Damp coming up through concrete slab

[gigakid]

Hi, hope someone can help me here.

We recently removed laminate flooring in a new property, uncovering wooden floorboards downstairs, and found that where a chimney hearth had once been there was a flush concrete slab. About a quarter of the slab is prone to damp and you can feel moisture coming up through it especially when weather has been bad.

We now wish to lay a new solid wood floor, but I am convcerned about putting this down, whilst there is a potential damp problem - a floor fitter recently checked moisture and said it was quite high and that he could fit the floor but not give a guarantee whilst there was this small area of damp.

I am getting conflicting info ranging from one extreme - "this is natural in houses like this and is nothing to worry about" to the other "its very serious, and if you dont address the damp issue it will
penetrate the whole of your ground floor as et then dry rot"

Would a damp proof membrane help if spread over the area which is prone to moisture?

One of the flooring guys I had round said that f the damp hadn't affected the laminate florring (it hadn't) then it wont affect wood flooring?

Any adfvise on this would be helpful - I have stuck my head under the floor and can see that under the hearth and the floorboards is a brick construction which is very wet - is this normal?

 

[Softus]

Has the chimney been removed?

 

[gigakid]

no, the chimney is still there but has been opened up and plasterboarded as an alcove.

No damp is coming through the chimney breast walls, only up through the concrete slab which the hearth would have laid on.

 

[Deluks]

I'm assuming this chimney is on an exterior wall. If not then the weatherproofing around your chimney stack could be hookey.

 

[gigakid]

Yes its on an exterior wall of an end terrace

 

[Softus]

No damp is coming through the chimney breast walls, only up through the concrete slab which the hearth would have laid on.

In that case it's easy - take up the slab and put a DPM down before re-laying it.

It would also be a good idea to find out what it's so wet down there and put a stop to it.

 

[gigakid]

Thanks,

I had thought of removing slab putting in dpc then relaying, but wondered if this was avoiding a bigger problem underfloor.

Once my new wood floor is laid, access underneath would be a problem

 

[captain_scarlet]

would be worth having a look under the floor to see if anything is going wrong. i've got/had water coming into the foundations from multiple places and has been for a very long time. damp patches over various walls etc. maybe water penetration occuring in your house?
a few years back i discovered repeated damp patches next to the radiator in the downstairs bathroom with a concrete floor. could'nt find what was causing it until i saw the wifes new cat using the floor as a litter tray

 

[memor maplin]

Are there any drains or down pipes on this external wall?

Memor

 

[LexL]

Problem 1: Leak - that is a crack in the slab or an exterior foundation wall. Check both for any indication of water such as stains or scaling and also check the bottom of any interior walls for signs of water.
Solution 1: Have all cracks sealed with concrete sealer before proceeding.
Problem 2: Wicking - that is the water is coming through the slab itself as it would through a sponge. This indicates that the rain water is not draining properly from under that part of the slab which could indicate a blockage in the drainage system around the house. A company specialising in drainage systems can varify the problem and tell you what you need do to correct it.
Solution 2: Have the inspection done and complete whatever repairs are necessary.
In either case repair any water damage to interior walls before laying the new floor. The lay down a vapour barrier over the entire floor area to be finished and wrap the edges up the walls at least 2 inches. This will provide a waterproof basin in which to instal the sub floor and finished floor. I would also strongly suggest that you install "sleepers" between the vapour barrier and the sub-floor. These are usually 1 x 2 or 1 x 3 wood spaced at 16" on center and they are not nailed down because the nails will perforate the vapour barrier. They do allow any moisture that may get in to evaporate before it can do any harm.
I hope this is of some help and good luck with the project.

 

[IJWS15]

On the flooring section they normally recommend having the same kind of sub-floor for timber/laminate.

Would probably make sense to take out the slab, sort out the cause of the damp and fill the gap in the floor by extending the existing wooden floor across the hole rather than replacing the slab (may affect any future fire installations).

 

[joe-90]

I'm with Softus on this one. Dig up the slab and put in a damp - proof membrane. Rising damp is a nuisance.