How fragile should I expect new lime plaster to be?

[worms]

Just to add to Biff's comment above, during an idle moment a couple of weekends ago, I mixed some home-made lime putty with some very dry topsoil 1:3 (quite sandy hereabouts but had some compost a couple of years ago and the dumped out ash from last year's BBQs). It didn't mix very well and I added too much water (fluid off the top of the putty) and made a real sloppy mess. However having let it settle, I pushed the settled stuff into a piece of the wall that needed pointing and painted some onto the surface of one of the stones - just playing really.

The amazing thing is how well it has set and how well it matches the old original pointing! Wish I hadn't painted the stone as it was difficult to remove again with a wire brush, but shows it would have worked well as a coating. Also where I painted some into the loose mortar in the wall, it has firmed it up remarkably.

I guess this confirms the local-is-best trend and that 1:3 is much stronger than necessary.

 

[biffvernon]

A fairly sloppy mix...

That really sounds as though there was too much water in it.

Interesting experience, worms. Of course it's the worms that are the problem if you use top soil rather than subsoil, and all the other organic part of the soil. You don't really want that in your mortar as it will eventually be eaten up by the bacteria, leaving a lot of voids.

Hmm, thinks... if you made a mortar with a lot of organic matter in it and the organic matter then decayed leaving tiny voids, would the lighter weight, better thermal insulation, greater vapour permeability etc be advantageous :?:

 

[worms]

The really sloppy bit was because the original mix went into marbles of putty coated with soil and I was having difficulty breaking them up. The water was from the bin of putty (had to break the salt crust on the water first to get into it!). As I say I was just playing. At that point I wasn't sure if the putty was viable as it was my first attempt and since I had used bagged hydrate lime from our local outlet of one of the big chains, I wasn't sure if it had adequate turn-over for the lime to be OK. Now that I know that the putty is good, I have no excuse but to go on and mix some properly. (Must also lay down next year's putty!)

As to the organic matter in the mix and its potential for decay/insulation, this is surely part of the theory with your straw and no different from the cow dung approach. The lignin fibres will bind but the rest might decay? Although it will be a pretty harsh environment for bacterial decay!

 

[biffvernon]

The 'salt crust' is, I guess, calcite, formed on the water surface where dissolved lime meets atmospheric carbon dioxide.
Yes, you don't want marbles of putty coated soil. Thorough mixing is required. It is much easier to mix dry lime than putty.
You're probably right about the unhealthy environment for bacteria. Dead bodies have traditionally been covered in lime and the straw in 200 year old plaster looks pretty much like it was harvested last ear.

I've been trying to make some insulation board by mixing lime putty with oak shavings and sawdust. It's a matter of getting the proportions right - just enough lime to fix it all together but still leaving a lot of air spaces. The result looks like wheetabix.

 

[Moreteyne]

You can seem to have it all covered , what can I say - lime rendered last year 1x lime to 2.5 sand. It's very important that you get sand that is of a mixed grade. You want lots of small grains to fill the voids the larger ones leave. This helps the overall strength.

Too much water can make this brittle, as can over working......

I would suggest you remove what's not sound and re-apply.

Good luck

 

[Ian Bond]

The following guide may be of interest.

http://www.womersleys.co.uk/pdfs/limeplastering.pdf

Best of luck.

 

[rshartley]

I would suggest you remove what's not sound and re-apply.

There's just the small matter of recovering some of the £2,500 labour costs to put it on in the first place, plus the costs of the materials used, plus the costs of the limewash now covering the walls, plus the costs of removing and reinstating architraves, skirting boards, radiators, etc, etc.

Added to that, all the work was done when the property was empty but now it is in use so things are not quite so simple.

For those reasons, I was hoping that perhaps the plaster was still in the process of hardening and that such drastic action would not be necessary.

 

[Ian Bond]

I sympathise, and I bet many PPUKers have been in similar situations, I know I have.

You have plenty of material to refer to in terms of the proper methods to use for lime plastering. Mike Wye is your best source for information regarding best working practice for using his products.

If the materials were up to scratch, and stored properly, the success of the job will depend of the competence of the plasterer. Unless there were circumstances beyond his control, such as a fire or flood damaging the job.

Is every wall / room affected or is it just in patches? I expect that you've already made a record of all the problem areas anyway, so that they can be assessed and repaired.

