Chemical DPC above or below existing DPC...

[paulbandler]

I have a rising damp problem that is almost certainly due to the bridging of the original DPC by hard render. This is to be removed, bellcast reformed above exposed DPC and bare bricks repointed below. However the builder suggests a 'belt-and-braces' approach of also injecting a chemical DPC below the original DPC, even though it appears in-tact as far as one can tell.

From what I've read the 'state-of-the-art' of chemical DPC's are the cream based ones that are injected into the mortar, so if I were to use this then to avoid disturbing the existing DPC I'd need to inject in the mortar bed one level below the existing DPC or alternatively one level above.

Does anyone have and recommendation as to whether injecting above or below the existing DPC would be most effective? Also any suggestions as to whether the 'belt-and-braces' approach of having both is sound?

 

[Gervase]

Neither.

To be honest, having an injected DPC is likely to be a complete waste of money, even a 'state of the art' job injected into the mortar. All chemical DPCs are supposed to be injected into the mortar. The majority don't work. If you have a DPC and the damp as caused by mechanical bridging (and cement render) then there is absolutely no need for a chemical DPC; you would be throwing money away.

If you have any doubts, get hold of a copy of Survey and Repair of Traditional Buildings by Richard Oxley, a well qualified member of the RICS and a respected independent historic buliding consultant. He very effectively debunks the injected DPC industry.

What sort of wall have you got - is it cavity brick or solid? What age is the building and what sort of foundations have you got. What's the ground like around the house? Any one of these could be as relevant to the dampness as another, and depending on the answers, replacing the cement render may simply perpetuate the damp problem.

 

[NT]

Your builder is ignorant or worse. There is no sense at all in injecting a dpc. Your problem is not likely to be rising damp. And even if by some freak chance it were, injecting would still be a waste of time and money.

NT

 

[paulbandler]

Thanks for the feedback. I'm a little taken aback by the categoric nature of the assertions but appreciate and respect your views. I've read quite widely regarding the questionable effectiveness and necessity of a chemical DPC and can relate to those concerns as I have had it installed twice before on other properties. My own 'distillation' of that input is the view that it has some effect albeit often mis-sold.

The walls are 9" brick walls, no cavity, 1920's build - not certain of the foundations but can identify a 'spreader' (not sure if that's the right term) course of bricks just below the ground level.

The injection cream looks straight forward to install and the materials would cost about £200 for ~30m of the perimiter according to the manufacturers info - so not a huge investment in the big scheme of things. Given that the below DPC brickwork needs repointing anyway if it can help even a little by supplementing the existing DPC then it seems a prudent step at this stage.

So again, given that I wish to do this, can anyone advise whether it is better installed above or below the existing (mostly in-tact) DPC?

Thanks

Paul

 

[Nemesis]

I'd simply remove the bridging and allow the wall to dry.

What is the existing wall built from - lime or cement?

What is the existing DPC?

 

[Lime]

The render outside is preventing the wall evaporating the water inside it.
(called breathing by many people here)
Once the DPC is unbridged the wall will have to dry out via the inside wall surface.
This will take some time.
Adding a chemical DPC is not really going to change things that much, however what it may do is trap water in the wall.



I remember when chemical DPC became popular.
At that time it was asserted that it was to replace the DPC constructed with a woven felt as this felt was known to degrade and rot away sometimes.
Now it is used as a catch-all for damp everywhere.

Does a chemical DPC work?
IME no. Often where there is a long-term problem of damp there has been a chemical DPC inserted at some time past which has not stopped the damp. Sometimes drilling through the DPC (particularly slate) has made the situation far worse causing much damage.

no matter what is asserted about it, a chemical DPC always takes the line of least resistance.
If the chemical has wet brick on one side and dry the other it will penetrate into the dry brick and not the wet even under pressure.
An old brick wall is not a homogeneous substance and therefore is never treated equally throughout.

 

[Penners]

Paul - leaving aside the question of whether a chemical dpc would actually work, I'm puzzled as to why you would need it.

If the mechanical dpc was bridged, then you have a clear-cut reason for your damp, and a straightforward way of resolving the problem, by eliminating the bridging.

It's your money, but personally, I can't see the point of the extra cost.

 

[paulbandler]

I'm puzzled as to why you would need it.

Because I feel there is some (albeit limited) risk that the existing DPC may have deteriorated in places (it is very 'friable' - if one can use that word for it) in areas where it is exposed. It will be much cheaper to do it now when the repointing work is being done and I have tradesmen on site, than to leave it, suffer the effect of damp on new decorations or joinery later and have to fix that and inject later.

If the chemical has wet brick on one side and dry the other it will penetrate into the dry brick and not the wet even under pressure.

I believe the low-pressure injection of cream based DPC's relies on the water in the brick to dissipate so maybe an improvement in this respect?

What is the existing wall built from - lime or cement?

The mortar is quite soft and lime based I believe. The internal plaster is/was lime, but the external render seems cement based - certainly the low plinth that was bridging the DPC is a hard cement based render.

What is the existing DPC?

It seems to be a black, thin slightly bitumenus material that has become quite brittle and breaks very easily where some bricks have become dislodged (another story), or where from under the internal floor I can get my hands on the bits that protrude from the brickwork.

