French drain and lowering external ground levels

[Estane]

Hey folks.

I'm currently in the process of dealing with a high water table at the back of a 1910s mid terrace house. The property sits at the bottom of a steep hill with steps up to a raised garden with a retaining wall. The previous owner had tried to treat symptoms internally rather than stopping the water getting into the brickwork - there doesn't appear to be a continuous DPC in place around most of the house, only the front bay window which is much newer. They'd laid a layer of gravel running from the bottom of the retaining wall to the back of the house and sealed it up with pavers and cement which was channelling ground water from the hill directly into the foundations. Now that I've removed the pavers and gravel I can see it pooling constantly, even in dry weather.

When investigating I found an old outhouse sewer connection (just underneath the downpipe thats hanging loose) that was buried underneath a few feet of soil. Theres now a french drain running along most of the retaining wall to divert the water into this with a P trap installed to prevent any gases escaping. The original drainage was relying on the pavers directing rain water into the drain and straight into the sewer in the third photo, but this didn't help with any of the ground water.

Underneath the steps also seems to be holding a lot of water, so I was thinking of digging a second trench around the remaining perimeter and boring a hole into the drain access where most of the water is going already. Once thats done I'd be looking to dig some of the clay out to bring ground level down a little, drop a membrane and some gravel before relaying the pavers.

Essentially it'd be mostly the same setup as before, with the pavers directing rain water into the drains, but also having the underground pipes to carry the ground water away as well.

I just want to make sure I'm not missing anything obvious before I go digging the rest of the garden up.

Many thanks.

 

[Estane]

I was looking for an edit function but I can't seem to find one. The yellow arrow shows the current french drain in place which is draining properly. The blue is a proposed second drain as water is still coming through from the bottom of the steps. The uncertainty is the depth of the blue arrow drain. I'm not sure it will have enough slope to still function with the water level inside the trap only being about 6 inches below the top, which would still be above the level of the foundation.

 

[malcolm]

Sorry to miss this one. I think you are on the right track. The really important thing is to make sure exterior ground level is maybe 4 to 6 inches lower than interior floor level. In your case where you have water puddling you'll also need drainage. French drains will tend to become blocked up over time and a normal yard drain might be an idea while everything is accessible (just stick a normal drain in with a plastic pipe going to somewhere like a sewer/ soakaway/ next door's garden where you can get rid of the water). Making it so you can rod it out if it gets blocked makes life more easy.

Previous attempts at waterproofing the walls to sort this sort of thing out are never a good idea. Where you have discolouration on the external walls and paint pushed off that's a sign that the walls are wet and water is trying to get out from the walls. The waterproof paints and cement renders won't be helping with that and it would be well worth cutting the bottom 12 inches of render above finished ground level out and leaving it as exposed brick to let the wall dry out.

 

[Estane]

Thanks Malcolm.

There's definitely moisture in the walls. When I stripped the internal render I found the brickwork wet from floor to ceiling in the kitchen, which is the room the door pictured leads to. Further to this, when escavating the outside ground I discovered the underground clay pipe running from the drain into the manhole access was cracked and had been leaking water into the ground everytime a tap is used or rain falls. This has now been replaced with a low backed pvc P trap (gully was too tall to maintain a downward gradient into the sewer) and all the waste water is going where it should.

I'm a bit limited for soakaways. The ground is solid clay well over a metre deep around the back of the house and I'm completely walled in by the properties either side and a hill with high ground water to the rear. The only way I could see to get rid of the excess water was to run it into the existing sewer system. The french drain I've put in does have a geotextile wrap, so I'm hoping that'll keep it working at least for a few years so I can clean up the house and move elsewhere.

I'll remove the bottom of the rendering this weekend to help dry things out.

 

[malcolm]

Sounds like a french drain will be the best option. Geotextile wrap should go a long way to preventing clogging. Surface water isn't a problem if it can drain away. Most of the time damp problems happen because the walls are covered in waterproof materials so water getting into the walls from damp soil can't escape again. People double down and try to make the walls even more waterproof after they get damp. Best fix tends to be to strip the waterproof coatings off and allow the walls to breathe and dry out. So long as exterior ground levels are lower than interior floor levels and drainage and guttering work you should be good.

 

[Estane]

Well, this got a bit more complicated. Turns out the old clay pipe running from the drain under the manhole was damaged on the underside and has been leaking both rain and tap water into the ground for at least several years, possibly even decades. It's now been replaced with a upvc pipe and clay adaptor. In addition, up until a few months ago the neighbours drainpipe at the other end of the house near the front door was doing exactly the same thing - broken pipe underground, spilling water into the foundations (and everywhere else, it was like a fountain...but this is the kind of landlord that rents out HMOs without a license...)

