UKC

I lifted the floorboards for the first time today, expecting to find a small void as the suspended floor is about a foot above ground level.

Somewhat surprised to find a four foot deep void under the floor with standing water at the bottom.

The house is 100 years old so presumably the foundations are reasonably OK with the water. 

However I had been planning on retrofitting an insulated slab floor with UFH.

Obviously I need to speak to a structural engineer (I'll be getting one in anyway as I want to take the chimney breast down) but is it likely possible to fill this in with aggregate and put a damp proof membrane and insulated slab?

Google indicates you shouldn't be filling in more than 600mm, but I get the impression that might be 600mm above ground level so you're not creating a retaining wall out of something that wasn't designed for it.

I think I’d want to be finding out where this is coming from and why it’s not draining. It’s unlikely to be the water table so either through flow is getting trapped, surface water is draining in or you have a burst pipe/ drain somewhere. Given it could be a sewer I think some investigation is required before you seal it in.

Happened to a friend who, just as you had done, lifted flooring to find an open river below the house. Turned out to be a leaking water main and the engineer who came round reckoned the company had known there was a leak for over a decade but never found it. The foundations needed considerable underpinning.

Reads your post, looks at the picture, raises eyebrows!

I can't really answer your questions but my first thought was, 'If it's that wet under there now, after months of exceptionally dry weather, what's it normally like?' 

On the plus side, you've now got somewhere to put all your chimney breast rubble. 

Oh boy.

The white coloured lower bricks and the damp sheen lower down make me think you’ve happened on the water at an unusually low level?

I think you want a local surveyor who is going to know the local building style and has an idea what is below that brick course that disappears in to the slime to advise further.

Do you think the wall to the left looks like it’s had water running down it?

If you ask the water board they might test it for chlorine to see if it’s a leak from their pipes nearby.

Good luck!

Now you've got access, if possible monitor the situation over a couple of weeks. Looking to see if the rain causes i rise. 

In all honesty it's below ground level so this can happen. Witnessed this many times. 

Speak with the neighbours and see if they have a similar situation. 

The neighbours reckon there's a stream running under the whole street.

Yes - I looked at the pic and thought "looks like low tide at the moment"!

Yes, I think so - We've had a long dry spell then a couple of days of rain, so if it's ground water it would make sense that it's low just now.

This the house you bought at auction? Bet this wasn't in the details.

Obvious things to do - speak to all the neighbours, find out whether it ever gets bad enough to be a damp or flood problem. Also see if any of them have implemented a solution that works.

Good luck.

I'm no expert but if the water had been there for a long time surely one would expect rot in the floorboards and joists at least due to humidity?

I had a similar problem in the house I lived in. I think the house had been built in the wrong place. Owners dug a ditch in the garden and put in drainage pipes.

I've no idea if that is an option.

I think my approach would be to dig a hole in the middle of that space and put a sump pump into it, then attach a pipe running into the drain.

My approach would be slightly different as why not make a positive out of a negative.

Is your house built on clay, by any chance? Clay doesn't allow water to soak through so spaces like this can hold water for a long time. Also be aware that if it is clay then drying it out completely makes the clay shrink and can lead to other complications. A neighbour of ours found a stream (and resulting lake) under his newly built house and managed to re route the stream. The house baked in the mid summer temperatures and his floor slab and walls cracked.  

The 600mm thing is more about settlement. Even with good compaction things will settle, particularly in a space like that where the corners and edges are difficult to fully compact. This can crack a slab and UFH pipes.

There must be a reason it was built that way

Do you need a ground bearing floor? UFH and insulation can work with suspended timber too. Doubling joists would likely be quicker and cheaper, also less embodied carbon

It may be the water table is at that level, in which case that wouldn't help.

It is an area of "moderate" shrink-swell potential so yes.

That's why I thought if it's stood for 100 years, and the neighbours are the same, it might be better not to do anything about the water.

With climate change, baking summers may become more normal.

I was mostly thinking of it for thermal efficiency:

150mm PIR, a large thermal store, no bridging and no chance of air leakage.

Quite common to have PIR between the joists on UFH on suspended floors.

I don't think thermal mass is important. The reason it's thought to be important is it can reduce cycling of ASHP which kills efficiency. But with a suitably sized modern unit, which can ramp down its output controlled with well tuned weather compensation you can avoid cycling without thermal mass.

If you have wood or carpet above the pipes, really go for narrow spacings between the pipes, 100mm if possible. This will allow colder water temperatures. Also the aluminium spreader plates help, but they are very expensive. I suspect they aren't needed for 100mm spacings, but I'm struggling to get an answer for that myself.

Indeed, but the joists are only about 11cm deep limiting the thickness of PIR, the bottoms have to be exposed lmaking it hard to get an airtight seal below the insulation, and the joists themselves are less insulating and a thermal bridge.

I'll need to think more about that. 

The other thing I was thinking about with a large thermal store is being able to make more efficient use of agile tariffs.

I'm thinking tiled ground floor, although I was also thinking of aluminium spreader plates under sanded floorboards upstairs.

