Damp - Slate 'v' Modern DPC's, Cavity? Advice Please.

[Anonymous]

Hello there, this is my first posting on the forum so please be gentle..... : )

I have a 1901 brick built mid-terrace house, in a high ground water level area. The house has been injected with a chemical DPC two years ago (specialised no longer in business). I now know, that they’re not very effective, especially where the walls are damp (kind of defeats the object really).

As the damp has not stopped, I am now considering putting in a physical DPC, as a first step. However there are a couple of things that are proving difficult to find out about.

Are the walls solid or cavity? The bricks are 4 x 9 inch & as the courses show the ends of the bricks as well as the face; does this mean there is a 1 inch cavity? Or is this not classed as a cavity in its’ true sense?

In which case do I have to treat each side as a separate skin & put two separate DPC’s in or as one wide DPC that covers the full 9 inches?

Also is there any guidance or wisdom on the pros & cons of slate v modern DPC?


I would also like to install air vents high up in an under stairs cupboards to remove any moist damp air. So when installing the vent, again do I have to fit 2 separate air vents, 1 in each skin (at the same position of course) or 1 that is 9 inches deep?

As only part of the house has wooden floors & the rest is concrete (inside & out) is there a way I can make it easier for the damp below the DPC to evaporate?

I would appreciate any advice you can give.

 

[Lime]

The first question in my mind is can you reduce the high ground water level?
The second thought is that if your house was built in 1901 it almost certainly has a slate DPC already.
Is it covered up outside by soil?

 

[NT]

I thnk you may be barking up the wrong conifer on this one. Damp at the base of the walls is only rarely due to rising damp, if at all. Of all the approaches to sorting out damp, adding a dpc is probably the very last on the list.

For a rushed introductoin to resolving damp in old houes, check out the damp faq:
http://periodpropertyshop.co.uk/phpBB2/viewtopic.php?t=6777


NT

 

[Anonymous]

Thank you for the replies;

I hope to cover all the queries;

Lime, thank you for you reply you said

...reduce the high ground water level?

Unfortunately the high water level is due to years of the surrounding fields flooding from the near by stream (last time house flooded was over 40 years ago, to my knowledge). My internal & external ground level is the same as everyone else in the row, some have damp, other don't appear to have damp, but who knows what lies beneath?

... 1901 it almost certainly has a slate DPC already.

Having taking up the wooden floor to check & change any rotten joists, the only slate that was visible was the new I fitted to bed the joists on. As the ground is down approx 3 foot under the wooden floor I would have to say no slate DPC. Also no signs of wider mortar lines to allow where the slate DPC would be.

NT I promise I use your thread as a checklist (very useful thank you) & believe I have already checked for the other problems.

No signs of discoloured bricks due to unnecessary ingress, all gutters, window ceils checked for cracks & throats. Roof slate & flashings have been check & I had to rely on someone else, as I don't do heights, but I do trust them.

The damp is not in corners & have checked with the old tin foil routine, so condensation doesn't really seem to be the problem. The only real area would be under the stairs & that's why I will be installing a couple air vents.

I've checked all pipe work & pressure is not dropping on boiler, so not doing it when I'm not watching. :wink: Also all drains are clear & not signs of any damage, thankfully.

No peeing animals either, badgers & foxes prepare my grass to my walls, little devils. :roll:

No spalling on exterior brickwork or mortar. Mortar is however cement, which brings me back to evaporation. As internal walls have sand & cement plaster.

Also half of the internal floors are concrete & internal floor levels are roughly the same level as that of the exterior. Whatever they are, they are definitely not 6" above the external ground level. However as before, neither are any of the neighbours'. I seriously think it is an evaporation problem & I will undoubtedly have to deal with the evaporation issues. (cannot reduce external ground level as if flooding does happen it will be me who becomes the sink, so I imagine I will have to raise the internal floor level & replace the concrete floor with suspended wooden ones.)

Hope that has answered all the necessary queries, I'm sure I would have left something else out. Trying to cover everything, although it's beginning to look like an essay.

I’m extremely grateful for the comments so far & welcome anyone who can advise on the cavity, air vent & slate 'v' modern DPC queries. Also any help with evaporation below the DPC would be good too.

Catch up soon. Somerset Maid [/quote]

 

[Nemesis]

Not all DPC's were slate.

 

[Mr.C]

The bricks are 4 x 9 inch & as the courses show the ends of the bricks as well as the face;

This sounds like a solid 9" (225mm) wall in English bond ............ consists of alternate courses of headers and stretchers.

Can you confirm?

 

[Lime]

Not all DPC's were slate.

Quite right, some were a form of felt.
Unfortunately the felt is often very difficult to see as it deteriorates and crumbles where it is exposed.
Often there is only a vague dark line amongst crumbly mortar.


