Water under house

[andy990]

I wonder if anyone can help?
We plan to have an extension built but I am a bit worried as I have on occasion seen water under the house.
There is a gap of about 3 foot below our floorboards and you can gain access to this void via a 'trap door' in the cupboard under the stairs.
Directly underneath the trap door is what appears to be a drainage hole (or a sump, not sure what the technical term is, sorry!) of about 1.5 foot square and 1 foot deep. Whatever it is it appears to have been put there by the original builders for a reason.
Anyhow, on the odd occasion I have had to go under the floorboards I have noticed that there has sometimes been standing water in this 'hole'. Once after some very heavy rainfall I noticed that the water was a good six inches or so above the hole. However, after a few days the water gradually went down and subsequent checks revealed no additional water. Most of the time there is little or no water in the hole, particularly in summer/dry weather. The water when it's there doesn't smell.
Has anyone any ideas what the purpose of this hole is?
Is it common (the house was built in the 1930s and there is a brook at the bottom of our sloping garden) and has it been put there for a reason? Does it suggest anything that might lead to problems when building the extension or is there anything we should be doing before proceeding?
Thanks

[Tony McC]

There's a good few other posts on this subject. Standing water in a sub-floor is summat that comes up every few weeks, and there's no straightforward answer as each case is unique. If you search through the Drainage section, or use the search tool above, you'll find the previous threads and they might just contain the nugget of information you need.

I'd guess that the 'hole' is as you guessed: a sump, and possibly created to make pumping out or baling out that bit easier.

[andy990]

Thanks for your swift reply Tony. I used the search function as you suggested and have found that water under floorboards is indeed a recurring topic. Some of the replies to the other posts have certainly given me food for thought!
I guess when the builder starts work on the extension I'll just let him know about the fact I've noticed water under the floorboards in the past and ask him for his thoughts/advice.
One thing I've noticed is that we only have one small grid at the front of the house which is used to drain away all of the rainwater from the roof and also what comes off the sloping drive. It could be that its not up to the job and that the excess water is finding its way under the house before finally draining away. Does this sound feasible? Maybe the builder putting in a wider drain when he does the extension would help?
Also would the fact the sump is there suggest its a common problem in the area?
Finally, I'd just like to congratulate you on an excellent website. Sites like this are worth their weight in gold.

[Tony McC]

Do you know if the property relies on a soakaway for surface water? That could be part of the overall problem.

The sump doesn't suggest anything other than the fact that it may have been pumped/baled at one time.

Worth its weight in glod, eh? You can find my address on the About Us page. Bullion is always welcome!

[andy990]

It's me - back again after 6 months.
We've now had the extension built and thought we'd cracked the water under the house problem.
We've had all our guttering replaced (on the existing house and new extension) and a number of new grids fitted that connect to the main drain. In the process we discovered that one of the old drains was blocked and thought this may have been causing the problem.
We've also had a gulley (I think that's the right word) fitted across the garage/porch to collect any water that comes down the drive.
However, after the last week's pretty heavy rain the water's back again, about three inches of it.
I think we can now confidently say that the drains are all functioning correctly (they don't appear to be overflowing when its raining).
The water under the house is clear (not muddy) and it always gathers in and over the sump and it only ever comes after heavy rainfall - after a few weeks dry weather (not that we get much) it goes.
When digging the extension footings, we also hit water outside the house at a similar level. As a footnote, we had to have the extension piled and the pilers didn't hit clay untit 3.5m down.
Does it appear that fitting a sump pump is now the only practical answer?
I've looked back at some correspondence that went on with the previous owners that mentions something about the water table in St Helens rising due to waning industry. This seems a bit odd, we have a brook at the back of our house and the end of about an 8m long gradually sloping garden, and the brook is 2m down from the end if the garden?
Can anyone offer ant advice?
Thanks

[Tony McC]

Water tables rise and fall all the time and despite Pilks closing down, I can't see how that would have a direct effect on the Water Table in any particular part of the town.

If the water worries you, then a sump is probably the simplest answer, but I'd strongly recommend you get the Environmental Health bods to take a look first, just to find out what their thoughts might be.

[ABILITY]

Sounds most likely to be ground water showing itself after rain, however maybe worth dye testing the rain water system just to make sure.
You comment that the drains are all functioning, not overflowing, this does not definately mean its going where it should.
During heavy rain place some dye in the gullies and wait for any sign under floor. If seen you can start to trace where from.

[danensis]

Just out of interest, what did the pilers go through before they got to the clay?