If you look near your front (or back) door, do you have a small circular cast iron manhole cover, about 10 inches diameter?

Edit, if not.. do you have neighbours with a similar house who has a little circular manhole cover?

Could be the natural level of the water table - it may fluctuate.  I've seen it often enough.

However, get all the drains dye tested. 

Is your foul and surface water drainage separated? 

There will be, you can see they're a different colour near the walls.  I had a winter lake under the floor of my previous house (c.1895), and the joists were simply sat on the granite sleeper walls with no slate DPC.  They'd all suffered minor woodworm over their lengths in the sapwood which wasn't problematical, but the bearing areas had all suffered rot and the whole floor had sunk at least a centimetre.  The 'dead air' corners had suffered bad rot and already had joist ends replaced with greenhouse timbers.  So the floors came out and were replaced with treated joists whose cuts were re-treated and the whole sides and undersides painted with bituminous paint to form a barrier against condensation and prevent woodworm. They then sat on thick DPC curled up around the ends to protect from the walls, or formed into pockets if they're in a wall socket. Should beat the 120 years the originals lasted.

Mine is the same.

The water table was locally raised by leaky drains but years after I fixed that it still never dries out. I thought about backfilling it but didn't like the added mass, work or cost involved. I'll fit a sump and pump when I get around to fixing the back room suspended floor (rotten and in the way of comfortably digging a sump). I figure it's been like this for a century and while the damp has clearly taken a toll, particularly where damp-proof courses were neglected or bridged, its not that bad for the age of it.

jk

Put a heater in there and you'll have wet UFH!

Thanks, if I can't put in a slab floor I was wondering what's the best way to protect a new suspended floor so that's a useful post!

The house has been empty for years, and at some point before the water was turned off there was a leak in the bathroom that rotted out one end of the kitchen floor. The rest of the floor is pretty uneven, presumably because of the years of condensation. So one way or another it'll be getting a lot of work!

You must be excited about what tomorrow’s weather is going to show you!  Some versions of the forecast have had close to 100 mm predicted rainfall in areas near us.

In not sure we're going to get that much here, but yes it will be illuminating.

It's gone down a couple of cm since my OP.

Northumbrian water are on their way round to check there's no leaks coming into my property from outside. No idea how they determine that, but hopefully the engineer will have knowledge of the local area to impart.

As mentioned above, they have a meter for Chlorine, if there's chlorine in it then it's from treated water supply and they'll chase it further from then.

You are doing the right thing consulting a structural engineer. From the photo the brick walls are well constructed and well pointed - perhaps because the builder knew the water was there? You may have found the top of eg an old well. Take professional advice before dewatering - you may initiate different problems like ground shrinkage / settlement.. houses 100 yrs old tend to have shallow / almost non existent foundations. 

Lived for a while in an ex-pub in Beligium, we had a sump pump running full time in the cellar to keep the water down, used to lie in bed on the first floor and watch barges go along above me if the tide was in!

My current house the water gets up to half a meter below my floor and it's been worse, when firewood starts floating out of the barn it's time to get pumping! All caused by a road improvement/realignment for a new bridge where they infilled which stopped the higher surface water draining, once you know what's going on it's not such a problem, you just accept it, them old walls like it a bit damp!

That seems to be the general feeling amongst the neighbours.

One said they installed a sump pump because it was smelling foul, the rest seem to leave it be.

Wishing you all the best with your reconstruction, of course.

It just reminded me of a medieval cloister that actually depended for its stability on the water level being above its wooden foundations, preventing wood rot of them.

And it's foundations are still submerged intentionally, as to this day. Quite an architectural feat, even if it could have been much easier without any of that on other ground.

Which is amplified if you have an old house on boulder stones or whatever and a modern extension with modern foundations, and the two move differently when the ground hydration changes.  

The other thing for the OP to consider is disclosure and discoverability from an insurance perspective.

Well well, that was a surprising find. Have you considered a suspended floor using concrete block and beam construction. They have less deflection than a timber floor but more than a ground bearing slab (suitable for tiled floors). The build up is the structural floor, then rigid insulation (often extruded polystyrene) and then UFH pipes either in a screed or a tray system. A screed will have higher thermal mass and slower response time. How much thermal mass is right depends on what the property's response time is like and how you heat it (consistently stable or quick blasts)

Retro-fitting a solid slab floor is deemed ill-advised in an old property without modern DPCs - the slab simply raises the damp level in the walls, beforehand the walls were able to evaporate off the damp before it got above floor level.

Salisbury Cathedral.

https://thefogwatch.com/the-miracle-of-salisbury-cathedral/

I had very similar in a 1970’s bungalow. No problems but make sure you have able air bricks for ventilation to protect floor joists and boards against rotting. Otherwise don’t mess with it

Cheers, seems like this is not the option!

Northumbrian Water have been round and confirmed there's no chlorine, so I think a new, well insulated suspended floor it is. And no messing with the water that's always been there. 

I have high ceilings so can afford to raise the floor a bit.

Thanks all for the helpful advice!