It's now been several weeks since the fix was put in place, but the clay under the concrete floor of the house is taking a long time to dry - it's still saturated anything more than a few centimetres below the surface. Dehumidifiers are pulling in excess of 20 litres a day out of the air, assisted by fans pointed at the exposed areas. If I leave them off, humidity climbs back above 70 within a matter of minutes.

If anyone has experienced this sort of issue, is there a rough ballpark for how long it would take to bring the soil back down to normal moisture levels?

 

[DaveG]

How long has it been since the drains were repaired? I’m no expert but from what I’ve read it can take a year or more for a building to fully dry out and contrary to what you’d think, winter often dries better than summer due to the colder drier air. Its a slow process but you will get there most importantly

 

[Estane]

4 months for the front, but only a few weeks for the back. There was also an old bore hole for a pipe that had been removed near the back drain just below internal floor level, uncovered this evening. This had been filled with soil rather than any kind of masonry and appears to have been giving the water escaping the clay pipe a much easier path into the building than if it had been exclusively soaking through the brickwork.

Rear external ground levels appear to have been raised several times over the years. I'm working on reducing this now that the french drain is in place.

 

[Glimma]

I'm reading this post with interest as I've got a similar issue here.

I'm in a Victorian End Terrace, at the bottom of a very steep hill with steps up to garden. The previous owners built a small extension at the rear for the kitchen and had 2x retaining walls built to hold the garden back. Behind the retaining walls french drains were put in and these connect to plastic 110mm pipes which run down the side of the house. The retaining walls we're pushed back from the house by about 2m, so coming out the kitchen ext or the dining room patio doors there is a small patio area, which leads to the steps to take you up to the garden.

What we've noticed is that in very heavy rainfall we end up with about 30mm of water under the floor in the dining room. The water doesn't seep through the walls, but from the actual ground about 500mm in from the walls. The water always eventually drains away and we're left with damp patches in ground, which by the look of it have been covered with tar.

I think there are a couple of culprits causing this or certainly not helping:

1- The original downpipe for the main house roof was removed to make way for the kitchen extension, so the gutters empty onto the sloped roof of the kitchen extension. The kitchen extension gutters then empty via a downpipe that connects into the french drain behind the lower retaining wall. I'm thinking that possibly there is too much water flowing through the french drain that it can't cope and then seeps under the small patio, has no where to go but into the house.

2- The small patio area is too high and has probably bridged the original damp proof course (if there was one).


My solution to help is:

1- Add a new downpipe at the side of the house and connect the main roof gutters to this, so taking the rain water away from the kitchen ext roof and therefore removing the water from the retaining walls. The new downpipe will connect to the 110mm underground drainage pipe at the side of the house.

2- Lift the small patio area, dig down to below the internal ground level of the dining room and install another french drain and connect to the existing 110mm drainage pipe at the side of the house.

3- I can't lower the ground/patio level at the rear of the house as the steps up to the garden are probably too close to the house, so dropping the ground level will either result with a larger 1st step up or adding another step. So I was thinking of using a dimpled membrane (Like this) on the back of the house.

I think 1 & 2 are no brainers but I'm not sure about the dimpled membrane, has anybody used them before?

Thanks

Glimma

 

[a twig]

The dimpled membrane works but you still need to provide a drain of some sort at the base of it to drain the water away otherwise it will build up and find a way through eventually

 

[Estane]

I actually had sheets of that membrane in the kitchen between the exterior wall and plasterboarding that had been installed to hide symptoms.

Exactly as you say, it did stop the water coming through the kitchen walls above ground level when combined with the membrane over the concrete slab that had been laid, but it still had to go somewhere. With no escape from the walls and the ground completely saturated, the brickwork soaked it laterally along the party wall through the house. It kept going until it found older lime render without a cement component, and also up to the level of the bathroom above where it was causing huge mould issues even with ventilation.

 

[Glimma]

Thanks a twig and Estane.

I'll have the french drain about 800mm from the rear wall of the house and the dimpled membrane up against the foundations. I'm hoping the french drain will deal with most of the water and with the downpipe being re-routed I'll be taking most of the water away from the existing drainage behind the retaining walls.

 

[a twig]

Just a thought but if you are going to the effort of installing a French drain and the membrane, would it be much more effort to install another drain at the base of the membrane and tee it into the French drain? Potentially belt and braces but rules out a future complication

 

[Glimma]

Thanks a twig - Yeah I'll probably end up installing another drain at the base of the membrane.

