Help - Concrete floors

[Jellyfire]

Hi, having recently moved into a 16th/17th Century timber framed cottage, (or moneypit as I now like to call it), we were about to have solid oak flooring put into the downstairs rooms. Upon removing the carpets and rubber underlay, the flooring people have discovered that the floor is concrete, and also damp in one room. We left it to dry out overnight, and it has dried out considerably, but the moisture reading in the walls is high. Our exterior walls are also concrete rendered, which we will be dealing with over next year or two when money permits, this hasnt caused any problems to the timber so far as we can tell as yet so the lime renderers advised us that it wasnt as urgent as we'd first expected.

The flooring company are suggesting they lay a membrane over the concrete and then fit the floor. My wife phoned our surveyor (who specialises in period propertys), and he didnt suggest removing the concrete floor. Im just really wanting some reassurance (or otherwise) that this isn't just going to make for long term problems, as the whole concept of waterproofing a concrete floor fills me with unease!

Im sure somebody will suggest laying a stone floor, but Im afraid we wouldnt be able to afford that so wood seemed the next best option
thanks
Lee

 

[Andrew Bodenham]

Hello, Jellyfire,

Tricky one this without seeing it. But it would seem that your house has been visited by a cement enthusiast at some point in the past. Presumably the concrete floor has been laid to cover the original earth floor or stone flags or whatever was originally there - a typical reaction to finding an 'old and unsatisfactory' floor. Really, you wonder why these people buy ancient houses instead of a nice 1970's bungalow. Anyway, it seems that the concrete was either laid without a damp-proof layer under it, or the damp-proofing has failed in some way. So you either have to remove the concrete and review the situation, or keep the concrete and deal with the dampness. The underlay and carpets have clearly done a good job of preventing the moisture from dispersing, leading to localised dampness, which is now being allowed to dry out naturaly. The dampness in the walls may be related to the concrete floor, but there are more likely to be external causes.

In deciding what to do you need to ensure that moisture rising through the floor is able to escape and disperse rather than being concentrated or driven up into the walls. Covering the floor in a continuous slab of wood is unlikely to help, particularly if an impermeable membrane is laid beneath. However, if the wood floor was laid on battens to leave a ventilated space below this might solve the problem, but the increase in floor level might cause problems with your doorways etc. I feel that you need to do a bit more investigation here and have a long hard think before committing time and money to any flooring scheme. I would be keen to find out about the relationship between floor levels and your wall timbers, and to find out what lies below the concrete layer, perhaps by digging a trial hole. Only then will you able to make an informed judgment about the best way to proceed.

 

[Penners]

(I was typing while Andrew posted, so there are many similarities in our suggestions!)

This is a common problem, and always a tricky one.

Concrete floors laid on to earth have a tendency to force the subsoil moisture to migrate sideways, where it inevitably ends up coming into contact with the walls. This is probably what is responsible for the damp in your plinth walls.

Removing a concrete floor isn't always possible, for technical or financial reasons. And although such removal is usually desirable, there's no guarantee that it would cure the damp walls - only a likelihood.

If the concrete floor is damp, it probably hasn't got a damp-proof membrane incorporated within it. So, however much you allow it to dry out now, the probability is that as soon as it's covered up with a membrane, it will start to become damp again.

So that's a sort of analysis of what you're faced with. The solution? Well, it's impossible to specify a hard-and-fast one. Damp is dreadfully unpredictable. But here are two different approaches for you to consider.

1. Remove the concrete floor and replace it with limecrete or a suspended wood floor. I know you're trying to avoid doing this, on cost grounds, but it has to mentioned as the most likely option to solve your damp problem in the long term. (Note that I said "likely" and not "definite").

2. Leave the concrete in place and do your best to manage the damp by other means. If you want to lay a wood floor, rather than covering the whole concrete slab with a membrane, I think it would be preferable to do the following. Lay joists/battens of pressure treated timber across the concrete (within reason the biggest in section that your ceiling height will allow), with strips of damp-proof membrane underneath them. Then lay the wood floor on the joists. Engineered timber boards would be best for avoiding the risk of cupping or warping, but if this is too costly, just be sure that you don't stint on the battens and fixings.

Ensure that you make provision for plenty of air circulation under the wood floor, to avoid a build-up of moisture (and its attendant risk of various forms of rot).

This won't solve the damp in the plinth walls. All you can do for this problem is to ensure that you allow the walls to breathe as much as possible. Either leave the brickwork bare or plaster with lime. And don't be tempted to slap black waterproof gunk on the exterior. I should also check that exterior ground levels are below the interior floor levels. You may also find that a French drain around the base of the plinth on the outside will help.

 

[Moo]

Do you know how thick this concrete floor is?

If it's a foot or so, it would certainly be a big job to get rid of it. But what if it's only an inch?
And - (pause for excited intake of breath) what if it's only an inch and there are wonderful pamments or flags underneath?

Please, put me out of my misery and do a bit of excavation in an unobtrusive spot.

 

[FamilyWiggs]

I second Moo - before you lay another floor on top, investigate what lies below the concrete.

If, as it sounds, there is no DPM beneath the concrete, then it seems equally likely that the original floor was not lifted before the concrete was laid. You could have a lovely (breathable) natural floor that will only require some TLC to reinstate...

Go on - take a cold chisel to a square foot or few in a corner. if you find nothing, don't give up - try a second spot to be sure.

 

[MdB]

Go on - take a cold chisel to a square foot or few in a corner. if you find nothing, don't give up - try a second spot to be sure.

I think I got up to 11 test holes in total both inside and outside the house before we started work....

 

[Jellyfire]

Thanks for advice, were in a bit of a dilemma now. Im afraid this is all a bit new to me, I've started chiselling through one of the floors, have got to a depth of about 3 inches, with no sign of breaking through the concrete... it's not easy stuff to get through, Im just using a hammer and chisel and it is very, very, very slow going, any tips breaking through concrete? Should I be taking this slowly or just whacking it with a sledgehammer instead?

I've attached some pics, not very clear Im afraid. The concrete (I presume it is concrete?)View attachment 2View attachment 1 is very different to the stuff in the other sitting room, it was formerly 2 cottages, one room in each cottage, so Im thinking one may have been laid much earlier. The one I'm working on is the one that is letting the damp through, and the other one is the one thats bone dry but the walls are damp. The floor thats damp, does this mean its actually breathable and I could lay a stone floor on top of it?

In response to Penners' comment, the plinth is painted black on the exterior, presumably with 'black waterproof gunk' already, top add to the lovely cement render!

 

[Flyfisher]

Well done for persevering enough to get down 3 inches! I'd expect a minium of 4 inches, so you might be close . . . or not. Who can tell?

The difficulty if because all the concrete around the hole is holding together the concrete you're trying to break apart. As soon as you get a 'free edge' it will become much easier to break away lumps of concrete into the free space of the hole - if that makes any sense. if it really is hard-going though, it might be worth using a hammer drill to make a series of small holes (6/7mm -ish) and then chisel out the concrete between them. But don't drill too deep as you may damage some nice underlying tiles.

It's interesting that you say the non-damp concrete floor is in the room with damp walls. Conventional wisdom is that the impermeable floor is causing moisture to migrate to the walls, but there could be a number of other causes, external (e.g. leaking gutters/downpipes) or internal (e.g. condensation against a cold wall).

As for the black plinth, it seems very common throughout East Anglia. We have exactly the same.

Good luck with the hammer-work - you'll have everyone eager to see what lies below!

 

[FamilyWiggs]

Yes keep going - and do let us know if you come across Sahara's cellar down there!