I do not remove architrave and skirting to replaster, but can get away with it in my semi-smooth (ie rough) cottage. Much of the old plaster abuts the edges of the architrave rather then the architrave covering the plaster at the join with the door frames.

With the walls being so wobbly, it is easier for me to leave skirting in place and plaster up to it. Small gaps can be packed and filled. Perhaps you won't have to strip the skirting off to repair the plaster either?

It must be very frustrating and annoying for you. It's very difficult living in a building site - I'm slowly getting my house in shape, but every room has plaster off the walls and holes in ceilings etc. I'm now trying to tackle one room at a time, and at least get a habitable and clean-ish retreat from the chaos elsewhere. Things may not turn out to be as bad as you first fear - I've had plenty of those moments in this house.

I hope that you get this sorted without being out of pocket, as it is not your fault. Best of luck, and please let us know how you get on.

Ian

 

[Ian Bond]

Just one more thing I've remembered.

Grand Designs, new build, limecrete floor. Wales. Clients at odds with architect. The builders had no experience with lime. Many months after the floor was laid, you could dig it up with your fingers in places. Disaster.

The floor had not set properly. I thought they were going to have to dig it all up and start again - not easy as the rest of the building work has been practically completed.

When the programme makers returned for the follow-up many more months later, the floor *had* hardened up and all was well.

Obviously this may not be applicable to your walls - different materials, different circumstances. But it does show that what looks at first to be unmitigated disaster can (perhaps) turn out OK.

Could it be that the thick, and rapidly applied layers of plaster have retarded the carbonation process but not stopped it completely?

Pure speculation on my part here, and may be of no use at all. I wouldn't suggest this idea to the plasterer to let him off the hook.

 

[Gervase]

Could it be that the thick, and rapidly applied layers of plaster have retarded the carbonation process but not stopped it completely?

Certainly carbonation is a painfully slow process, particularly in dry conditions. I've heard various rates bandied about, but 1mm every 10 days steems to stick in the mind. If your plaster's laid on pretty thickly, then under that rule of thumb the rearmost parts of a 3cm layer would take around 300 days to reach full carbonation.
As a vivid illustration of the slowness of carbonation, take an apple-sized lump of lime mortar the you may have dropped two years ago and break it open; you'll find that the core will still turn bright pink of you add a few drops of phenolphthalein indicator, showing that it is still strongly alkaline and hasn't yet carbonated.
None of which goes very far to providing a definitive answer to the failing plaster, I'm afraid. My own course of action would be to send a sample off to Ty Mawr, Mike Wye or one of the other lime companies for analysis, then get the plasterer back, show him what's wrong and ask him what he intends to do about it. £2,500 is a lot of money to put down to experience! Good luck.

 

[biffvernon]

Certainly carbonation is a painfully slow process, particularly in dry conditions.

Which is why a bag of hydrated lime keeps for quite a while so long as it is kept dry. It's the water which allows to CO2 in the air to dissolve to form carbonic acid which react with the calcium oxide to form calcium carbonate. Without water the reaction is much slower. Furthermore, crystals of calcite (the calcium carbonate) can only grow if the ions can migrate to the growing crystal surface. This happens in an adsorbed film of water. This is why damp conditions are good for lime work.
Maybe your plaster suffered from two, apparently contradictory problems: too much water in the sloppy mix that was applied and then too rapid a drying out preventing either or both of, lack of carbonation and lack of crystal growth.

 

[Ian Bond]

I agree with Gervase - far too much money to put down to experience.

phenolphthalein indicator

I can't even pronounce that, let alone know where to get hold of it! :lol:

Seriously though, at least rshartley should now be in a better position to deflect any more excuses, particularly if a proper analysis is made.

Ian

 

[dmcquade]

This link is of interest to this thread. Shows there are many ways to the same end.


http://www.buildingconservation.com/articles/plaster/plaster.htm David

 

[Ian Bond]

Thanks, that's just the thing I was looking for - I've seen it before, but had not realised that it was written by Ian (of Restored to Glory).

http://www.buildingconservation.com/articles/plaster/plaster.htm

In fact that's where I'd read about the finish coat being thicker than the backing coats in an example of historic plaster.

The plaster in my house contains bits of leaves and grass as well as hair. And it was never scratched before receiving the finish coat, not even on the ceilings - just left with a coarse, open texture to give sufficient key.

