Water table under the house

[Aunt Maud]

Are you 100% sure you have a combined system?

Even though you can't see a manhole and separate drain for the surface water, doesn't mean they aren't separated.

[Aunt Maud]

"Wooden floor is level with path outside, with about a foot air-gap to the soil below."

There you go, that's why you have water under the floor.

[Canardly]

Wouldn't disagree. How old is the house or extension?

[fd3]

House is about 100 years old?  (give or take, I think).

Maud, do you reckon that
* Concreting up to level with outside will sort the problem
* some sort of membrane/wall thing would stop water coming in (how deep would you have to go?)
* if we just dug a hole outside the back door (say 3 or 4 foot) filled with gravel, then membrane then re-laid path, sure the water would flow there, but then would it not cause a problem of wet earth under the house? (back to the idea of a really long french drain to the garden).

[Aunt Maud]

You need to get your level out and check the fall of the paving. Once you've done that and made sure the water flows *away* from the house you can start thinking about french drains and the like. The ground level below the floor is acting as a sump and that's why there's water there. I'm not sure if there is a lower ground level somewhere in your boundary where you can send the water to, but that's where you need to think about having a soakaway ( big hole 1.5mx1m filled with either rubble or fancy plastic blocks depending on your technological tastes/budget).

Don't worry too much about wet floor joists at the moment, as we can deal with that once the water has gone. If you dry timber down to less than 18% moisture content most timber decay organisms will cease to be active, so that can be fixed without doing anything.

Just think about getting the water to flow somewhere else other than inside the house.

Easy, eh?

[Canardly]

Sounds to me is case of getting the table below the FFL via French drains. As to why they  used timber joists is interesting given the external levels. Betonite slurry could be used to form protective walls but would be expensive and still leaves the issue of reducing the WT if that is indeed a factor. On the face of it just looks like poor construction detailing and relative positioning.

[fd3]

I'm not sure if there is a lower ground level somewhere in your boundary where you can send the water to, but that's where you need to think about having a soakaway ( big hole 1.5mx1m filled with either rubble or fancy plastic blocks depending on your technological tastes/budget).

Easy, eh?

Wouldn't this work no matter the ground level, just possibly with a deeper hole?  (As long as it's 1.5m lower than the level in the house?)
How far from the house should you go?
You didn't comment about sump pumps, should this be telling?

[Aunt Maud]

Firstly and you have to do this yourself before anything else. Confirm that it is the water coming off the paving, be 110% sure.

Just go around the house with a long, accurate level and check what the water on the paving is doing. If you don't have one, I suggest you get a long Stabila, failing that you could spray a lot of water on the paving and see where it runs. You'll need a level for laying the pipe to the soakaway anyway and Stabila are worth buying, as they are accurate.

Pump: Gravity is your friend, it's free, cheap to run, easy to install, quiet, never breaks down nor goes wrong. No pump can beat that unless you want water to run uphill.

Soakaway: Building regs say 5 meters away from the house (or is it 4m, can't remember precisely, but something like that). The drain needs to fall the right way and if you have lower ground, it's less digging to put it there. You can do all this yourself and save thousands to spend on a nice holiday instead.

A deeper hole will work, as long as the pipe isn't laid flat and connect into the bottom of the soakaway, I'm sure somebody must have tried that before.



You're right Canardly. I think it was common in the 80's/90's to still have suspended timber floors. Whoever laid the path should have dug it out to be lower than the ground inside the extension or built the extension higher than it is, but then you'll have a step up inside the house.

[Aunt Maud]

D28015F8-F228-4DA8-8FD8-35DE0DE549BC by belgiangoth, on Flickr
Brick-paved path slopes towards gutters.

Does the path fall towards the house ?

[fd3]

Hi Maud,

in reverse order:
 - The path slopes away from the house and towards the drains.  The building on the left is the kitchen with a poured concrete floor, it is level with the wooden floor in the living room.  It doesn't show any damp issues, so a poured concrete floor could well solve our problem in the living room.
 - So you reckon it needs to slope as opposed to have some sort of step action to the soakaway.  I struggled to break ground under the brick path and did a crap job of re-laying it, so not sure that this is something I will be able to do - especially with inquisitive 3 year old twins hanging about.
 - Path sits just above the drains from the house, so lowering the path would mean moving the drains and also the pipes, which I think would be impossible to re-attach to the network.
 - I'm not even 50% sure it's coming in from the paving.  It's not rained in days and rain does not appear to be the driving factor.