I did some exploratory digging last night and there is an existing gully/trap in the small patio and the area around this is absolutely soaking wet. So I poured a couple of buckets of water into the trap and the water and you see the water seeping in the soil... So there's something wrong with the trap/gulley but I'll need to do more digging this evening to see what's going on.

 

[a twig]

I found two gulleys that lead to broken pipe less than 1m away from the walls. So many problems come from basic failures of rainwater goods so always worth a check, correcting those is definitely the cheap option!

 

[Estane]

The back drainage installation/repairs seem to be functional now. It's not pretty to look at, but the aesthetics can wait. I'm now working on the front and came across yet another hidden problem that probably isn't helping with drainage issues.

Patio had sunk in places so I opted to relay entirely and add some surface drainage with the help of a carbide drill bit. Removed the 450mm paving slabs, sand they were resting on and sub-base and hit saturated clay. It seemed like a really bad idea to start reconstructing on ground that was clearly unstable so I tried to find drainage. The old front garden wall has no weep holes and sits about 1.5 metres off street level. When I stuck a pitchfork into the clay I hit stone - another patio, hidden underneath, which I assume had been covered up years ago to hide symptoms just like the interior of the kitchen.

These stones are far, far too heavy for me to manage on my own. I was just about able to lift the one onto its side using a spade and rake for leverage, and thats a small one. The one still laying flat is almost double the size.

I can drill some weep holes, but I'm worried that won't be enough to give it proper drainage and the problem will just come back. Right now any water that goes into the ground seeps out through damaged mortar up to about 50 cm from the floor.

 

[a twig]

Feel like I’m missing some context but is digging out the newer patio and keeping the original stone patio not an option?

 

[Estane]

That would be the ideal solution. I don't use the front for anything as it's a busy road with a lot of vehicle and foot traffic - no privacy. There are some voids and I'm not sure the weep holes are going to work properly without checking it from the internal side of the wall though. I've drilled one through the stone work from the outside and it's hit yet more clay. Not the one you can see in the photo, that was going to be acting as surface drainage for the planned level before I found this.

I'm trying to avoid having to deconstruct the ornamental wall on the top as I've only just rebuilt it, but it may be necessary.

 

[Estane]

Sadly not an option. Only the front half of the old slab patio next to the retaining wall is still there. Closer to the house it seems to be just clay. I've run out of space to work with though, need to hire a skip to get the rest out of the way.

Edit : These are old limestone slabs as per the vinegar test. Seeing as I only have half a patio there I'm thinking I should fill in the void underneath the ones I have to secure them in place again, use some of the clay I've dug out to level it out, membrane, and then standard patio with drainage as originally planned. Without the extra water being pumped into the ground from the broken pipes at the front and back, and the french drain diverting the ground water from the hill into the sewer system I'm hoping that'll be enough to stop any further sinking or loss of support material. It could just be the case that it was laid with voids in the first place no way of knowing.

For science I guess?

 

[Estane]

So the front is almost done. Just some paving slabs to go down, the weather looks ok for a few days so the weekend should see it done.

New complication with the back, however. The wall at the base of the kitchen on the neighbours side has been consistently wet below ground level since it was exposed. I've had fans pointed at it for months and it's made no noticeable difference, humidity shoots back up after I turn off the dehumidifier. The french drain does appear to be diverting the ground water from the rear of the property, but its not stopping the water coming in from the neighbours side.

I nipped over to check the rendering myself (currently unoccupied). The damage to the wall itself seems to be superficial, just the paint flaking off. Towards the floor, however, two of the neighbours steps are flush against my wall. When I chipped off the bit of rendering about the steps I found wet soil sandwiched against the wall and it appears to go much further down than I can see.

All of these properties are built on heavy clay soil and the rear gardens are raised to the height of the first floor. This causes drainage issues, as I saw when I dug up the lower. Next door dont have this lower area at the rear. Instead, they have a small extension - picture 3 is the most clear for this - that is presumably tanked else the back room would be absolutely soaking. This leaves me questioning where all that rain water is going...and the only answer I can come up with is the steps. There's nothing substantial underneath them, just soil and a couple of boulders, so I suspect this is acting as the natural drainage and since the base of the steps are concrete...into the wall it goes.


A damp specialist recommended that I tank the affected wall, but this seems like a really bad idea. I can't get at it externally with the steps in the way, and tanking internally will just push the water somewhere else and not necessarilly solve anything.


Any thoughts on how to proceed?