Floor void water removal

[ThunderGuts]

Morning,

So we have an "interesting" problem at our house; it's a 1920s semi with a fairly substantial floor void beneath the suspended floors; anywhere between 60 - 90cm of space under the floor to the earth. Except, most of the time at least part of the earthed area is actually a puddle / small lake, anywhere between 10% and 90% coverage of the underfloor area. It's definitely ground water, plenty of tests/investigation - it appears to be a "feature" of the area affecting others too. General consensus online is that it's not that unusual, many people simply ignore it . . . but it's making the downstairs rooms have a very high humidity (e.g. the living room, approx 12ft square, has a dehumidifier running in there and set to 70% humidity, it's pulling 2 litres out of the air per day!).

I am going to increase ventilation (there are currently about 9 airbricks) to help preserve the timber floors from the damp air, but really I want to lower the water level too. The thing is, I'm a bit concerned about what effect doing so might have on the foundations of the house as they're used to sitting in saturated ground. The other thing I'm concerned about, is if I make the area under the house effectively a locally lowered water table (it's clay so I won't be fighting a massive flow of water, e.g. if I lived in a sandy area) there's going to be a natural movement of water into that void from the surrounding area - my concern here is this might, over time, erode pathways through the ground which could also introduce problems.

I see two possible routes;

- The simplest, cheapest and least destructive is to install a sump pump in the lowest point of the underside of the house and pump the water out. Installation would be a pain as I'd need to dewater while I did it (simple enough install, dig a hole and drop a perforated chamber, then the sump pump would live inside that chamber - there's a gully immediately outside the house for the gutters which could be used to take the water - quantities would be fairly small once the level has been established)

- Install a French drain all the way around the perimeter of the house. It'd need to be 1m deep realistically to catch everything, installation would be pretty destructive as there's imprinted concrete drive around 2 of the 3 faces of the house and a patio at the rear, plus dodging services. Care would also be needed to avoid damaging the foundations. Advantages is it'd be a better overall solution as they'd be no issue with water flowing into the floor void. It'd still need a pump of some description though.

Any thoughts on the above? Anyone encountered a similar issue?

Cheers
____________________
TG.

[Riejufixing]

Where's the water level in relation to the damp-proof course? Got any pics?

ASSUMING that the DPC is well above normal water level, I'd be tempted to infill as needed and then place a polythene DPC over.

[ThunderGuts]

[Riejufixing]

[dynax]

Leave it alone the house has been built with all this in mind, tampering with it will do more harm than good, it has survived a 100 years with very little attention so why start now
____________________
Mike.
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[doggone]

[-]

[ThunderGuts]

[Riejufixing]

[Sister Sledge]

This ^^
Something has changed. Yes they're known for it as you say earlier on but now it's too much - there has to be a reason.

Firstly check for a leak on a pipe. Then check for broken drains. I know someone who'd broken a hot water pipe and it had been leaking under the floor for a year. It had caused massive problems aside from higher bills!
You mentioned concreted ground outside. What else was done? Were drains moved or re-done? Perhaps not correctly or perhaps in a way which means ground water has to run a different route and that route is through the floor void.

Half my street has underfloor pumps (4 1/5 foot gap) because we're sited on the edge slope of an old pond. We have actual foundations etc and they cope well with being pumped. Mine doesn't have a pump but next door and the rest do. They work via a float valve in a sump.
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CCM 404 DS

[ThunderGuts]

[BTTD]

I had a similar problem in the dining room at the rear on my 1908 terrace. Next door neighbours gutter drain was blocked and cracked and effectively flooding my floor void - you could see the water coming in from that corner during heavy rain. Neighbour fixed the drain, problem solved.

Friend had a similar problem in his detached house (similar age, but more rural). His gutter drains went into a soak away right next to the wall. Over decades the soakaway, well it wasn't, so the water flooded the void. He put new drainage in taking the water away from the house and has not had a problem since.

Do not fill in. Additional air bricks won't hurt, but will not solve the problem. Get rid of the water.

If you do decide to fill in, use gravel, not dirt. When the water level drops the gravel will drain and not hold moisture. Slugs and all sorts of mould like damp areas........

[Riejufixing]

[Riejufixing]

[ThunderGuts]

Here's a plan of the floor void area. Sleeper walls in black, party wall in green. Existing (functioning) airbricks are in red, although all are old style so not the most efficient compared with modern plastic ones. I suspect, but can't be sure, that there were airbricks at the end of the kitchen and possibly where the porch is (although with the latter, the porch height is only 1 brick course and I can't see any, plus I'm pretty sure it's original with the house).

I think, from memory, that there are gaps between the joists and the sleeper walls, i.e. airflow can move across the top of the sleeper walls and isn't limited to the openings . . .
____________________
TG.

[-]

I'd be tempted to find out if this is really just the water table, or a blocked or old soak away from a gutter causing this.
Could you not dig a small hole somewhere on the outside, away from gutter drains, to measure the water table depth outside the house, using the damp course as a guide?

See if the depth is relevant to the depth inside, after heavy rain.

I just get the feeling that if the void has always been an indoor lake, there would be a lot more signs of historic damp in the house.
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[ThunderGuts]

[Riejufixing]

[ThunderGuts]

[Diggs]

You are on clay. It has a capacity to hold water, and its volume is dependent to a degree on the level of saturation. Take the water out of it completely and it will shrink, potentially causing you structural problems.

Looking at the photo it is clear that you do have an issue that needs resolving though. If it is something that has worsened recently it will either be that surrounding ground has reached saturation point and can't drain quickly enough after heavy rain, or you have a leak. You can find and fix a leak. If it isn't that, you need a sump pump as you suggested.

Assuming your surface water goes to a drain, I'd suggest you pump it to this via a length of hosepipe in the first instance to see if it resolves the problem. If it does, only then look for a permanent solution.

Note that if you connect to a public sewer (i.e. after your drain has connected to something else), you may need Building Regs and consent from the Statutory Undertaker. Far simpler to connect to the drain....
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[ThunderGuts]

[Diggs]

Think about the weather also. If the ground outside the house has reached saturation point because of a period of heavy rain then this is likely to contribute to the problem. The water in the void has nowhere to soak away to...

This is a common problem where I live and I have a pump under the floor for the same reason.
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[BTTD]

[Riejufixing]

[ThunderGuts]

This is kind of the logic behind my original suggestion; use a sump pump to remove surface water, then cover with heavy grade plastic sheeting, not a full "bathtub" but lapping maybe 100mm up the walls. This will mean there is very little wet surface to get evaporation from, while airflow will be unaffected. Reducing water to just below the surface shouldn't affect the foundations as I'm not drying out the ground, just removing water that sits on the surface.

It's still a heck of a job though . . .
____________________
TG.