Plastering damp proofed walls

In message , John writes

Strong chance. I remember having some dampproofing done, getting the wall plastered by someone else, then calling the damp proof firm back when the wall got a bit mouldy.

They quickly pointed me to their advice and specification for the plaster mix on the back of their guarantee - clear as day!! Did I feel like a prat??

I seem to remember it is some kind of cement mix with waterproofing additives (lets water vapour out, but not water), then plaster on top for the final skim.

Never had it properly explained why this is necessary if their damp proof course was any good... It is called damp *proof*, not damp resistant course.

Bob

There were some special plasters for damp walls, but I'm not sure if they exist anymore. The company's guarantee will stipulate the type of plaster required. If it doesn't, they're pulling a fast one.

Most chemical damp companies are a scam anyway. They misdiagnose the cause of the damp in the first place, so the treatment doesn't fix it.

Scratch (base) coat of sand and cement (and lime for a lime mortar building) with waterproofer (and plasticiser if you like).

You can use a normal plaster finish coat on this, but avoid getting any of the finish coat bridging across the waterproof scratch coat (e.g. at the bottom edge).

Sovereign rendermix is the stuff:

What was the initial cause of the damp?

Hm. I've often wondered whether it's the new render that stops the damp, rather than the dpc injection. IMO cement boards are an easier d-i-y option. They bond to brick well with plaster adhesive and there's no drying time. You can stick a few long screws in to be on the safe side, or maybe use mortar as the adhesive.

Most damp proofing companies seem to insist on doing the plastering themselves to ensure that the correct stuff is used. There are all sorts of plaster and they will have chosen something very specific as part of the treatment.

Christian.

Few more facts please: Was the work done when you bought the house? What did the remedial company specify for re-plastering? Was this specification adhered to or was the work 'dual-responsibility' with the dpc company doing the (presumably) injection and someone else doing the re-plastering?

An injection dpc is not a cure for rising damp. It is part of a system which also involves replastering. The new plaster not only prevents moisture and salts from migrating to the surface, it also ensures residual hygroscopic salts are removed. If these are'nt removed then you will forever have damp patches.

There are many kinds of base coat which will work, the one recommended by BRE (defect action sheet 86) is 1:3 cement sand). There are other proprietary mixes of course which may have been recommended by the dpc company. What will not work are light weight gypsum plasters such as Carlite browning, weaker sand cement mixes and those with lime in them. These all make for a more porous mix.

If you have one of the former then the dpc system should work, if you have one of the latter, it won't.

To find out for sure you'll have to get the plaster analysed. If you don't, you're never going to find out the truth. If you want to do this, email me directly.

(IANAL)

As for the legalities if you get into an argument: if you have a base coat which is normally acceptable but it isn't the one the dpc company have specified, you may still have a case for asking them to re visit. I would guess however that you'll be flogging the dead horse because BS6576 (dpc installers credo) says it (re-plastering) should be to the installers specification.

Dual responsibility is a recipe for disaster whereas employing a remedial treatments company in the first place was simply a dreadful mistake!!!

HTH

Xav