How fragile should I expect new lime plaster to be?

[rshartley]

I have a question about the resilience of new lime plaster.

We recently had most of our walls replastered in lime (having stripped off cement render/gypsum and unsound lime throughout the house).

Materials were purchased from Mike Wye & Associates last year and half of the work was done by me following Mike Wye's recommended methods. This involves a harl coat, scratch coat and then a thin finising coat.

In January, a broken right hand prevented me continuing with the plastering so the remaining work was contracted to an experienced lime plasterer.

We were very pleased with the finish achieved and all seemed well. However, a few months down the line, the plasterwork which was done by the professional still seems very fragile and prone to damage. The slightest knock will take a great chuck off the surface and the plaster beneath is like sand and will simply crumble off the wall. It is possible to take the surface off the plaster with a thumbnail - like you're breaking the skin - and once you do so the plaster beneath has little or no strength to it.

The questions I have are:

1) is it normal for the plaster to be so fragile?
2) will the plaster strengthen with time as more carbonation takes place?
3) how long will it take for the plaster to become more durable?

I am concerned that at the current rate of attrition we'll have none of our precious and expensive plaster left on the walls in a couple of month's time!

I would be grateful to hear others' experiences.[/list]

 

[biffvernon]

It sounds as though something is seriously wrong. Although lime plaster will continue to carbonate and gain strength over weeks and months it really should feel quite hard and strong within a few days. You may need to get a second opinion on site but my guess is that your man was a 'professional' in the sense that he worked for money rather than in the sense that he knew how to do the work.

 

[Ian Bond]

Hi

The plaster should not be that soft and fragile, especially after this amount of time.

I take it you used the tubs of pre-mixed plaster? There should be no need to add anything to it for internal work, except perhaps hair for extra flexability / durability (can't store the hair for long periods in the lime, as it dissolves away, so you have to add more!). Leaving out the hair would not make the plaster weak like this though - I've rendered directly onto the stone walls with no added hair with good results. You should not need to add a pozzolan for inside work, either.

I find that the scratch coat can shrink and crack a little during the first week or so of curing, but lightly spray with water rub over with a wooden float to close the cracks.

By the time I get around to the finish coat (usually a few weeks later, as I'm a great starter of jobs!) the scratch coat is very firm and not easy to damage at all.

I have had early attempts at the setting coat / finishing plaster fail becase I used far too much water, and had the plaster very runny. It did initially appear very smooth, then cracked horribly and was easy to pick off.

When you say you used the technique recommended by Mike (which incidentally differs from another lime plastering technique I've been shown) - did your pro appear to do it the same way? Did you manage to see how he went about the job?

Can you ask the pro back in to have a look and see what may have gone wrong?

Ian

p.s.
The technique for the finish coat that I've seen from Mike Wye is similar to the way I used to apply the gypsum finish - 2 thin layers applied in quick succession with a steel float.

The 'other' way I've been advised to apply the lime finish, is to have it in a stiffer mix, trowel it on, then use a wooden float to flatten and smooth it. This leaves a more open grained surface suitable for a lime wash, rather than a hard, polished surface that you can get with the steel float.

You can polish the finish to a hard, marble-like surface - not suitable for my rough cottage, and I'm told it reduces breathability.

 

[dmcquade]

Hi,

It maybe that the plaster has been over worked with a steel float bringing a lime rich layer to the surface, leaving a lime depleted sandy weak layer below. It could also be that too little lime has been added.
Rather than guessing or going by what others have used, you would be as well to do a void test on the sand to see how much lime you need to add. Dry some sand in an oven then place in a jug. Add water slowly from another jug until it saturates the sand and comes to the surface. This will give the % lime you need to add to the lime.

All the best.

David

 

[Ian Bond]

Perhaps, but if he's used ready mixed stuff from Mike Wye, it should work out OK, unless there was something fundementally wrong with the material or the workmanship.

 

[dmcquade]

Sorry, i assumed he was using putty and mixing it himself.

David

 

[Ian Bond]

You could be right about that - he's not specified what he got from the supplier, or what the pro plasterer used, yet.

It may well be lime putty and sand, but be too weak a mix. Your suggestion would be very useful here. I'm not as scientific - I just shovel about 3 parts sand to 1 lime onto me spot board and mix it all in with a bit of hair.

