Need for chemical DPC once root cause removed?

[paulbandler]

I have just aquired a small 1920's detatched house that requires extensive modernisation (including adding central heating and rewiring, new bathroom, etc). It is empty and I will not move in for another 2 months so I want use this time to undertake as much disruptive internal works as possible

Whilst generally in good condition for its age, it has some damp in lower walls that I want to address as completely as possible. I spent much time reading this and other sites on the net about damp, and have also had a survey and quote from a local damp/rot renovation specialist (who unsurprisingly recommended a chemical DPC and replastering). I am willing to invest in this if it is the best/safest option, but I am still unclear and confused by some things, hence this post...

The property is rendered. Beneath the bellmouth is a particularly hard smooth cement based render that extends to the ground in places. Concrete slabs abut the wall and in some areas these slabs are as high as the DPC. In some areas the hard render is clear of the ground for a few inches and there is evidence of earlier repointing of the exposed brick at the base. Yesterday I went looking for a DPC by lifting some floor boards close to the wall where damp is evident and cut off some of the external lower hard render. From the inside I found what I think is a bitumen impregnated flexible material protruding from a course of bricks above the air vent that I take to be the DPC. From the outside I found a corresponding black line at the same level - i.e. I think original the DPC is there where it should be but bridged by the hard render which in some places extends to the ground.

It seems clear that it would be a good idea to remove the concrete slabs that abut the wall and install a french drain to stop the damp 'wicking' up into the render. But what else...?

Q1. Should I remove the hard render to stop it bridging the DPC? The small area I removed yesterday using an angle-grinder to make cuts and then block bolster was not easy and damaged the face of the underlying bricks. I've read elsewhere of concerns over such an excersize (although also found that an SDS chisel hammer was effective). However there are some areas where this hard render can be pulled off easily - indeed I did so and exposed a large area of rough bricks in needs some kind of protection - should I re-cover it with new render, re-bridging the DPC there...?

Q2. As an alternative to removing enough hard render to stop it bridging the DPC, would it be enough to remove just to stop it going right down to the ground?

Q3. Should any such exposed brick at the base of the wall be painted in a black bitchumen paint? I've seem this (well some kind of black paint) on the base of many house walls, but would it not have the effect of sealing in any damp?

Q4. It seems reasonably clear that the bridging/ground height is the cause which will be fixed, but is it still worth investing in injection and water-proof internal replastering? I only want to do this once and I need to make the decision now to make the most of the building being empty. OTOH, I'm wary of the internal disruption to the skirting and some plaster that is in otherwise good condition.

Thank you for reading this post, and thanks in advance for any advice.

 

[middi]

If the house has a DPC fitted, and presumably in good condition, your first priority is to make sure that this is not bridged inside or outside with any renders plasters etc..to create a capillary action for water then to sit and stay above the DPC.

If the DPC is doing its job , to inject anything in the wall is a complete waste on time and money.

Make sure there is a clean bell shape run off above DPC , that the render above this is not cracked and generally in good condition, including gutters etc..

As to the hard render below DPC it does not really matter what you wish to do, as whatever you do should not affect damp getting past the DPC above as long as this is clear and free.

You can remove the render below DPC and leave the bricks exposed if they are in good condition, just repoint and leave them as they are is probably the best solution .

If you paint or rerender these it will only be for your prefered appearance and will not improve anything. Similar to a french drain, this will achieve nothing if the DPC is working and damp should stop below this anyway.

 

[paulbandler]

Thank you for the reply - so quick!

Just to clarify - the 'hard' render goes from the ground (or nearly) up past the DPC line until it meets the bell of the main render. The height of this lower render before it reaches the bell varies due to the house being on a slight hill - it varies from about a foot to over 2 foot below the bell, but above the DPC. Should I make a second bell at the DPC?

I _think_ the main render is also of a cement form, but it has at some stage a while ago been covered in what looks like a spray-on top coat to give it a rough-cast appearence.

My major problem is time - I can't really afford to wait to see if removing the breaching and removing the paving stones will fix all the damp - I mean how would I know it was fixed? Would the upper render not tend to trap whatever moisture had made it in?

Thanks again for any futher thoughts...

 

[Gervase]

My major problem is time - I can't really afford to wait to see if removing the breaching and removing the paving stones will fix all the damp - I mean how would I know it was fixed?

The problem with nearly all damp remedies is that time is essential. bricks, mortar, stone and plaster all take weeks to reach a state of equilibrium when anything is done to change their relative humiduty/wetness. Ther is no way of knowing that a problem has been solved without waiting, I'm afraid.

It does sound as though the problem is down to the hard render. But, as to the original question about chemical DPCs, you could fork out for one if you are certain the damp isn't a result of the existing physical DPC being bridged.
I have to say that they rarely work, but one area where the success rate is slightly higher than zero is in brick walls. Make sure the chemical is gravity fed into the mortar, as this is the only way it stands a chance of working.
Too many cowboys simply drill holes in the brickwork, stick in an octopus of probes linked to a compressor and squirt the stuff in. That way is bound to lead to failure.

 

[NT]

quote="paulbandler"

> I have just aquired a small 1920's detatched house that requires extensive modernisation (including adding central heating and rewiring, new bathroom, etc).

While we're here, as well as fitting a new electrical system you have the option to leave the '20s wiring system in place, disconnecting the old fusebox from the mains and connecting it to a 12v supply instead. That way you get to keep all the ancient kit and can use table lamps with 12v bulbs and other small appliances on it if you want in future. Old sockets can somtimes be tatty with paint and scuffs but bakelite cleans & polishes up very well. Historic electrics are becoming increasingly rare today, and often have lots of character.

