Strange concrete floor problem...

[Pecanfan]

Hi all,

I've started on our latest renovation escapades and have found a strange problem with the kitchen floor. It's a 1930's brick-built semi and the kitchen floor (downstairs at the back of the house) is concrete. The rest of the downstairs floors are suspended timber by the looks of it (there's laminate down at the minute).

Anyway, just ripped the shockingly-laid laminate up in the kitchen and found that part of the floor is solid concrete but the rest feels suspended - there's significant give in some areas and it feels more like a timber floor when you're walking on it. It's definitely concrete though!

So I knocked a big hole in it to see what's going on (there's serious damp on the back wall and was worried any joists suspending a concrete floor might have rotted, hence the give) but the floor would appear to be suspended... on nothing! There's no void under it so it wouldn't appear to the suspended - and I couldn't find any joists. The concrete is about 4-5" thick and I managed to get my arm underneath to feel at least a 1" gap between the concrete and the earth beneath (in that area anyway - the area with the most give).

As I say, parts of it feel perfectly solid, a couple of bits have significant give (5-10mm) and some are solid enough but feel like a normal timber floor when you jump on it.

Anyone come across anything like this before? Seems very odd. Read something about Sulphur attacks but couldn't find much more about it. Should I worry? I need to level the floor anyway - I gather latex is the way forwards with a 'bouncy' concrete floor?

TIA!

Andy

[meow2222]

Never come across that before. If you ram some stone down there when you fill the hole you're giving it an extra solid support, and halved span means much less give. If its bad enough you could put a few new supports in this way.

NT

[Roger]

The message from "Pecanfan" contains these words:

Not seen anything like that.

I take it you don't have any damp protection under the floor not any reinforcement in the concrete.

The void could be as a result of a serious water leak a long time ago or perhaps it was a jerry built replacement floor without a proper sub base and whatever fill was used has since shrunk.

Personally I would be tempted to dig it out and put in a proper insulated concrete floor. Concrete isn't good in tension so as it is there is always the possibility that it will fail some time in the future. Digging out will also give you a chance to do something constructive about the damp wall.

[Pecanfan]

No - there doesn't seem to be any form of DPM under the floor (from the bit I've taken up) and it's not reinforced. The floor itself is perfectly dry though.

Tempted with that option - looking at the price of self-leveller it may actually be the cheaper option anyway! And as you say it would give room to get the damp proof injection below floor level.

Cheers,

Andy

[Phil L]

Anything other than what Roger said is wasting money - rip it all out, dig out 150mm and put in 150mm of polystyrene, then DPC membrane, then 100mm of fresh concrete...the ground has sunk away, this is what happens when the ground isn't properly prepared prior to laying concrete, or the infill isn't properly compacted, you may also want to bung some steel mesh in there, for £15, it's not worth leaving out.

[auctions]

Had something very similar in our first house. The kitchen floor had an old water pipe underneath which had been folded over to seal it off, but was still connected. I think it was the supply for the outside loo before the room was knocked through to be the kitchen. The slow leakage over the years had caused the ground to drop by about 2 inches leaving a cavity.

We removed the concrete and fitted a new floor. The upside was that it made getting the concrete out easy - one bash with a sledgehammer and the whole thing dropped!

A

[Brian Sharrock]

This may not directly address your point .... in my 1935 built house the kitchen floor was constructed in different ways . At the two end corners there were plinths of concrete laid over with quarry tiles - the plinth in one corner supported a Rayburn cooker/heater while the plinth in the other corner supported a thirties-vintage gas cooker. Separated from these plinths was another longer one under the window. This longer plinth supported a 'Butler's Sink' - on brick piers ! The other areas of the floor were suspended timber - as is the rest of the house. AIUI flooring need not necessarily be all of one type.

[The Natural Philosopher]

It can happen..I agre with everyone else. A day with a hired kanga a sledge and a barrow will net you some nice hardcore that you can use later to ill the voids. Up it all comes, damp inject whats needful with full access - heck even underpin if you want, and lay a new slab over sand filled hardcore, and stick some mesh in it too, Then a nice slab of polystyrene and a screed with UFH. Magic!.

Or if its a big space and the subsoil is unstable, lay some strip fundations along the insides and put a block and beam concrete floor in.

[Weatherlawyer]

I am almost certain that your house predated widespread use of concrete. In fact cement wasn't even widely used. Perhaps the original floors now concrete, were clay tiles at one time. These were laid on earth and would have fallen with it, if there was any subsidence.

Then the occupier tiring of any unevenness laid concrete instead.

Or perhaps it was a genuine good job not just a bodge up but the ground fell away without taking the floor with it. As well as happening as the others have suggested settlement of the hardcore, water erosion and that, it might be a geological fault where a layer of clay over a sand (or something like sand for shrinking when drained) has collapsed as the aquifer -or water table, fell.

It's not that uncommon in the UK. I'm not sure that is a slowquake but you get the idea. Before going to any expense I would be prepared to shell out a few hundred finding the cause of the fault.

If it was due to land drainage, it could come back up one day. Of course in this weather it should be doing that now.

[Pecanfan]

I'm fairly sure concrete has been around longer than the 1930's. My last place was early 1900's and had original concrete floors. Wasn't the Pantheon made of concrete? ;-)

Andy

[Pecanfan]

Cheers everyone for the advice - up it comes! :-) To be honest, now that I'm looking under the floor I'm wondering if it's actually the concrete floor that's causing the damp problems in the wall - the 'soil' base to the concrete certainly bridges the DPC and it would also explain why this is the only room with rising damp.

It's a relatively small area (1.1m3) so happy to tackle it myself - anyone come across any good step by step guides for concrete laying? Presumably there's no need for formers since the walls will do that, with the DPM extending up between the wall and the concrete of course. So how do you get the concrete to the desired height - just marker pegs or something? But that would penetrate the DPM... ?

Out of a matter of interest, does the polystyrene not introduce an element of 'bounce' to the floor?

Andy

[Weatherlawyer]

Yes, according to the price the customer was willing to pay, the needs of the structure would incorporate whatever was known in the time of its knowledge. We know Wren used Portland Cement. But then he was rebuilding London and water freight and the mass market might have stood for it. Or there again perhaps he just used it on St Paul's.

But it is unlikely an expensive import would be carted by train and by horse to your house in the 1930's when a clay tile floor would have sufficed for the servants.

Do you really think it would?

There was hardly the draw on qualified builders that a national disaster would create and most dwellings even now just used the best that is available according to the work skills in the area.

And not even always then. A surprising amount of rubbish goes into a building. Consider fly ash and asbestos. One was a locally produced crap material the other a cheap import ...but an import no less.

Today its tin studding held together by plasterboard and black screws or aerated blockwork.

[The Natural Philosopher]

Portland cement is a victorian invention..

Prior to that other lime based concrete type materials existed, but lacked the strength. You can see such in old castle ruins and so on..the walls were filled with cement and flint. Well mortar and flint anyway.