Damp proofing - Are they doing it right?

Jon_Toast

Hi,
I'm currently having some damp proofing done and I have a question about the method being used.

We are having the old plaster knocked off, a plastic membrane installed, damp course injection, and then re-plastered.

The house is stone built and so the walls are pretty uneven, in one room this caused long delays while the membrane was installed. I'm not sure why, but this is what I'm being told.

For the next room they would like to knock back the old plaster, render the wall, install the membrane, and then plaster.

Is this a valid method? I can't help feeling that the membrane should go direct on the wall, however I've read a few of the membrane product leaflets and they seem to say that installing over render is fine.
Can anyone confirm!?

Thanks.
JT

littlesnuggy

When you say plastic membrane do you mean a cavity drain membrane (one with studs on the back which go against the wall)?

If they're installing that, then you do not need an injected dpc as well.

These membranes can be fixed over defective old plaster (to reduce mess/time) but you then shrink the room dimensions, so normally you would remove the old plaster, fix the membrane and then if it has mesh on it, plaster straight onto the membrane or dot & dab plasterboard over it, or if it doesn't have mesh fix battens & plasterboard.

There is no point putting new render on behind the membrane as it won't be seen and is not needed.

Who's doing the work and where did you find them?

Jon_Toast

Thanks for the reply.

The membrane has a mesh front, and a sort of bubble wrap style backing, so I guess that would be cavity drain.

I believe that the DPC injection was something that the manufacturers wanted in order to provide a guarantee. To be honest I'm not too concerned about that, I'm happy to have both, even if not strictly needed.

The reason given for rendering behind the membrane is to provide an even wall to fix the membrane to. If as you say below that its possible to use the membrane over old paster then it sounds like this would not be a problem.

I realise that it may not be the best way to do the job, but I've have a few issues, and now I really want to get it finished, and providing that it will not compromise the damp proofing, I'm happy to let them render under the membrane.

Right now I'd rather not name the company involved.

Thanks,
JT

dampdaveski

Hi Jon,
If the wall is really uneven, this implies that it is relatively poor condition. the reason they will want to put a render coat on is 1) to straighten the wall up as they've already indicated and 2) to allow for a decent fixing with the mesh membrane.
this in my opinion (as a preservation contractor/surveyor who is a member of the PCA) is perfectly acceptable
It is definately worth having the wall injected as well as the membrane being fitted, if you didnt this could cause problems in the future, fitting a membrane virtually eliminates any evaporation of moisture from the actual wall, therefore it is not impossible for rising damp to continue higher up the wall than it otherwise normally would.

The height that rising damp can 'rise' to is determined by the water table, evaporation rate from the wall and what the wall is made from.
Cavity drain mesh membrane systems applied above ground are a very effective way of controlling aggressive ground salt migration problems in the futre
Sorry for the waffly reply, but I hope this helps
good luck

DD

thebaldwindowfitter

most stone walls are uneven on the inside of the house if like you say they are injecting stone first ,then cement render to straighten the surface (with waterproofer in mix) then applying membrain and plaster this is the correct way to install.one thing i would add is make sure they dont plaster to the floor ive known some plasterers do this

olias

Surely if they are going to the bother of rendering the surface first, then they may just as well have tanked it and not bothered with the injection or the membrane? or am I missing something?

Olias