New damp proofing failed - replastering help needed

[Mike]

Hi Long story but i'll try to not to go on for ever!. We had a 'pro' in who injected DPC and re - plastered bottom metre or so of our ground floor to solve rising damp. 6 months on we still have some damp and have now given up on trying to get hold of the guy that did the work. This is our first house and we simply cannot afford to pay anyone else so need a DIY fix. To be fair the job is not an entire disaster as it has worked in areas where cement / sand undercoat was used. We opened up a chimney breast before dampwork was done (to about 6 feet for a cooker) and for some reason the guy has used plasterboard in this area plus in one other small area of wall. We now appear to have penetrating damp in these plasterboarded areas. you can clearly see damp circles where the dabs are holding the plasterboard to the wall which get noticably worse on wet days and salt crystals have formed on the surface of these patches. My guess is that the DPC is fine and the plasterboard is 100% the problem (i know chimney breasts are problem areas but the other area of plasterborded normal wall has the same problem and either side of it is fine) I'm guessing i need to rip out the plasterboard and re plaster as per the rest of the house. Looking for a blow by blow run down of how to do this, in particular how to treat / prime the brickwork, ratio of mix for the undercoat and which damp proofing additives to use in the mix (also not sure if i need to use additives in the skim coat? Thanks to anyone who got through that lot and any advice will be much appreciated.

[meow2222]

I recommend you do some reading first. The problem needs to be understood more clearly if its to be solved.

[cynic]

The sand cement render would contain a proprietory damp/waterproofer and form a containment barrier. As long as this containment also crosses the chemical dpc it "works" Exactly why the numpty didn't use the same technique in the problem areas is beyond me. Your idea is exactly right as to the way to deal with the problem. It is possible that the new dpc is now exacerbating the original problem as the damp is now rising in a concentrated area. Regarding the preparation, chip away any old render or plaster as this will wick the moisture upwards past the dpc if left in place. (I assume he "did" put a dpc in the problem area???) It should be perfectly fine to get adhesion to freshly chipped surfaces without using pva or other adhesives. Sand and cement form a perfectly good adhesion to brickwork as you can see by a properly built brick wall! Don't forget to use waterproofer in the render mix. If there is a problem with getting a starting adhesion "spatter" the wall with mortar and let it dry (and stick) then use this as a key for your render coat.

[The Natural Philosopher]

My guess is yo are wroing, and the chimney isn't properly proofed, and never could be,which is why he used plasterboard, knowing it would last long ENOUGH before you noticed and stopped the cheque.

If this is an *outside* single brick wall, then this may work..rising or penetrating damp can escape outwards and as long as you waterproof rendering goes up high enough, it will have gone before it gets inside.

What worries me is the word 'chimney' - is it capped/vented? It may be rain coming *down* the chimney, in which case you may bed up sealing it in... Another solution for outside walls is to stick up a vapour barrier (poly sheet) and studwork with insulation and dry line with foil backed board. As long as the timber is away from the wet walls, it won't rot, as long as the walls are not waterproofed OUTSIDE to prevent them breathing. Actually it will still work then,. but rising damp plus frost may make a nonsense of the bricks..

[The Natural Philosopher]

Indeed, but that side understand it even less..

So do the reading elsewhere.

[Andrew Gabriel]

Chances are it wasn't rising damp (almost never is), so it's not surprising that it didn't cure the problem.

Yes, sand/cement/waterproofer scratch coat is a good barrier against penetrating damp, so it looks like you may have had that. You should also investigate the cause of the penetrating damp and try and fix that. If moisture is getting in to the outside of the wall, it will deteriorate.

Was the fireplace damp when you opened it? They often are. If so, this should have been left opened and ventilated for a few months to allow the brickwork to dry, before applying any finish.

Yes. But when you've stripped it back, leave it for as long as possible to allow the brickwork to dry. Also, you need to investigate why you have penetrating damp -- what's on the other side of the wall?

PVA the brickwork with watered-down PVA. If the brickwork is very absorbant, do two coats, allowing first coat to dry, but do the final coat just before you start putting on the scratch coat plaster so it's not completely dry.

A good scratch coat mix is 1:1:6 cement:lime:sand. Add a mortar waterproofer to the water (or a combined plasticiser/waterproofer). Make sure any metal edging strips you use are a type which won't rust (same for any nails which hold them in place temporarily). Devil-float (score) the surface of the scratch coat when it all on and flat to give more key for the finish coat.

