulation. I'm not sure of the fire regulations regarding stuffing your walls with tinder though, they may have changed since it was done.

 

[Moo]

some of our lath and plaster walls have hay in the void to provide insulation. I'm not sure of the fire regulations regarding stuffing your walls with tinder

We discovered a foot-deep void, behind a lath&plaster wall, that had been filled with sawdust. Over the years (decades? centuries?) it had settled so that by the time we found it the top half of the void was completely empty.

 

[skier-hughes]

Maybe it was always a void and over the years, decades, centuries it had gradually filled up 1/2 full with sawdust?

 

[Moo]

It would have had to be a masssive woodworm attack to do that, surely? The wall is about ten feet high.
And I incorrectly described it as lath&plaster: actually, it was reeds or rushes under the plaster, and no sign there of any insect attack.
I suspect it was put in for sound insulation when the original house was sliced up.

 

[irina]

Hello again and thank you for the various pieces of advice. I've been using one of those plant misters to wet my plastering as needed, not that it needs much since the climate here in the south-west is rather damp, and we are not heating the affected rooms at present. Since you asked for some photos I dare to post the following examples of my first-time efforts at lath and plaster:
View attachment 2
This is the laths applied on one side, view up to gaping hole in lath&plaster ceiling, which I'm practicing for restoring ... unless the ceiling comes down first.

For needs of sound insulation more than historical accuracy we then inserted 50mm of rockwool in the gap between laths, applied other side of laths, and then started rendering using my 'cowpat' method, see here:
View attachment 1

The finished scratch coat of render looks like this (side of the wall in the other, more habitable room).


I'm going to practice applying the second coat and final finish coat on this small area, since it will probably be hidden by storage units, so my feeble efforts won't get too noticed.

I've since plucked up the courage to start patching up other areas around this room that need re-plastering, as in a walk-in cupboard that had most plaster coming off the wall if you so much as brathed on it; this is rendering onto brickwork, which is working a treat, and allowing me to get a much more even finish. I've asked my regular plasterer to apply the top coat using lime, that way we get a decent finish and he gets to learn about working with lime.

 

[Schoolmarm]

What is the white showing through the laths in top pic?

And in the cow pat pic, does the lime go through enough to curl over and key onto the laths?

I have never plastered onto laths but always though it was a bit sloppier and squished through.

Am impressed by your getting on with it though. 8)

 

[irina]

Thank you for the kind comment Schoolmarm re. getting on with it. I've learned a fair bit about haired lime plaster in the past week, and I've actually come to really like working with the stuff. Unlike modern pink plaster I can actually get lime to stick to walls, for starters, and while doing corners on another wall I discovered that if you use a fairly dry mix you can actually mold the lime - I used to like clay modelling at school, so perhaps I'm reminded of that a bit....

As to white showing through in top pic, that is just a dustsheet temporarily fixed on the other side before the lime went on.

I can assure you the lime I 'squidged' onto the laths was pretty, well, err 'squidgy' and went through the gaps in the laths. That's why I used my hands to apply, since I could control pressure application better - also left lots of finger marks in the 2nd pic, before scratching the coat.

And now back to this weekend's work....

 

[Schoolmarm]

I see. Well it looks brilliant.

 

[Gareth Hughes]

I'd always get someone in to do that sort of thing, so well done you!

But I'm interested to know if what I was told by another enthusiatic amateur plasterer is true - does lime plaster stick best when it's been on the floor twice?