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Plastering after damp proofing (and another house renovation blog)
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# 1
Jonsend
Old 03-02-2010, 7:02 PM
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Join Date: Jan 2007
Posts: 6
Question Plastering after damp proofing (and another house renovation blog)

Hi all,

We bought a repossessed house in December knowing about damp and woodworm issues (this is our first house and also a massive project). We've just had it sprayed for woodworm and a chemical damp proof course injected.

About 1m of plaster was removed from the walls all round the outside walls for the course to be injected and I am looking for some unbiased advice on re-plastering(. All the sites seem to be selling one type of salt inhibitor/waterproof additive or another).

From what I can tell I need to leave it for a couple of weeks first (though no central heating yet so maybe longer), then apply in layers 3:1 or 4:1 mix of washed rendering sand, cement and some of the additive stuff (is this necessary?) at about 1/2" thickness at a time then leave it a few months to dry out.

Does this all sound right or are there alternatives such as the damp proof (foil backed) plasterboard and how would this be fixed/flush with the plaster above etc? I really have no idea so any help is appreciated.

Here's the blog if anyone is interested (it wont let me post links)

jonsend.blogspot.com

Jonathan
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# 2
David Aldred
Old 03-02-2010, 8:44 PM
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Join Date: Nov 2009
Location: Thornton Cleveleys, Lancashire
Posts: 366
Smile Damp proof course re-plastering

Hi Jonathan,
There have been similar posts on this issue before and the following points as previously mentioned by myself may be worth repeating for you to consider:

The wall should be left as long as possible before re-plastering to aid drying down. However a drying down rate of 25mm wall thickness per calender month is the best that can be hoped for so an external wall will take months if not over a year to dry down and you cannot practically wait that long but the longer the better.

I appreciate that you might want to re-plaster yourself or get a plasterer you know to do it for you as a cheaper alternative to the price the damp proof course (dpc) contractor may have quoted. However you need to think long and hard about the pro's and con's of somebody else other than the dpc contractor doing the re-plastering.

Chemical dpc's where they are justified are a system that relies heavily upon the re-plastering system meeting its design function of holding residual salts within the wall and some degree of dampness that will continue to rise up through the wall due to the inherent limitations of the chemical dpc system. The manufacturer of the dpc system will issue a specifcation for the re-plastering which the dpc contractor should have issued to you if a third party is going to undertake the plastering. If they have not done so contact them to obtain such and do not under any circumstances take advice from the forum posts about the specification. It needs to be word for word perfect to that issued by the dpc manufacturer or any claim at a later date will fail.

If you look at this specification for re-plastering from the chemical dpc manufacturer you will see that preperation of the substrate and the re-plastering requires a high standard of workmanship. For instance typically the mortar beds should be raked out at least 15mm for sand / cement base coats and there are lots of other things within the specification I will not bore you with - If you are not doing this yourself you are hoping the plasterer you choose will work to this specification but probably you will have no idea if they will have done so or not. It is unfortunate that many plasterers will disregard the specification and simply re-plaster the way they always have.

The plasterer wants paying whilst the wall is still wet and if it fails to dry down the plasterer will blame anyone but themselves. If you go back to the dpc contractor they will deny liability and blame the plaster / plasterer since if it is damp they will argue and often quite rightly that the plaster / substrate preperation is not to specification. Remember the plaster could initiallly dry down then begin to fail at a later date - even years down the line and think of all the hassle that would be to correct. Because of this the recommendation is always for the dpc contractor to re-plaster no matter what the cost since the liability for the complete system is then with the dpc contractor and you are not bouncing damp claims between different contractors.

There are alternatives to sand and cement base coats which have better insulating qualities and are more sympathetic to the original construction but some contractors find problems with these plasters and they are not suitable for all substrates but also neither is Ordinary Portland cement suitable for all substrates. There may also be a requirement under the Building Regulations to use an insulating plaster depending what % is being re-plastered. If the dpc contractor is not going to re-plaster it at least have your plasterer sign that they have prepared the substrate and applied the plaster all in accordance with the dpc manufacturer's specification issued to them so at least you have some documentation shoudl a claim arise at a later date.

Wet re-plastering introduces a lot of water into the property when the aim is to dry the property down not make it wetter. You can dry line the areas instead of re-plastering but there are advantages and disadvantages with such an alternative:

Advantages are you as a layperson can see a physical barrier going on the wall and you are not guessing whether a plaster has worked to specification plus there is far less water introduced with dry lining. To cold external walls you can also include insulation with the dry lining system.

Disadvantages are the inevitable increased thickness over standard re-plastering and most of all the system must have an effective vapour barrier or you will be allowing warm moist air behind the system where it may simply condense out and trickle out from the base of the wall.

Setting up a semi closed void against a wall where it is already damp and will to some degree continue to be damp will often cause the moisture vapour to rise within that void until it becomes a saturated enviroment and you need to appreciate how to deal with the risk of this issue.

If your dry lining system is only up to 1m high then not only will you have to work out how to make a feature of the different thickness between plaster and dry lining but there could also be problems where the moisture vapour behind the system starts to cause damp to the original plaster where it meets the dry lining at this joint. For this reason and that of making insulation over a wall height consistent insulated waterproof grade dry lining systems incorporating vapour barriers are better undertaken to fall wall heights which adds to the cost of the project.

Standard grade dry wall adhesive is hygroscopic (attracts moisture form the air) and should not be used to fix plasterboard against damp walls under any circumstances unless you want nice damp circles to look at in the room when finished.

You pays your money and you makes your choice - please have a look at my other posts on this forum regarding damp, cavity wall ties, cavity insulation and woodworm which may be of use to you - hope this helps kindest regards, David Aldred Independent damp and timber surveyor.

Last edited by David Aldred; 03-02-2010 at 9:20 PM.
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# 3
Jonsend
Old 04-02-2010, 3:47 PM
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Join Date: Jan 2007
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Thanks very much for your help David. I've been in contact with the company and they are sending through the required spec, I'll update when it comes through.
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