 

[rshartley]

Many thanks for all the advice - much appreciated.

Perhaps you won't have to strip the skirting off to repair the plaster either?

I left some architraves on too but the skirting generally needed renewing (it was missing on some rooms anyway). The “possibly dodgy” plaster extends behind it although having managed to get fixings all the way through into the bricks I suppose it doesn’t matter so much about the stuff hidden behind the skirting.

Could it be that the thick, and rapidly applied layers of plaster have retarded the carbonation process but not stopped it completely?

I remember that Grand Designs programme – I seem to recall that they put the floor down in the middle of winter!

The carbonation issue is an interesting one and I am hopeful that it really might just be the thickness of the coats combined with the speed with which the coats were applied that is the main cause. At least that may be part of the problem. The reason for suspecting (and hoping) this is that I’ve noticed that the plaster inside the wall is very fragile (i.e. once scarred, a small area will easily spread to become a large area of damage and you can simply rub the inner plaster away with a finger) . However, if you come back to a scar a week later, the exposed plaster inside has gone off and is much stronger.

is every wall / room affected or is it just in patches?

The degree to which the plaster is vulnerable varies quite a bit. As I mentioned, the areas I did are all OK but these were applied over a much longer period with much thinner coats: a harl coat, a scratch coat of about 1cm and then a couple of skim coats of finishing plaster applied as thinly as I could get away with. Only the skim coats were applied in quick succession.

The worst wall is the one where the plaster is the thickest: a wall which had been affected by damp, had huge holes in it and a really uneven surface that had to be dubbed out. The bricks were incredibly crumbly at the bottom of the wall and the substrate sucked water up like a sponge. Here the plaster is really quite thick – I hacked off at least an inch and a half of cement render from the bottom section so I guess there is the same or even more lime on it in places.

In the dining room the walls started out straighter, the coats used are thinner, the bricks are less porous, and the resulting lime plaster is more durable.

Finally, the professional did one wall in a bedroom which was over the top of original lime plaster (rather than onto bare bricks). Here the plaster is quite thick in places but this wall seems the most durable of all.

I am therefore wondering if it’s a combination of the thick coats together with the degree of suction behind the initial plaster coat. Certainly these soft Norfolk red bricks are incredibly “thirsty” and I used a hose with a sprinkler head on it to wet the background when I was plastering over them – I found nothing else would get sufficient water into them.

The harl coat had the effect of cutting down the suction and made subsequent coats easy to apply. Then I lightly sprayed the surface of each coat for a few days running to make sure they coats dried out slowly. I don’t know if this was done with the coats the plasterer did.

Maybe your plaster suffered from two, apparently contradictory problems: too much water in the sloppy mix that was applied and then too rapid a drying out preventing either or both of, lack of carbonation and lack of crystal growth.

This would seem to make sense - perhaps where the bricks are particularly soft the background was too dry and the mix too wet. That might have meant that the bricks sucked all the moisture out of the first coat very quickly and it dried out too fast for proper hardening/carbonation and crystal formation to occur – at least deep down.

If your plaster's laid on pretty thickly, then under that rule of thumb the rearmost parts of a 3cm layer would take around 300 days to reach full carbonation.

That sounds encouraging - perhaps, given time, the coats will set more strongly.

I haven’t yet got around to talking to Mike Wye but I need to order some insulation for the loft this week so I will do so at the same time. I will let you know his opinion.

 

[Ian Bond]

OK, that sounds very encouraging, I cannot find fault with the technique you used.

There's actually an excellent guide to lime plastering and rendering on the PPUK Shop page (see Practical Guides).

The plasterer does seem to have completed the job very quickly (10 days) and applied the plaster in thicker layers than I do, but from what you've said, maybe it will carbonate and firm up if left alone.

I wouldn't worry too much about the plaster behind skirting boards, so I hope you'll be able to leave them all in place.

 

[Gervase]

Hmm, sounds like you did a rather better job than your plasterer!
Certainly a good wetting of the substrate is essential, particularly if it's a high suction material like soft brick or old lime plaster. That way you can get away with a stiffer, drier mix which will have less risk of cracking or of the lime being pulled to the surface when it's worked.
It may be worth applying a mist spray to the professionally done stuff every few days or so to aid carbonation (I find the Kilaspray or Hozelok 5-litre pump-up jobs from the garden centre to be ideal for misting). Otherwise, fingers crossed...