Or I used too - I've found it much easier to get the plaster ready mixed in tubs and just knock it up as needed. That's OK as I'm only patching now, but for large amounts I think I'd do it the putty (or hydrate) and sand way and mix it myself (for economy). In fact, I'd probaby get myself a mixer, because by the time I've knocked the plaster up I'm too knackered to do the plastering!

 

[Nigel Watts]

Lime plaster in tubs doesnt have an infinite shelf life (the hair especially will disolve). Could it have been past its best before date?
I have used Mike Wye's premixed lime paster and have found it excellent, and Mike very helpful. Why dont you give him another call?

The only plastering of mine that failed was due to lack of adequate adhesion to the brick wall (plastering over lath worked fine). I pinned this down to excessive suction. On my second attempt I wetted the wall more thoroughly and added a bit more water to the plaster. After that it was fine. Where I had to apply the plaster over modern blockwork (which was ultra absorbant) I harled it first, on Mike's advice, than continued to dampen it over an number of days. This worked fine. Could the wall or plaster have been too dry, or allowed to dry out too quickly?

 

[dmcquade]

Couldn't agree more. The first time i have ever plastered was the gable of my house about 3 weeks ago. I used a belle mixer and NHL, worked the full day mixing, moving and rendering. I woke at 3.00am with cramp in both hamstrings and a right hand that was moulded into a hook! I didn't get my hand straightened for two days after. I am getting slightly faster and without the pains, the main trick IMO is not to play with the render - get it on straighten it with a feather edge and rub it up. If using a steel float don't put it flat on the surface but keep it at an angle to open the surface on the final pass.

One question why do people use a wooden float and not a plastic one?

David

 

[Ian Bond]

I've had lime mortar stood around in tubs for ages, and it just needs knocking up again before use. It should be OK for a long time in sealed tubs with a layer of water over the top.

The hair does dissolve, but you just chuck more in if necessary. I can't see it going off any more than lime putty stored in a similar way, but I could be wrong. Perhaps if the air got to it and it had partly carbonated in the tub?

I wonder how old the material used for the failed job was? We're still not sure what he used.

Too much suction is a problem, so you do need to wet the substrate., I've bought one of those pump-up garden sprayers, but I note that others chuck the water on from a churn brush to good effect. When I made my early mistakes with insufficient wetting of the background, the plaster would not even stick to the stone, and what did stick fell off the next day.

Far too much water seems to cause problems too. After a while you get a feel for how much water to chuck on, the consistency that the plaster should be etc.

Plastering / building work should save on gym membership fees, anyway.

I've never tried plastic floats - perhaps they're too smooth, not having the grain of the wood to give the required finish.

 

[biffvernon]

Lime plaster in tubs doesn't have an infinite shelf life

Yes it does. Well, OK, infinity may be stretching things a bit, heat death of the universe and all that, but so long as there is water covering the putty in the tub it will last for ever. As you say, the hair will not survive in wet putty.

I've been experimenting with various mixes recently and have come to the conclusion that for much general plaster work (I'm not talking fine cornices) this mix has certain advantages:

Six parts coarse sharp sand passed through a 10mm garden sieve
One part clay-rich subsoil
One part dry bagged hydrated lime from builders' merchant

(Use a finer sieve for the top coat.)

I mix a wheelbarrow full at a time, dry, and then add as little water as possible to make a workable mix. Then add a bucketful of chopped straw and mix a bit more.

Advantages:
Cheap; being half the price of a 3:1 mix (I dig up the aggregate for free)
Eco-friendly; half the embodied energy and CO2 production of a 3:1 mix (no energy/CO2 cost of aggregate dug up withing wheelbarrow range)
Traditional; vernacular building relied on the cheapest local materials available. Over-specification is the bane of modern building.
It works. Really.

There is a good theoretical reason why a sand/earth/lime mix is better than just lime and sand. The aggregate should be of a range of grain size, the coarsest grains just touch each other, sand fills the spaces, silt fills the spaces between the sand grains and clay fills the spaces between the sand. The less rounded and more angular the particles the better because they lock together instead of slipping past each other. Clay minerals are extremely angular and do such a good job of locking silt particles together that most of the world's houses are made of earth and actually stay up better in earthquakes than concrete houses. Adding a very small proportion (one eighth of the total by volume in the recipe above) allows calcite crystals to grow amongst the clay minerals, firmly fixing them together. The water adsorbed by the clays allows the growth of calcite crystals to proceed further, as the hydrated lime carbonates, than if there were no clay present.

