Friday, March 12, 2010

When your lime plaster cracks

Hi some one was asking about cracks, so I'll have a go at trying to explain. Without getting too scientific about it, cracking is due to the plaster loosing water in the first step of curing. You want to aim for it to slowly loose water, in order to minimise shrinkage. So the first step is thoughly wetting the background. This is best achieved with a mister type spray. You spray repeatedly until the background can absorb no more water and the spray just begins to flow down the wall. If its a dry or windy day, or the background is very porous, such as old bricks or cob, this will have to be done again and again. The plaster can pull moisture from the wall as it dries, and that will result in delamination. Secondly the plaster should go on as evenly as possible, so that thin areas dry at the same speed as thicker areas. If the plaster is made from lime putty as opposed to hydrated powered limes, the plaster will have to loose a lot of water and will shrink. This is why plaster with lime putty is made up months before it is used, and also why traditionally some type of fibre is added to resist shrinking and cracking. Thirdly, it is important to keep an eye on the plaster after it is applied. It helps to protect it against wind and sun, (and rain!) Again to slow down the drying. It's a good idea to very carefully mist the plaster when you finish and possibly again if you see tiny cracks appearing. Finally tiny cracks aren't the end of the world, lime is somewhat self healing, and a second coat will help.


A short history said...


Very interesting site. I am currently renovating a stone barn in the Lot-et-Garonne in France.

I would love to use a hemp/lime mix to insulate the internal walls.

I think I have identified a local source for the hemp. I now just need to work out the technique.

I wondered if you could help or point me to a site where I could learn the ins and outs of insulating the place.

Many thanks

Gerry O'Neill

toffelnigar said...

The typical homeowner will try to fix concrete crack repair with caulk. But this is only a superficial crack repair. Water will fill the inside of the crack and cause efflorescence, which will eventually loosen the caulk. In couple years, the caulk will start peeling. An alternative foundation crack repair method is to excavate and patch the crack on the exterior. This may not be permanent because all surface crack repairs will eventually get loose or crack.