I agree with Mr.C that this appears to be a solid wall with no cavity.

Almost certainly the outside ground level is a major cause of your damp.
If there is a DPC it would have been put in at or slightly lower than the interior floor level.

 

[Anonymous]

Good Evening all,

Thank you Mr C. I assume Headers & stretchers, is the correct terminology for what I have classified as ends & face. Still learning & more than happy to learn each day. If I have misunderstood please correct me.


Unfortunately nothing is straight forward..... one part of the wall has alternate headers and stretchers in every course. Another part has the same headers and stretchers combination, but only on every third course. So two courses of stretchers (face) & one course of alternate stretchers & headers (ends). I hope that clarifies.

Lime, yes I too think it's the outside ground level, but unfortunately the main problems area adjoins a neighbours property, an alleyway that all neighbours have right of way over. As the alleyway is level with the highway & the rear gardens the neighbours have no intention of lowering the ground level. Also as the other side has a internal concrete floor there is no where for the damp to evaporate, apart from behind the skirting. So need to find a different way to aid the evapouration in the rooms that have concrete floors.

Would venting the skirting be enough to allow the evapouration? Really rather not have to dig up the concrete floors, if there is another way that would be great. I may be a belt & braces & not a bodge it & scarper kind of gal, but if there is something else I can do in the meantime I will try that, at least until the spring.

 

[MrWiddles]

Hi

Just to say my house is of similar age and mid terrace. I also have the headers and stretchers showing. There is a cavity between the two
skins as you will see if you go up to the loft. You will also notice it if you drill a hole in the wall to run a pipe for a garden hose.

Wrt to a damp course, I had the front garden up removing several inches of concrete and replacing blocked drains, you could see the foundations quite clearly.
The damp course appears to be a black bitumous layer on top of the bricks (which are of course frog down in those days)

regards
steve

 

[NT]

Thank you for the replies;

I hope to cover all the queries;
...
Catch up soon. Somerset Maid

From what you say its apparent where problems lie, and that not everything been done that can be. And that resorting to dpc injection is an unlikely candidate.

Reducing a narrow strip of ground level around the house then backfilling with gravel would maintain visible ground level, but allow air permeation, thus allowing some evaporation. Not ideal, but much better than constantly wet earth. I'd ensure pointing is ok at dig out time.

If your ground and floor levels are the same, how is the underfloor void vented? 3' is quite a lot of area to evaporate from, if ventilation is adequate.


> The damp is not in corners & have checked with the old tin foil routine, so condensation doesn't really seem to be the problem.

I dont know what you mean there.


> Also all drains are clear & not signs of any damage, thankfully.

There wouldnt normally be any signs, since its underground. Century old clay pipes can not be expected to be watertight, and sometimes there can be significant damage, while the drain still works but saturates ground.


> Mortar is however cement, which brings me back to evaporation. As internal walls have sand & cement plaster.

Some major room for improvement there then,


> (cannot reduce external ground level as if flooding does happen it will be me who becomes the sink, so I imagine I will have to raise the internal floor level & replace the concrete floor with suspended wooden ones.)

occasional presence of water against the walls does not imply interior flooding. Especially so where the removed ground is not above floor level anyway.


If your underfloor void is not vented, the damp can only go 2 ways: into the walls & into the interior air, both of which will make matters worse.

A dehumidifier woiuld be a much better spend than a dpc, as long as its not used aggressively.


NT

 

[NT]

When you say the bricks are 4x9, you dont by any chance mean the visible brick height is 4" do you? I expect not, but if perchance it is then you've got cavity walls.

NT

 

[Anonymous]

Hi Steve (MrWiddles), thank you, unforunately the houses have had a blockwork firewall installed in the loft & cannot see down through the wall. So after reading the replies I decided to take a good look by removing a brick. Yes there is a gap between the bricks, however the mortar from each skin nearly touches, maybe half an inch gap. So the question still remains, is this a true cavity, as not clear all the way up due to the brick construction (see previous posting for construction).



NT

If your ground and floor levels are the same, how is the underfloor void vented?

Voids have been left in the external concrete level & modern plastic airvents are installed under the internal floor level, only where wooden floors are present.

> The damp is not in corners & have checked with the old tin foil routine, so condensation doesn't really seem to be the problem.

I dont know what you mean there.

Whilst researching & having been given advice from a retired plasterer I followed the guidence mentioned on many websites.
Eg;http://www.informit.com/articles/article.asp?p=426766&rl=1 "tape a piece of plastic or tin foil to the wall where you suspect water is coming in. Make sure you tape down all the sides, and then wait a few days. If beads of moisture have formed on the inside of the foil/plastic and between it and the wall, you have a leak coming through that wall. If there is moisture on the outside of the foil/plastic, your basement is suffering from a moisture problem." Other site provide the same instructions for regular walls as well as basements.

