Replacing suspendes timber floor with concrete - pros and cons

21 Jul 2019
Reaction score
United Kingdom
We live in a typical 1930s house with suspended timber floors.

Underneath the floor we have discovered several inches of water that comes and (slowly) goes after heavy rain. The void is approx 1m at its deepest and ventilation is poor due to a rear extension. Done some checking of pipes/gullies/drains and the problem appears to be due to a high water table.

Filling in the floors with concrete has been suggested as a long term solution but research is also suggesting this could then cause damp in the walls.

Any pros and cons of filling in the floors with concrete?

Also any ideas of potential costs (2 x 4m2 reception rooms), time taken and the exent of the mess - will I have to redecorate everywhere? Where will I have to put my gas and electric that currently go under the floor - up the walls, under the new concrete, in ducting?

Thanks all!
Sponsored Links
If the water is not causing any issues, why change it?

What sort of condition are the timbers in?
I'm not really sure what issues it is causing (yet). Possibly none. Not convinced the ventilatiom is good enough as the extension on the back blocks the old airbricks. I have been told by a neighbour that the previous owner used to pump it out, so assume it can get high enough to worry about. Some electrics run under the house and I have seen the water cover some of the wires (which we have now hooked higher up).
Humidity in the house seems quite high (65/70% showing on the dehumidifer) which concerns me a bit as my son has asthma. Essentially I am a bit worries abour it being there and don't want to be fretting over the amount of water coming in everytime it rains for the rest of my life and pulling back carpets and checking under floorboards...
Timbers look ok though have a moisure meter on order to check the levels. To my (untrained) eye the joists and floorboards look pretty good, possibly newish so could be that the original floor has been replaced at some point.
Sponsored Links
Filling in the floors with concrete has been suggested as a long term solution
A solution to what problem? This is normal in hundreds of thousands of properties.

And who suggested this solution? Was it the same person who may be providing the solution?
No, not consulted any 'professionals'as yet, just talking to friends and family and trying to do a bit of research but so many different opinions, its a minefield.
Its just something I don't want to have to be worrying about in prolonged rain (like at the moment) and something that will make it difficult to sell (unless we just don't mention it like our sellers didn't...)
I'm no expert, but do have a 1930s house with suspended floors that I've spent some time clambering around ...

my concern would be the damp walls. If the water table is causing the subfloor to flood, then pouring in concrete risks breaching the damp course in the walls and damp will become a problem.

Also, cost will be huge - 1m depth of concrete is not going to be cheap.

If you go ahead, make sure the best quality damp membranes go in and up each wall. My neighbour did do this, but didn't have ground water problems.

Could there be another solution? Maybe more air bricks on the sides of the house to ensure that the wood remains well ventilated?

If the house is 1930s and the floor has been changed once, then next floor change might not be due for 60 years...
Before fixing my air bricks and cleaning out the floors in general (over an inch of muck in places) I did consider using plastic joists like these
but as I had some free joists from a garage roof I dismantled I decided to use those instead.

Humidity in the house seems quite high (65/70% showing on the dehumidifer)
compare with a local weather station (I use this in Chelmsford) as humidity has been high lately, currently 83% outside.
I think unless you go for a very engineered (read expensive) solution like that, you are likely to cause more problems than it solves.

I'm no engineer or builder, but I would suggest a registered damp consultant, a proper one that you pay, not a free consultation from a damp proofing company, would be a good place to start.

They can often suggest suitable remedial measures. It maybe something like a sump pump or similar may be the easiest solution, or more airbricks. As said, this is not an uncommon situation, and I dont think a few tonnes of concrete is the best solution.
Have you thoroughly checked drainage? Because if it is worse when it rains, this suggests not ground water / water table, but poor drainage. Maybe because of a collapsed clay rainwater drain (I had this in my back garden) and the water will find its way to the easiest route - down into the large void under the house. I might not be ground water at all, but poor drainage sending water from the roof to the floor.

If water table, then surely the problem would be all the time, not just when it rains - and it would be worse in autumn when the ground becomes more waterlogged.
We live in a typical 1930s house

Then you will have clay gullies, traps and underground waste pipes and rainwater pipes, and they will be cracked and leaking.

Show us some pics of them, and of any manhole covers and, especially, any concrete or paving that is cracked, sunken, or patched.
Thanks all for your responses.

Causing more problems than I cure (eg damp walls, subsidence) is my worry really! The more I hear the more I think concrete isn't the answer.

We can put in more airbricks but only to the side as the back is blocked by and extension, the side has concrete floor. A fan has also been suggested.

Good point about the humidity being high in general and will look into an independent damp consultant.

I think the drainage is poor as the ground slopes towards the house and the driveway is rubbish. I have had the council clear and check the road gully outside. Also had the water pipes checked for leaks and cctv'd the drains. We have replaced one cracked gully to the side and I have my suspicions about the front one, may chuck some food dye down it and see if that turns up under the floor - the water however is the opposite end to the front gully and the driveway - the end nearest the front of the house is dry (though the ground level is much higher here.) We intend to put in french drains when we redo the drive. Will post some pictures.

Am not sure if its an all year round problem. We discovered it in April after heavy rain (3 months in 3 days or similar) and it took a long time to go. We then had a really dry spell and the ground dried out, its rained heavily the past couple of weeks and has filled again. Was assuming the water table rises with the rain/saturation of the ground.



Last edited by a moderator:
I would have thought that the red airbrick is too close to the ground and more likely to allow water IN than ventilate air in/out

I'd either move the brick up or remove the concrete that surrounds the area and add gravel
The void is approx 1m at its deepest and ventilation is poor due to a rear extension. Done some checking of pipes/gullies/drains and the problem appears to be due to a high water table.
If there is a 1m deep underfloor void and high water table that will be why it is filling with water , doesn't need to be any leaking drains. I am not suggesting the best solution for your situation but building up would be done in hardcore below a new concrete slab not in approx. 1m depth of solid concrete.
Thanks all, I'm thinking its maybe a combination of factors relating to the rainfall - poor drainage on our drive that slopes towards us, maybe another cracked gully, high water table/clay soil, and the council have just checked the road gully outside and confirmed there is a problem with it (which definitely won't have anything to do with our issues). Hopefully if we can tackle it all we can monitor and if it continues to be an issue install a sump pump or fill with gravel/dpc/gravel and consider concreting in as a last resort.
Thank you all for taking the time to reply to my post, its really appreciated. Any other thoughts are very welcome!

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links