I use a wooden float because I can make it myself. A plastic one requires money and a petrochemicals industry.

 

[Ian Bond]

Good idea- I may well try mixing earth with the mortar I use for pointing the house where I need to repair it - that should help blend it in.

I'm a bit worried about ending up with a big hole in my back garden if I use too much soil for other jobs - perhaps I could make a feature of it? Sunken garden, pond (nice for the wildlife)?

It is a shame that all the lime tubs are plastic for such an otherwise eco friendly (friendlier than cement) product. I wonder if my supplier will take back the ones I can use / recycle elsewhere? I definitely do not want to be chucking them away once used.

 

[rshartley]

Thanks for all the valuable comments and advice.

I will try to give more background on what was used and to answer the other points raised.

Firstly my professional plasterer appears to know what he's doing - when I first discussed the work with him and met him on site he gave me every confidence that he works with lime materials on a regular basis. He also carried out some restoration of brick floors and that work was done very competently. I've had my fair share of cowboy tradesmen on this project but I really don't think the plasterer was one of them. Having said that, when I called him to explain my concerns about the plaster, he did say that the plaster is meant to be that fragile and that it's normal for lime to be like that. This left me somewhat confused, particularly as the work I did myself (using the same materials) is not easily damaged. I really want him to come and have a look for himself as I'm sure he's misunderstood just how delicate the resulting finish is.

The materials from Mike Wye consisted of 2.5 tonnes of premixed 3/1 mortar/plaster for the harl and scratch coats and 0.5 tonnes of premixed 2/1 finishing plaster. It was delivered in bags within crates last October and then transferred to very large tubs and kept sealed up. I don't think carbonation was a problem as I've still got about 4 tubs of coarse stuff left over and have used the remains recently to patch some damaged areas and also for repointing. It behaves exactly as it did back in October and has hardened up fine.

No hair has been used as most of the original plaster in the cottage was unhaired and the coats were all quite thin.

The professional completed all of his work over a period of about 10 days. His technique was quite different to the one I was shown on Mike Wye's course. He did not use a harl coat at all but instead put on quite a thick dub coat/scratch coat and a similarly thick finishing coat. For the finishing coat he mixed some of the 2/1 with the 3/1 as he said he would be able to manage a good finish with that blend. At some point, he told me he had run low on the Mike Wye supplied stuff so he used some of his own mortars which I believe he makes up using hydrated lime. I am guessing it was the fine stuff that he ran out of as there was still original Mike Wye coarse stuff left over at the end.

I never saw the plaster being applied but from looking at the work as it progressed it seemed to me that the plaster was being applied very thickly and possibly with quite a lot of water in it. When I plastered I kept the mixture quite stiff mixture (no added water) and really pushed it hard into the wall. I get the impression that the mix used by the plasterer is somehow less dense - as if there are lots of voids in the mix.

I do know that the plasterer used a large water spray to wet the background and the walls did not dry out very rapidly (they were damp for at least a week after the finishing coat went on).

I guess my best course of action is to make another phone call to the plasterer and try to persuade him to come out and have a look for himself.

 

[Ian Bond]

Hmmm the plot thickens.

The (absence of) hair should not be causing the problem with softness of the plaster anyway.

I have found that my plaster does take good while to dry, but depending on the weather, the surface dries out quite nicely after a few days. When I mist spray (to help prevent cracking) the plaster sucks all the water up. Shows how good it is at managing moisture in the building, too!

I don't use a harl coat inside, just a spray of water to control suction. As I understand it, Mike does recommend the harl coat for rendering to aid adhesion.

As others have mentioned, if the substrate's too dry, and you don't control the suction the plaster will fall off.

I am concerned if the plasterer added water into the mix - that should not be necessary. The plaster should also be well mixed / knocked up. You probably noticed yourself that straight from the tub it can be very stiff, but beat it and it pasticises and sticks to the hawk / trowel / wall / cat.

I've used bagged hydrated lime, but I got it from a merchant with high turnover of stock, and in plastic lined bags. That way I could be reasonably sure that the lime had not carbonated in the bag already, and become useless. That would make for a very weak mortar/ render indeed!