Use standard multi-finish gypsom plaster for the finish coat. Apply 24-48 hours after the scratch coat, whilst it's still damp, without using any PVA. If you leave it longer and the scratch coat dries, you need to plaster as for a reskim, which means PVA'ing the wall first. Do not allow the finish coat to bridge the waterproof scratch coat layer anywhere, as it will wick moisture through.

[The Natural Philosopher]

If its a property pre 1900, with no DPC at all, chances are that it almost always IS rising damp, for which injection is an excellent remedy.

Post the period, your statement is largely correct.

[mail]

On 2 Sep, 11:38, Mike wrote:

Dear Mike You were right to put the "Pro" in inverted commas as he most certainly was not what is implied by the term! The most sensible reading you can do is on the BRE documentation. Some websites are quite good and you could try that of RTS on

Period Property one is reasonable in its technical content but you may get a bit lost in the purist parts some of which may not be apposite for your problem. As I understand it and I am happy to be corrected you have dampness that might be on of three possible sources a) rising b) hygroscopic due to the burning of fossil fuels (SO2 to SO3 to sulphurous acid to sulpuric and then sulphates) c) lateral penetration My guess is that it is likely to be a mixture of a and c First Check the external ground levels and make sure the existing dpc (it has been part of building regs since 1886) is 150mm above ground levels and lower ground if needed Second Get rid of any plasterboard and dabs which will be gypsum-based and a direct coduit of damp and hygroscopic salts in the wall Rake out any mortar 20 mm internally check that the dpc has been injected in the correct location AND in the mortar not brick See the BS on this or the PCA (ex BWPDA) Code of Practice as to how and where to inject a chemical dpc If not - RE inject the mortar (I suggest it is easy to DIY with Dry Zone from Safeguard Chemical You can easily do this yourself Then replaster in a sand cement render 3 to 1 with a silicatious rather than hydrophobic water "proofer" - I suggest SIKA No 1 Two (thin?) layers of render followed by a thin finish coat of multi finish will do the trick Try to overlap with his existing render by angle grinding a half lap join and using Feb SBR glue to effect the join Any questions - feel free to come back If you were to send a photo I could be more specific Chris George

[Mike]

Hi, Thanks for all the help so far. Most of my questions answered but should have given a bit more info in the first place:

1870s terrace with no existing DPC. Pretty convinced myself that original problem was rising damp because the bottom 2-3 feet of all downstairs walls were visably wet with plaster falling off, rotten skirtings, doorframes etc and no damp problems anywhere else in the house. Also all areas where cement mix was used are now fine, only plasterboard areas affected ( and this included a piece of board right in the middle of a wall which is fine apart from the boarded section. The problem chimney breast has had DPC injected. Also, our other chimney breast which board was not used on is fine. I'm still feeling that the plasterboard is the problem here and just wasn't suitable for the job. No idea why job was done in this way but suspect it may have been because they had used all the sand they had or that side-kick who did the boarding wasn't able to do plastering. Anyway, it would make sense to me to try and match the mix for new plaster to the areas already done. I'm sure that only sand, cement and liquid dampproofer was used, so i'm after a ratio of sand (builders?) to cement and a recommendation for which dampproofer to go for. not much plastering experience so any general tips would be helpful. Thanks again

[Mike]

Sorry Chris, Posted last effort before reading yours. Have read some of the suggested stuff before and my guess would have been option B, because there is no damp in any direction around the problem areas, the DPC appears to have been done properly, internal wall, problem is all over where dabs are rather than just bottom couple of feet, and salt crystals are clearly to be seen on the damp patches. The chimney was very sooty and damp when opened up and probably not given long enough to dry out so i'm thinking dabs have leeched salts into board which now suck in moisture on wet days. Does that sound plausable? If the cure is the same i guess the cause doesn't matter so much - if proposed replastering will sort the problem either way? Cheers

[Rick]

Have you checked if your cavity (if you have one) is full of stuff ? Take a brick out, and have a look. Its an simple, but time consuming job to clean it out.

I am using a render, with Sika-1 to damproof my cottage, its easy to do - the materials are not cheep. There are alternative render additives, if you are above ground level.

Rick

[John Rumm]

Many places of that age (and earlier) will have had a slate DPC, which is usually very effective. Much depends on the area of the country and the style of construction.

IME, rising damp comes after a long list of more probable causes.

Penetrating damp from splashback against the wall over the existing DPC is more common in solid wall properties of that age, and there are plenty of other things to check (pointing, rainwater goods, ground level outside, airbrick obstruction etc).