>There wouldnt normally be any signs, since its underground. Century old clay pipes can not be expected to be watertight, and sometimes there can be significant damage, while the drain still works but saturates ground.

Pipe & drainage company have used a camera gismo look up & around all pipes. All are sound & not leaks.

>When you say the bricks are 4x9, you dont by any chance mean the visible brick height is 4" do you? I expect not, but if perchance it is then you've got cavity walls.

The stretcher is 9" & the headers are 4".

Thanks again all. Somerset Maid.

 

[Lime]

So the question still remains, is this a true cavity, as not clear all the way up due to the brick construction

In many other walls this gap or cavity would be filled with mortar.
Assuming the headers go right through the wall, from one side to the other, the cavity does not serve to keep the damp out as this can be transmitted by the header brick.
I have seen this type of wall construction before and considered at the time that it was a combination of the brick dimensions
(stretcher : header) and/or economy of mortar.
Some older bricks must have been quite difficult to build with because their dimensions do not make wall construction easy.
I have seen similar bricks in totally solid walls with one face of the wall perfectly flat while the other side had all the headers sticking out about a quarter of an inch.

 

[NT]

> The damp is not in corners & have checked with the old tin foil routine, so condensation doesn't really seem to be the problem.

I dont know what you mean there.

"tape a piece of plastic or tin foil to the wall where you suspect water is coming in. Make sure you tape down all the sides, and then wait a few days.

I suspected you might mean that, but was far from sure. Is this a foolproof method?. Water on the wall side only tells you the wall is wet, but not how it got wet. Lack of water on the inside means youre not experiencing heavy condensation at the times you looked, which I expect was not 24hrs a day nor under all weather conditions.

Reducing interior RH effects walls in 2 ways: it reduces condensation, if any exists, plus it speeds up wall evaporation.


A half inch cavity is a (bridged) cavity, though air flow will presumably be lower than in a modern 2" wall. In principle forced cavity ventilation at times when it would evaporate damp, which is not trivial to control, would help with drying, but removing the problem does seem a much more logical approach.


NT

 

[Stephen]

The construction you describe and for a house of that age it sounds as if you have a fairly conventional 'solid' brick wall. The fact that only every few courses has headers running through does not really change this. In many Georgian and Victorian buildings the headers do not tie back through the wall thickness. This was to save money. The bricks on the outer face were visible and had to look good, but inside could be any old rubbish (misshapen bricks, etc). The rubbish bricks cost a lot less and to maximise the use of them they either did as with your house and used as few headers as possible, or they used snap headers - where the brick was cut in half and both halves used as 'headers' but neither going through the wall (cheap bricks used behind).

None of the above changes the wall from being 'solid' to cavity, it merely means you have a solid wall that probably has gaps and voids in places between bricks. As it has stood the test of time so far it is nothing to be concerned about.

The exception is if you have a particularly thick wall and the headers are extra long and clearly tie between an outer and inner leaf. This would be rare and I consider it unlikely in your situation.

As for the damp problem, you have been given a range of views, but the general consensus is that you should not go down the road of chemical treatment. I would agree with this. If such has already been undertaken there is nothing you can do about it so no point in worrying.

It sounds as if there is a reasonable void under the floor and if you can get air ciruclating in this area by way of ventilation to the exterior this should help to move moisture out and will help dry the walls at low level, thus reducing the risk of damp progressing higher up. If ventilation to the exterior is difficult consider vents in the floor surface under heaters/radiators, as these help draw the moist air through and dry it whilst the natural 'spring' of the floor creates a bellows effect.

In addition consider whether there are modern finishes on the walls that could be trapping moisture and if so remove them and replace with traditional lime plaster, etc.

If you have tried all the remedies suggested and there is a continuing problem have the walls properly tested to find out if there is true rising damp. If so, consider having a physical DPC inserted.

Although it is true to say that rising damp is rare (if not very rare), there are sometimes situations whereby only the installation of a DPC resolves the problem. It is very much a last resort and should be a physical DPC if it is to be effective especially if the wall is damp anyway.

 

[Anonymous]

Lime & Stephen, thank you for your informative replies, they are very much apprieciated.

In order to answer my queries I requested the presents of a RICS Damp specialist who has finally answered my concerns & would recommend that anyone with any such concerns to do the same. Call in the experts.

Thanks again to all those who have been helpful & others that have made it clear that a specialist would be the only way to find clarity.

Regards.

 

[PC]

Can I ask who your RICS Damp Specialist was?

PC