Diagnosing water under the suspended floor

7 Apr 2022
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United Kingdom
OK, I'm just going to list everything I know!

Village hall, built 1897 from sandstone & brick.
Floor - suspended pitch pine T&G on joists.
Water pooling in the substrate under the suspended floor - a few inches suddenly, after rain
Rumours that a stream runs under the hall (it doesn't, as there isn't one)
There is a drainage channel on neighbouring land, leading to a culvert which (at that time) was blocked.
It is possible there is simply a high water table - how do I find out?
It is also possible (I guess) that the culvert could be damaged and allowing water to seep and saturate.
Floorboards in a different position permanently wet, even in the recent spell of hot, dry weather, when the drainage channel at the back was completely dry.
The wall adjacent to this also looks quite damp/wet.
There is a rodding point near to the place that is continually and inexplicably damp.
Said rodding point may be blocked (I have enquired in a different forum).
There is another patch down the other end of the hall that also appears to get damp - and has had the floorboards patched before.
There are a lot of ventilation bricks and grilles, but really the pool of water was a bit much to expect to evaporate, I would have thought.
Someone appears to have put in a vent pipe, similar to those installed for bathrooms (not sure it has any effect, but it might have been the council attempting more ventilation)
Not sure if there is a damp course - there are two courses of bricks a different colour at the bottom of the walls.
The council (who I bought it from) reckon that it needs a concrete screed laying. However, if it is a rising water table, or a leaking culvert causing saturation, a) will concrete help? and b) if the cause is removed, will it not be a waste of money, given that removing the floor is going to mean replacing the floor, as the T&G boards won't come up. (Anyway, I don't think I can afford it.)
The tarmac outside the property has been re-laid, at the time they put in accessibility ramps, etc. I think it now comes too high up the walls (doesn't block air bricks though), and has affected the camber, and as I'm on a very slight hill the surface water run-off is probably running towards the building instead of away. I'm gradually dealing with the surface water coming from the road above, but I don't think that is a significant cause, because there was no obvious surface water flow when the pool appeared.
Water pooling at the footings at one end of the building during heavy rain (but not where the floorboards are damp).
The hall is rarely heated, but it gets quite a nice lot of sun.
Doesn't smell damp.
Plaster on the walls feel a bit moist.
It's had PVC windows put it, and although I use the ventilation things now, they may not have used them before.
It had a vinyl carpet, which I think was also affecting breathability.

I'd think about getting someone in to diagnose and quote etc, but a) I don't have confidence that anyone would definitely diagnose correctly, and I really can't afford to do unnecessary work, and b) I can't afford a top-notch professional job, as I'm doing everything on a shoestring. I'm working on the principle that if I can stop the water ingress in the first place, that's 9/10 of the problem solved.

Thank you *so* much for wading your way through all that! If you need photos, they may need to wait until tomorrow as it is too gloomy now.
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When a floor void is below ground level, it can have water in it. It's a feature not a bug.
Oh OK... my water feature! So... glass floor, watercress, frogs... Seriously, though: how do I stop the wood from rotting? Maybe it's just another feature (Victorian floor, probably been there since new)... I save up, get it all replaced, keep an eye on the ventilation, and it'll last another 120 years?
Yes. The thing is to ensure that the void is adequately ventilated, and this removes the risk of rot to the timber. But a damp void is not in itself a problem.

The water seems to drain away, so wont get stagnant

There are options for an external French drain, but this is really only for high surface water run-off, as this drain reduces water getting to the wall and leeching in. But be careful that you could end up spending thousands on a technical solution to a problem that may not actually exist.

Other option is a sump drain in the sub-floor, but this tends to rely on it being able to be drained to a drain system lower than the sub-floor level. But again, this can be costly.

If you can demonstrate that the excess water is from other's land, or a faulty drainage system or culvert, then you can claim against that landowner.

Damp walls above floor level are a different problem, and would tend to need dealing with as a separate issue - ie damp-proofing.

Do you have photos of the general external areas on all sides and of the floor void and damp walls?
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I can take a few photos tomorrow, of the external areas, but the continually damp patch is under the stage so it is really hard to see or get decent light for a photo. The floor has been repaired, although I do have a photo of the water. The exterior with the pooling water is nowhere near the damp floorboards (although there is damp at that end of the hall - the concrete floor in the little porch is really sweaty.

I've emailed the council about their maintenance of the culvert, and when it was last checked or cleaned etc, and they have simply not replied. I must chase them up.

You could always dig a hole in the deepest part and bury a large bucket with an automatic sump pump. You could even have it on a timer so that it only operates during the day time.

I think the problem with that is that there are several (four? five?) different sections divided by brick foundation walls (on which the joists are placed). Will the water definitely run to one deepest place, or is it not likely to just stay in these channels? Also, I'm not sure how I would be able to identify the deepest/lowest position without removing the floor, which I am unable to do at this stage. (I need to source some reclaimed floorboards, and/or probably sell my rental property for the funds). But if/when I do replace the floor, I'll have a look and see if something like that looks viable. I don't know much about sump pumps. I guess the pumps can be solar, and presumably they detect whether there is actually any water to pump, as it won't be needed continuously.

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