Water under the ground floor

[fex10]

Guys, the floorboards are up in my house during renovations, and the other day, after some serious rain, I found a small puddle in the underfloor space!

I cannot think where it could be coming from. We do have a high water table here.

The floor joists are all solid and I never found any damp. The only floorboards that are showing rot are the ones under where the old leaky sink was.

 

[JBR]

A mate of mine once assured me that if there is good through ventilation of the under-floor, there can be a swimming pool down there!

 

[ree]

The puddle will most likely be ground water.

Are you on any kind of slope or hillside?

Is there a retaining wall near the house?

Check all your air bricks by shining a light thro them.

How do you know that you have a "high water table"?

 

[fex10]

Thanks JBR, Ree. To answer questions:

Are you on any kind of slope or hillside?
- We are on a hillside. It's a fairly big hill and we're half way up it, approx 30m above sea level. Why is the hill relevant?

Is there a retaining wall near the house?
- No retaining walls that I can see aside from the cutting for the railway line a few hundred metres away.

Check all your air bricks by shining a light thro them.
- Can you elaborate please. What am I checking for? How do I shine a light through a brick?

How do you know that you have a "high water table"?
- When the holes were dug for the fence posts, approx 60cm - 90cm deep they quickly filled up with water. In some places I think even just 40 cm below ground level! Before that an inspection pit was dug to check the ground before the garage foundation was done. The pit was two metres deep and quickly filled up. I remember that one side of the pit was bone dry, while the opposite side had water coming out of it which made me think that the water is flowing in a direction in the ground. Also the alley way behind the house, approx 25 m from the house is always wet when it rains. I tried to determine where the water on the alley was coming from, and it looked like it was just coming out the ground like a spring!

House is 1930's, three bedroom (two rooms plus box room) next to London. All clay round here, clay clay clay everywhere!

 

[ree]

The water table follows the outline of the hillside, and when you cut into the hillside to create a flat plot for housing etc. you in effect slice out a piece of the water table.

The water table is now near the surface, sometimes it actually surfaces: "out of the ground like a spring".
The level under your house is probably lower than the local ground level, and the water table will easily enter into it as gravity forces it downhill.

Retaining walls are built to attempt to stem the flow of ground water and stabilise the soil.


Air bricks, as google will tell you, have multi openings to allow ventilation to pass thro. Shining a torchlight into them will reveal if they are blocked or open & free to ventilate the sub-area? They must be free from all debris.

 

[Nige F]

A mate of mine once assured me that if there is good through ventilation of the under-floor, there can be a swimming pool down there!

Very true but conversely ( as I have found in my latest house) if you have inadequate air bricks/ventilation and a small but regular amount of water, given 50 years you end up with floorboards that smell, yet have not rotted
but the smell comes through the carpet slightly I have addressed all the problems and stopped the water ingress and added airbricks.

 

[fex10]

Thanks everyone. I'll be checking all my vents and air bricks.