I also wonder about the thickness applied in each coat - I try to keep it to under an inch for the scratch coat, and build up in layers. You are also right to push it hard onto the wall. I have never put on such a thick finish coat myself, but I have read of this being found in some old buildings - the finish coat thicker than the scratch coat!

Perhaps the water, thickness of the coats and insufficient time between coats has caused a problem. The condition of the hydrated lime could have contributed to the problem.

The thickness of the finish coat is also something to question - perhaps you can investigate other areas in your house. Can you see how many coats were used, and how thick they were?

I don't understand the assertion that lime plaster is meant to be that fragile - far from it. You know this yourself from the work you've done.

I'd ask the plasterer to come and have a look and give him a chance to see the condition of the walls for himself. If he can't / won't sort it out you're going to have to consider a different course of action, but let's hope that's not the case if he is a pro.

Have you given Mike Wye a call to explain the situation - I'm sure he'd be happy to give the benefit of his knowledge and advise accordingly.

 

[Ian Bond]

pasticises?

I mean pLasticises.

 

[biffvernon]

Sounds like the main problem might have been the extra water that was added. That's likely to substantially weaken the finished work.

 

[Anonymous]

I would recommend you get in contact with Bob Bennett at the Lime Centre. I did one of his one-day introductory lime courses last year. He's a lime expert, runs a consultancy and, I believe, will mediate and provide a second opinion where property owners have had problems with plastering work done by professionals. He will analyse samples for you and provide advice on rectifying the situation - for a fee, of course, but probably well worth it. I was impressed by the course I attended there.

 

[Nigel Watts]

Shelf life of premixed plaster

I had two tubs of Mike Wye's premix left over from a job about two and a half years ago - a 3:1 haired coarse and a 2:1 fine. The course stuff definitely had gone off and the hair had completely disappeared. The fine stuff seemed less problematic but did not feel quite right. Both had use by dates on their tubs which had long since passed. Rather than take a risk I ditched both of them (although I did mix some of the coarse stuff into a fresh batch, with no ill effects).

My lime putty, from the job, and from another job I did a year before, is, as one would expect, still perfect. I used this to make fresh plaster.

 

[Gervase]

I have to say I'm a tad dubious about your plasterer. Lime plaster simply is not inherentlymore fragile; it's been around for centuries, and when carbonated properly is as tough as old boots.
It's odd that he should have felt the need to transfer it from sealed bags to tubs. I tend to leave ready-made stuff in the bags, and have found it perfectly servicable after two years (though it did need the mother of all knockings-up to get it workable again).
As has been mentioned, he shouldn't have needed to add more water (or if he did, no more than a wine-glass full for every mixer load). Lime mortars and plasters do need far more knocking up than modern OPC and gypsum mixes; the most recalcitrant, lumpy and dry mix will suddenly 'slump'and turn into a beautifully-workable mortar or plaster given enough effort and enough faith. Too much water will weaken a plaster, as will too much working of a wet plaster with a steel float (save the steel for when you're applying very thin coats and for when the worst of the water has already gone - otherwise stick to wood).
To resolve the problem, knock off a chunk of the offending plaster and send it to Mike Wye and ask his advice. He should be able to tell if the lime has leached or if carbonation had occured before application.

 

[rshartley]

It was me who transferred the plaster from the crated bags - about two tonnes of it by wheel barrow!

It had to be moved from the driveway next door where it had been delivered and I happened to have two enormous plastic barrels that had formerly served as massive water butts. I shovelled the coarse stuff from the crated bags into the barrow and then back out into the barrels/silos. The silos were lined with the big black bags that Mike Wye delivered the stuff in and they were sealed down.

I can't be sure about the addition of water - he used a mixer so may be this enabled him to get what was clearly a very plastic mix without any added water. And I am pretty certain he used a wooden float - even for the finishing coat.

I am wondering if the weakness is partly due to the fact that the resulting finish is quite an open and grainy one. A fairly sloppy mix was floated onto the wall in quite thick coats - perhaps without enough pressure being applied to compact it.

I will start off by asking the plasterer to come and have a look and if he refuses or insists the results are normal I'll get one of the suggested specialists to give me an expert opinion.