 

[Gervase]

The tarred paper DPCs are brittle when they're not held in a sandwich by the courses above and below. that doesn't mean to say they're not perfectly capable of doing their job.
I have, for my sins, installed injected DPCs, and I can assure you that they very rarely work. And, when they appear to have worked, it's because other remedial work has been done which has rendered them superfluous.
The chemical will always take the line of least resistance in the mortar, following pores and fissures, a process known as 'viscous fingering'. The damper the wall, the greater the risk of this, so if you must have an injected DPC, use a gravity-fed delivery rather than any pressure.
And try to find a company that has been around for at least five years, is a member of the Property Care Association (an offshot of the BWPDA) and ask them for a 25-year guarantee on the work (as is; without additional cement-based plasters, renders and waterproofing coats applied to the walls) backed by Guarantee Protection Insurance. I'd love to hear their response!

 

[Penners]

I have, for my sins, installed injected DPCs

We-e-e-ll! Now the truth starts to come out, eh?!

 

[Gervase]

Honest guv, I was only obeying orders. And this was after I told the bloke how bloody useless they were, but it was part of the architect's spec and he was terrified on not gettig signed off by the BCO if he didn't do it on all the buildings in the scheme.
It was interesting, however, to see the 'line of least resistance' theory actually working as the stuff dribbled out here and there and pooled in expensive puddles on the floor.

 

[Matt Green]

The chemical will always take the line of least resistance in the mortar, following pores and fissures, a process known as 'viscous fingering'. The damper the wall, the greater the risk of this, so if you must have an injected DPC, use a gravity-fed delivery rather than any pressure.

The silane/siloxane creams work by diffussion rather than gravity feed or pressure. They were originally designed to waterproof German bridges.

In that respect, at least, if you run a watering can over the wall afetr you install the DPC the wall surfcace under it beads water rather than absorb it. Which shows it waterproofs walls, though not that it stops capillary action. Looks good in the demonstration though. :roll:

Really, I'll join in the throng and chime: Don't do it- you'll cause damage to your house that no one will be able to reverse.

And try to find a company that has been around for at least five years, is a member of the Property Care Association (an offshot of the BWPDA) and ask them for a 25-year guarantee on the work (as is; without additional cement-based plasters, renders and waterproofing coats applied to the walls) backed by Guarantee Protection Insurance. I'd love to hear their response!

Definately! I reckon the caveat would read longer than the 'report'.


Matt Green

 

[paulbandler]

you'll cause damage to your house that no one will be able to reverse.

What actual damage will it do if I inject in the course below the existing DPC?

rather than gravity feed

What is an example of a gravity fed DPC?

 

[Penners]

What is an example of a gravity fed DPC?

Instead of injecting the chemical under pressure, each drilled hole is fed with a steady trickle of liquid from a bottle (by gravity).

 

[Lime]

What actual damage will it do if I inject in the course below the existing DPC?

Apart from a few holes, not a lot.

However my experience of the workmen doing the job is a very different one than that painted by the sales rep/"expert".

The installers/workmen will believe that their process is replacing the original DPC and therefore will think nothing of damaging it within the wall.
Once the drill has shattered the slate ANYWHERE the old DPC is damaged beyond repair.
If you try to intervene you will be told that they are the experts and you can *****.

 

[NT]

Thanks for the feedback. I'm a little taken aback by the categoric nature of the assertions but appreciate and respect your views.

I guess I just have a different way of putting the same information. DPC injection is a question that comes up over and over, and I think we could all benefit from a FAQ rather than rewriting parts of the book on every thread.


NT

 

[paulbandler]

Apart from a few holes, not a lot.


Thanks for that reassurance.

Once the drill has shattered the slate ANYWHERE the old DPC is damaged beyond repair.

Good advice no doubt, but I'm going to DIY it- well kinda. Am going to buy the materials and get my 'friendly' gang of Polish tradesmen living on-site under the (hope) watchful eyes of the builder running the project who seems to understand the merit of retaining the existing DPC in-tact but thinks it worth providing additional support.... Me naive - nah... :wink: I know I won't get a guarentee this way, but I tend to think they're not worth much anyway. If you try to claim on one you first have to shell out about £70 for someone to take a bored sample from your wall for lab analysis to determine if there is really rising damp. Funny how that isn't necessary in the first place when they're selling the treatment to you isn't it...

 

[Lime]

Me naive - nah... :wink: I know I won't get a guarentee this way, but I tend to think they're not worth much anyway. If you try to claim on one you first have to shell out about £70 for someone to take a bored sample from your wall for lab analysis to determine if there is really rising damp. Funny how that isn't necessary in the first place when they're selling the treatment to you isn't it...

:lol: :lol: :lol:

Doesn't that say something about the whole business?

 

[Nemesis]

Ask Matt about taking samples - and which kind is actually best. And consider many think rising damp to be not quite as prevalent as damp proofing companies would have you believe.

He's posted on the samples and guarantee scam in the recent past.

http://periodpropertyshop.co.uk/phpBB2/viewtopic.php?t=6374&postdays=0&postorder=asc&start=15


The thing is - by sticking that stuff in the wall, you will possibly be preventing the damp from 'breathing' out from under the DPC via lime mortar as it is supposed to.

If you want to waste time and money though, clearly there is no stopping you.

 

[Matt Green]

Apart from a few holes, not a lot.


Thanks for that reassurance.

Well, hang on. You are also saturating original bricks with silicone which can't then be removed from them. Purism, possibly. Also, if you are cement plastering the inside of the building, the faces of the bricks are going to get damaged when someone removes it. Again, one man's purism is anothers needless vandalism.

If you try to claim on one you first have to shell out about £70 for someone to take a bored sample from your wall for lab analysis to determine if there is really rising damp. Funny how that isn't necessary in the first place when they're selling the treatment to you isn't it...

....so why not find out if your wall is actually wet before you throw chemical into it and perpetuate an industry responsible for the irreversible damage of a vast amount of..... Period Property. :?

Come along that man: be part of the solution.


Matt Green