Identifying dpc & solving drainage issue - hel

[holly]

Hi Folks, this post 'posted' before I had chance to actually draft anything and I can't seem to delete it, so please bear with...

We have a serious surface water drainage and penetrating damp issue at the front of our house, resulting in pooling water in our subfloor and wet rot, woodworm etc in floorboards and joists. The house is early 1940's construction with cavity walls, half suspended timber floor, half solid concrete floor.

We have dug out the front of the house to discover thick heavy, waterlogged clay against our external wall. After heavy rain water is seeping through the porous concrete wall of our subfloor. We've ruled out leaks in local water pipes and drains with our water company - it's definitely a surface water issue.

We have found a dpc membrane in our inner cavity wall just below the floor joists, above the air bricks. We think we have also identified a bitumen dpc in our external wall, this time below the air bricks, but we are not 100% sure. Please could you help us identify? I would attach a pic but can't work out how - if you could let me know how to post an image that would be super helpful!!

We ideally want to be 100% sure of our external dpc level before coming up with a drainage solution for the surface water.

If we have correctly identified our external dpc, would we be able to;

i) apply waterproof tanking to our external wall, covering both the porous subfloor concrete and dpc, without creating a risk of rising damp in the future? (or would we have to also install a new dpc above the tanking?)

ii) Assuming the above is successful (with or without a new dpc), could we then raise the ground level above the original external wall dpc?

Context for questions: refilling / raising the ground level to above the current dpc (if identified correctly) is the only way we will be able to drain / divert surface water away from the area without the need for a pump.

Many thanks in advance for any help or advice you can give us!

Best wishes, Holly

[lutonlagerlout]

generally a linear type drain is your friend here Holly
there are lots of the type of houses around me you speak of and often the dpc (slate ) is at the bottom of the internal floor joists
the key thing is to get the surface water to run away to a suitable soakaway
painting walls with bitumen generally isnt a solution as the water still sits there looking for holes
LLL

[Tony McC]

The usual 'fix' for this sort of problem (we see lots of the very same issue in parts of South Lancashire with a nasty, nasty bed of clay we call "cowbelly" because it moves like tripe) involves creating an intercepting filter drain against the house, down to foundation level, along with lining the masonry with a one-way drainage composite and/or bituminous tanking. They *key* component, however, is creating or finding a suitable outfall for the intercepted groundwater.

This can become a bloody nightmare. Back in my contracting days, we'd 'line' one property in a street, and then find all the neighbouring properties we're unwittingly donating *their* groundwater to the system and causing overload. However, it usually led to enough of an improvement for the housing associations we worked for to continue with the process.

And then, as LLL says, collecting the surface water before it has a chance to add to the groundwater problem is an essential component of a fully working system.

[holly]

Hi LLL and Tony,
Many thanks for your replies, much appreciated! I believe your suggestion of a drain to divert water away from the external wall is our only option, plus controlling as much surface water run off as possible. The difficulty we are now facing is where to drain it to!

The house is on a slope and our front external wall faces up-hill. Our clay-filled front garden is not large enough to situate a soakaway 5 metres from the house (and I'm not sure it would work on the clay), and the surrounding road and pavements / rights of way are in line with or above the level of our internal floor / dpc. The only nearby drainage available to us is cased in concrete and also lies above the dpc. We are hemmed in on all sides! (We are actually in the processing of contacting the LA/Highways about a public right of way that is compromising our dpc along the gable end of our house...).

A few people have suggested that we somehow channel the water below our dpc through our subfloor, following the slope of the land, so that we can drain it into the back garden / rear drain. Have you ever seen or been involved in any work like this?

This whole process has also helped us realise that our rear patio is also above our external dpc. To fix this we plan to implement something from your super useful dpc page - thanks for all the info!

Thanks again for all your help,
Holly

[holly]

Hi again, I've found these useful pages on your website, am I on the right track re. filter and linear drains?
https://www.pavingexpert.com/drain03
https://www.pavingexpert.com/drain06

If we can lower the public right of way along our gable end to below the dpc, we could potentially use a combo of filter (at the front and along the gable end) and linear drains (at the rear) around the entire perimeter of the house to collect and divert all water into the back garden / rear drainage (being careful not to overload the system). Other than channeling water from the front through the subfloor under the house, I can't see many other options due to our low position relative to surrounding ground levels...

I'll try and upload some photos later. It would be great to know your thoughts, but no worries if you aren't able to help.

Thanks and best wishes,
Holly

[Tony McC]

I *have* seen the "drain it across the sub-floor" option - I saw one in liverpool last yerar, and while it sort-of solved ("eased" might be a better term) the saturation in the rear garden, and did successfully shift excess water to the front and from there out (very discreetly) onto the public highway, it caused no end of issues within the house...damp patches, musty smells, mysterious moulds...so it's not something I would ever recommend, other than as a fully-enclosed system.

I'd much prefer relying of drainage composites/fin drain to direct the water around the side of the house and into the vicinity of the existing drainage facilities, if that was possible.

[holly]

Many thanks for your response Tony - it's really useful to learn from your experience. I would also prefer to take water around the side of the house, rather than under it!

We'll see where we get with the LA/Highways. Fingers crossed we can lower ground levels around the front and side of the house to direct the water into our rear surface water drain.

One final question here. I've seen some advice on the internet saying not to create drainage too close to foundations. As long as we aren't digging past the foundation footing, will we be okay to install filter/fin drainage right against the external walls?

If we do end up having to do the sub-floor option we will definitely make sure the system is fully-enclosed. I don't fancy risking future damp / mould problems (or rats, who might fancy a nice sub floor void with a fresh water supply!)!

Thanks again,
Holly

[Tony McC]

The advice regarding drainage close to foundations is generally good advice, but there are always exceptions and your scenario could well be one of those instances.

A drainage composite down to but *not* beyond foundation level will not be an issue. Problems occur when, f'rinstance, saturated clays are droed out by laying draionage, even when the draionage is not intended to drain groundwater, and also when foundations are tunnelled to get pipes running beneath.

None of this applies to you, so you should be OK.