An open question to all:
One thing that really confuses me is that when I dig under the house the water level rises to 2-3 cm below the ground and does not rise higher.  I would expect that I would either get higher, if water is flowing in, or would potentially flow out and drop over time.  Any smart ideas about this?

[Aunt Maud]

When did you re-lay the path and is the down pipe attached to the part of the building where the problem is ?

And

Before I offer any kind of solution, you need to know if it is the water from the paving or the surface water drains which is causing the problem. Proper investigation by you is the order of the day.

One way to check if you have a combined system is to open the manhole to the sewer and run water into the surface water gully. A flow of water which stops and starts when the hosepipe tap is turned on will answer that.

[Canardly]

Has the external ground level been raised at some time? Is a dpc or a course or two of blue engineering bricks visible? Can you see any air bricks in the external wall. There also appears to be some sinkage to the paving towards the rear in the photograph when enlarged. If that is a RWP and a gulley, you might have a leaking drain.

Typical victorian/edwardian suspended floor

https://fet.uwe.ac.uk/conweb/house_ages/elements/section3.htm

[fd3]

External ground level has not changed, drains and black bricks suggest that.

I have dug up the nice brick path at the back, then dug down into the clay to the same depth as inside the house - and struck water!  This suggests that the issue is just the water level in the ground.  So current plan is to fill the hole in and then put a bit more earth on top as it appears that this is the spot where water is coming up.  With a bit more earth and an extra air brick or two there shouldn't be any wet coming up and threatening the floor.

Question: should I pile up more clay or should I put something else as the top layer?

I was really lucky, new friends are Architect&Geologist, who investigated the local ground survey stuff and suggested that the following is happening:



So the clay layer on top of the mudstone is not very permeable.  The water level tracks the slope of the road but could for a number of reasons find itself pooling slightly.

Checked the floorboards in the rest of the room and the floor is dry.  So a solution tot he localised problem should sort it out.

[Canardly]

Great that you now have a diagnosis. Personally I would want to get that ground water away from the house if at all possible. Not sure how raising eternal ground level will help. Can you discuss practicality of installation of land tile drainage with the architect  and see what he/she thinks?

[Moleman76]

Can you create any way for the water to continue to flow off to the left in your sketch?  Mini-ditches in the clay under your house, for the water to flow in, and then a route through your left-side foundation wall?

The glacial till layer is essentially impermeable to water passage.

[fd3]

That would be the neighbours' house.

[Aunt Maud]

Going back to your original post, has the carpet always been wet ?

[Moleman76]

That would be the neighbours' house.

ah, so there's another house to the left in the sketch, just not drawn?

Anything you can do to give the water a place to continue flowing toward would be good.

Perhaps dig a small recess / sump at the low point into the clay, and put a submersible pump with a float (to control on/off, so it doesn't try to run continuously) in the sump; direct the water in a pipe to some place you would rather have it go.

Of course, that requires having access to the pump location, wiring, etc.

[fd3]

Going back to your original post, has the carpet always been wet ?

No, but I'm not convinced that this wasn't a separate issue.

[fd3]

So, it's back!

I'm again, unsure what's going on.  Moving a box that was in the corner of the room it turns out it was wet underneath, closer inspection showed the boards in the corner are wet, wetter underneath and from an inexpert view it appears that the wetness is coming in from under the beams, from the side of the door.
Additional complications are that the skirting board is wet (imo room side as opposed to wall side) and there is damp on the inside of the door (black spot in photo) which is not damp on the outside.

I'm thinking that maybe some sort of damp proof wall under the back door would be the solution, or some sort of membrane beneath the path to stop the water flowing down into the ground and shifting it further from the back door.

I have had a quick look at this similar topic (beware there is a circular reference!) but there isn't a pipe or wires running under the door.


Photos:

(click to show/hide)

Where the wet comes in:

The rood, complete with "black spot"

[Davef]

Do you have a photo of outside the door ?

[fd3]

I took three, from increasing distances...

(click to show/hide)

[Kim]

It's not just that bag of compost (or whatever it is) keeping the wall soggy, is it?

Drip loop on that aerial cable looks a bit stingy, too...

[Davef]

When there is heavy rain doesn’t that whole area fill up to deeper than the air brick ?

Is it normal to run paving right up to a wall - I thought you normally left a few inches of gravel?


Sent from my iPhone using Tapatalk

[Canardly]

I would be very tempted to remove a course of the paving brick immediately adjacent to the house and see if this improves matters. You can always put them back. An air brick flush with the brick paved surface is not a good idea even if the path falls away. Does that dwarf wall bridge the dpc? (assuming there is one). Are those ext bricks above it wet or just a different colour?