A lime plaster question

[jbaldwin]

I've a 300 year old cotswold stone cottage I bought 4 years ago. An internal wall has damp because someone about 15 years ago chopped all the plaster off, had it injected and then plastered it with cement and gypsum. So I'm taking the cement & gypsum off so I can redo it in lime plaster and then limewash.

I'm trying to do as little damage to the stone as possible, doing it with mallet and chisel. The gypsum comes off ok - some of it needed no help! - but chopping the cement off is very tedious and time consuming indeed and I don't want to use a power tool. So I want to only chop off what I need to.

Will I get away with this: Chop back to the stone on the lower part of the wall, but not worry about small patches of cement left (to try and minimise stone damage). On the top part, just take off the gypsum and top layer of cement but leave the bottom layer. Will the Lime plaster take over the cement?

 

[Tom]

I see 64 people have viewed this but you haven't had a reply, so I'm just bumping you back up to the top. I'm sorry I can't help you myself as you seem to have such a positive attitude to the way you're carrying out the work. Keep it up. I'm sure someone will be along soon to help you

Jaqy

 

[Ian Bond]

You may prehaps get the lime plaster to stick if you roughen up the cement enough, hacking bits out of it with a chisel to provide a mechanical key. I've done this myself to small pockets of stubborn cement.

As you are aware, you'll need to get as much of the cement as possible off to avoid moisture being trapped in the walls, and let them breathe.

There are products that allow you to apply lime over unsuitable surfaces, such as :

"Prependuit"
Coarse grade preparation mortar which allows lime plaster to be applied to difficult to coat surfaces both outdoors and indoors, including gypsum plaster.

Scroll down to the bottom of the linked page...
http://www.womersleys.co.uk/acatalog/Building_Products_2_6.html

But that's just a cosmetic job, and I would not be happy just covering a problem up.

The unpopular answer is just to carry on doing what you're doing, and get as much of the cement as possible off without damaging the stone walls. The more cement you can get off the better. Not a pleasant job, but you'll be pleased when it's over. We've all been there I think...

Good luck ....

 

[jbaldwin]

I had a horrible feeling that was the case, I'll just have to get on with it.....might be finished by next year !

Thanks for the reply

 

[Ian Bond]

I've found that an SDS hammer drill with a chisel bit useful in places, but it has to be done with extreme care.

The sandstone my house is built with is pretty hard stuff, but it's easy to dislodge pieces of lime mortared rubble with injudicious hammering and drilling.

Have you tried power tools and found that they caused too much damage to soft stone?

 

[skier-hughes]

I tend to carefully hand chisel a small section. See how it's like and then get the sds drill onto it if I feel I can use it, otherwise it's carefully goes it by hand.

Though I am working inside on brick or lath.

Graham

 

[Ian Bond]

Just another thought - I would concern myself most with removing as much as possible from the inside face of exterior walls , and not worry as much about the internal walls.

For the internal walls I'd concentrate on the lower yard of the wall the most, remove as much of the rest of the cement as I reasonably could by hand, then hack the remainder to give a good key.

btw - 300 year old Cotswold stone cottage. I'm green with envy here, do you want to swap?

 

[jbaldwin]

Its only this wall thats a problem. You're suggestion of doing the bottom 3ft as well as poss and being less thorough with the rest is what I had in mind really. If on the top half I remove all the gypsum and top layer of cement, chop some of it back to stone and hack the rest to get a rough key, is that likely to do the trick?

 

[Ian Bond]

It's not ideal, but what you are doing is a lot better than what most would do - i.e. just cover it all with gypsum plaster or similar and forget it.

If you are worried about the lime plaster sticking, hack up a small area of the cement, wet in down and do a trial patch. If it firms up and remains well stuck after a couple of days you'll have a better idea of how the job will go.

I found that if I didn't control suction on the stone with thorough wetting down, the lime render didn't stick properly in places, and was hollow when tapped. This is far more likely to happen on a cement but hacking a good key into may help, but I've only done this with small patches of cement and most of the wall bare stone.

A scat or harled coat might be worth trying too, using unhaired lime mortar.

 

[Ian Bond]

btw - where's the damp at the moment - is it condensation forming on the cold parts of the wall. The lime plastered walls would cope far better with moisture and my not show signs of damp at all.

Someone had attempted to solve an apparent problem in the past, and succeeded in creating a much worse one by the sound of it.

 

[jbaldwin]

Most of the outside walls aren't plastered. The house is a long way up the side of a hill in a very well drained spot, facing south. The rooms the internal wall sits between have concrete screeds on the floor, with stone tiles on, so what damp there is under the middle of the house goes up this wall. The damp manifests itself by the gypsum falling off here and there on the bottom couple of feet, even round where there is a radiator. Just letting it out of thw wall by changing the plaster should do the trick.

 

[Ian Bond]

Ah, I see. The concrete screed forcing water up the walls is familiar on the forum.

Gypsum doesn't stand up to water anywhere near as well as lime.

Your proposed solution sounds good to me. How are you going to decorate? Limewash? Whatever you use will have to cope with moisture, so modern emulsions are out, as they'll peel off.

The colour of limewash will deepen where the wall is damp, but the variation is part of the charm, when you're going for the traditional appearance.

 

[inwood]

If you're lime plastering onto the remains of cement remember that that area will have virtually no suction. Last week I lime plastered (finish coat) a section of tanked wall and it took about two days for the first coat of lime to stiffen sufficiently before I was able then apply the subsequent coats. If you have a mixture of cement and stone backgrounds put a thin finish coat on the cement, leave it overnight to stiffen and then follow up with first coat on stone (making sure fully damp). When this has pulled in apply a second coat to the whole area.

 

[Ian Bond]

Good point about suction inwood, a thin scat coat would be able to cure much quicker than a thick layer of plaster. Perhaps that's what your finish coat method achieves?

I'd imagine that when spraying up the cement covered will, the water will mainly run off, instead of being sucked up as it does with stone / lime render.

I was interested to read about your tanking project, and read up the details on Mike Wye's website. If I'm reading this right, the Thoroseal guide says that you apply it directly to the cleaned up substrate. Did you do it this way and then render over with lime, or apply it over an existing render? Either way, it is encouraging to know that despite a bit of crazing you managed to apply the lime finish successfully.

Your experiences with using this will be very valuable to anyone else with similar problems of high external ground level and penetrating damp.

 

[inwood]

The room where this was applied was originally some sort of washroom - literally. There was a huge copper tap sticking into the room by about two feet and clay brick gullies around the perimeter.

Since there was none of the original plaster left (well about one square metre which almost jumped off the wall when I approached it) I'd already applied haired lime scratch and devil coats to all the walls.

Thoroseal tanking was applied to the devil coat in two thin coats (comes in two different colours - white and grey so you can tell the difference when applying the second coat). It's a pig to apply (by thick brush) and sets hard overnight. I applied it in two directions up/down and then across. Since it's applied by brush you end up with a very rough texture so this acts as a mechanical key. In hindsight I would have removed some of the sharp highspots before plastering the finish coat as these either broke off and dragged through the lime or always caught on the trowel.

I then applied the finish coat over the top. My mistake was to apply two thin coats across the whole wall. I then went to apply a third into this without waiting long enough for the first two to pull in. This explains why it took five days before I could trowel the lime on the tanked area.

As I mentioned in my post the best approach (apparently) is to apply a single coat of lime to the tanked area and wait overnight for this to pull in before then treating the whole wall as one. I'm off on hols for a couple of weeks so I'll advice how I get on when I return.