On 12v you can use items with the original wiring and plug on, and only change the bulb. Also a 12v supply can eliminate the use of many wallwarts.


> The property is rendered. Beneath the bellmouth is a particularly hard smooth cement based render that extends to the ground in places. Concrete slabs abut the wall and in some areas these slabs are as high as the DPC.

This will increase the water content of the earth as it prevents surface evaporation.


> It seems clear that it would be a good idea to remove the concrete slabs that abut the wall

maybe


> and install a french drain to stop the damp 'wicking' up into the render.

rising damp is very unlikely to have anything to do with the problem. It is hugely overdiagnosed, and rare at best.


> Q1. Should I remove the hard render to stop it bridging the DPC? The small area I removed yesterday using an angle-grinder to make cuts and then block bolster was not easy and damaged the face of the underlying bricks.

then alas no. Damaging the bricks starts the brick deterioration process.


> However there are some areas where this hard render can be pulled off easily - indeed I did so

this will provide areas for evaporation to occur.


> and exposed a large area of rough bricks in needs some kind of protection - should I re-cover it with new render, re-bridging the DPC there...?

Brickwork does not need protection If the pointing is bad, repoint in lime. If the brickwork looks awful, you can lime render it or lime paint it, lime is porous enough to allow evporation.


> Q2. As an alternative to removing enough hard render to stop it bridging the DPC, would it be enough to remove just to stop it going right down to the ground?

the more you remove, the more evporation will occur. But dont remove any that wont come off without damage, just take off whats ready to come away without a fight.


> Q3. Should any such exposed brick at the base of the wall be painted in a black bitchumen paint? I've seem this (well some kind of black paint) on the base of many house walls, but would it not have the effect of sealing in any damp?

this would prevent evaporation, so not whats wanted.


> Q4. It seems reasonably clear that the bridging/ground height is the cause which will be fixed,

is it? How high is your damp and how high does the ground height go? Condensation si many times more common.


> but is it still worth investing in injection

pointless. You already have a dpc fwiw anyway.


> and water-proof internal replastering?

counterproductive. Damp content of wall depends on the equilibrium between water in and evaporation out.

I get the feeling you think this is rising damp, but I'd say thats very unlikely.


NT

 

[paulbandler]

Thanks for the further input...

The outside ground is below the level of the DPC - at the worst point some concrete slabs are at about the same level of the DPC (which is hidden behind the hard render) but in the area where the damp is most readily identifiable internally the outside ground level is a brick course or two below the DPC. The internal damp is visible in patches around a chimney breast up to about 2 foot above the timber floor. The plaster seems most effected in the alcoves either side of the chimney breast. I pulled off some of the old paster here which was blown and the brick underneath gives a reading (well noise) on my simple resistance damp meter. The hard external render at this point will not readily come away - in fact it looks like its been repaired in the not too distant past and is well adhered to the wall and hard as nails.

So if its not a good idea to remove the hard render with force and if the bridging seems like the most likely cause of the problem then I don't seem to have any good options other than live with the internal damp for as many years as it takes for the external render to fall off - which I suspect will outlive me... But I do need to fix the blown plaster and don't like the idea of it going onto a dampish wall.

BTW I just this morning bought what I thought was an SDS drill and chisel from B&Q and tried it out to see if it would remove the hard render without doing too much damage following the advice elsewhere on this forum (angling it at 45 degrees). I couldn't make it work - the 19mm chisel would go in straight at 90 degrees but as soon as I tried to angle along the render I just couldn't hold it firm enough to engage and make any impression on the render. I'm not sure now I have the right drill - it is described as a hammer drill with an SDS chuck - not sure this is the same as an SDS drill - as you can tell, I've a lot to learn.... I'm back on the net looking for information on that one too...

BTW, I suspect the level of damp in this property is not as bad as others report on this site - there's no damp smell and the timbers near the wall look in good shape as far as I can tell. But there is some evidence of beetle burrowing on the joists in one place where I've lifted a floor board.. Just trying to add some perspective to my description.

 

[Lime]

Just to add that the outside ground level needs to be below the DPC by about 5 inches preferably more.
This is particularly so where there are slabs because rain readily splashes back up the wall from off them.

 

[NT]

Chimneys can suffer from salt contamination, which makes condensation more likely. Alcoves will suffer condensation more than the area either side, as they are thinner areas of wall thus less insulated.

The odds of it being rising damp are as close to zero as makes no practical difference.

High Ground levels will cause rain splash onto the wall, which will cool it and worsen condensation, as well as wetting the wall directly.

Damp meters dont measure damp, they measure resistance. Damp is one cause of lowered resistance, not the only one.

I think you may need to question some assumptions.


NT

 

[paulbandler]

I contacted the local damp-proofing company that surveyed the property earlier to clarify for me why I may need to install a chemical DPC if the cause of the damp was breaching of the DPC by the render. They indicated that it was because that is the only way the work could be guarenteed and accepted that it may not be necessary - and offered to come and take a second look now that I've opened it up.

However they also indicated that the likely build-up of contaminents (?salts) caused by damp would require the special replastering. So I have a second question for this forum if there are residual salts in the wall as a result of damp that has now been cured by removing the breaching - what is the appropriate type of replastering - the traditional lime-based that I hear so much about here, or the 'special' plaster mixes that these damp proof companies propose...?

 

[middi]

I should think the " special" plastering they are refering to is nothing more than a blo**y hard cement/sand mix with a generous amount of damp proofing in it.

Don't even think about it.

THe best thing you can do, as already suugested, give it time to dry out and if there isn't any more moisture that keeps feeding the problem eventually it will be ok .