Wooden flooring over a (damp) solid floor

[lorbight]

Hi there all,
I'm after some advice please. We've recently moved into a 1830's cottage in the Midlands and have found that the front room floor is a bit damp. We removed the cheap carpet to find a good quality underlay sat on what we believe are 1950's floor tiles - not good quality really. We removed them at the weekend and found they were sat on a sandy mortar mix which itself sits on a solid (but rough) concrete slab. The walls appear to have been chemically DPC'd as they have drill holes externally (plugged) and internally (found them after removing the skirting board).
After a bit of 'exploritary surgery' on the concrete floor, it appears very poor quality subfloor beneath.

Our concerns for putting a wooden floor in have been raised when we noticed the the sandy mortar is very damp. Now... can we put a DPM over the sandy layer and then float a battened floor (with insulation) over the DPM and then attach the floroboards to the battens?

How do I 'finish' the DPM to the DPC in the walls?

Could it be just condensation? We've recently opened up the chimney and installed a wood burner so will this help with drying out the room? Or will we allways have a damp problem and ruin quite expensive floorboards??

Thank you in advance (the flooring's arriving on Thursday!!)

Rob.

 

[Matt Green]

There are a lot of variables in the scenario you describe. We need to know:

What the external ground levels are in relation to the internal concrete floor, and how thick is the concrete?

What sort of plaster is on the walls?

Is the concrete floor a replacement- does it look like there was a timber floor originally that was filled and screeded over?


Cheers,


Matt Green

 

[lorbight]

Hi Matt,

Sorry, I'm new to all this!

The external ground floor level is three bricks lower than the DPC (assuming the DPC is the same brick level inside and out...) and therefore the internal floor. The concrete is varies in thickness between 30-50mm. If I've not mentioned already, the sandy mortar layer is about 25mm.

The plaster appears not to be too old (ie I don't think it's the original), although I can't be sure... The rough base layer is about 10mm and then a finished layer on top of that is about 10mm again. We found this when a radiator was removed (for good) and found the plaster had blistered behind it.

The thought that it was a backfilled suspended timber floor had crossed my mind, but in my small hole I've dig by the wall, I can't see any evidence - I'm expecting to see cavities in the wall where the joists would have been, correct?

Thanks again

 

[Penners]

Have you found any evidence of a dpm between the concrete sub-base and the sandy screed (failed or otherwise)?

 

[lorbight]

There's not even a hint of a dpm! If i could work out how to put images into my post i would show you the state of it.

After a couple of evenings of a warm well ventilated room, the floor SEEMS to be drying out a little.

 

[Cloudscape]

I know this is a little late to say this, but I was wondering if the carpet underlay you removed was rubber, or rubberized. A neighbour of mine found that the floor in a room where rubber underlay was used was saturated and the lower parts of walls were being affected.

Once the underlay was removed, the moisture began to evaporate away. What I mean is, your floor might be damp, but maybe removing the air circulation made it worse. Incidentally, the neighbour's floor had been damaged by being so soggy and did need remedial work.

 

[Matt Green]

Hi Matt,

Sorry, I'm new to all this!

The external ground floor level is three bricks lower than the DPC (assuming the DPC is the same brick level inside and out...) and therefore the internal floor.

:|

Morning Rob,

...3 bricks lower than the DPC.... The DPC is chemical, right? And there are holes on both sides? The DPC level can't be the same as the internal floor level, unless I'm missing something. Assuming, externally, the gap between the ground and the DPC is approximately 7 1/2" (3 brick courses + mortar ish) the internal floor must be less than that.

The point is that there should be a difference of at least 150mm (6") between external ground level and finished floor height. It sounds like you are very close to that and ground water is making the sand layer under the concrete wet.

Don't assume the DPCs were put in at the same height, at the same time or by the same company.

Sounds like it's always been a solid floor, possibly tiles on sand or similar. Putting a DPM over wet sand and laying a floor on top will probaly not cause much of a problem for the floor, but there's a good chance the ground water will evaporate through the base of the walls. This is probably why there 2 DPCs in the wall and so much water has been drawn through the plaster behind the radiator. The concrete will have let less water through than the tiles (or whatever) were there before it was laid and caused a damp problem.

The room is essentailly being converted to take a floor construction that it never had, which is not an ideal situation. I take it there are no air bricks either.

However. Your floor is arriving in two days time.

Take up the concrete. Lay a DPM. Lap it around the edges of the floor. Remove a channel of plaster at low level around the perimeter to a height of: "as much as possible that can be hidden by a skirting board" (about 50/60+mm). Trim the DPM to tuck into the channel. Stick your skirting boards on. It may be prudent to use treated timber skirtings, they are likely to have a pretty high moisture content.

Then: Reduce the ground levels outside as much as you can. the idea it to get the ground water trapped under the DPM to evaporate outside, rather than around the edge of it and up into your walls.

It's not a perfect specification, but it shouldn't do any harm to the building, should reduce the damp and you get your floor and it should be at low risk of decay once fitted.

Hope that helps.


Matt
-would have been a good application for limecrete, though.

 

[lorbight]

I certainly don't think it's too late! Yes the CarpetRight underlay was rubberized. I'm hoping the dampness will continue to dissappear, but am concerned putting a DPM down will have the same effect. That moisure will have to go somewhere - in to the walls perhaps?

 

[Penners]

Rob - if you won't have enough time to do what Matt suggests before the designated carpet delivery date, in your shoes I would delay the carpet.

Matt's given you excellent advice (he's a professional expert in such things) and you would be very sensible to carry out his programme before laying the carpet.

 

[lorbight]

Sorry I should have made it more obvious. We want to put an oak floor down...

Thank you very much for your advice, Matt and others! Really very much appreciated - :lol: I shall check out all the pointers re the DPC in the walls this evening as i'm unfortunately sat in an office Coventry at the mo.

Thanks again!

 

[CHRIS]

Think long and hard about putting the Oak floor down, A good friend of mine had a similar time of things when his wife wanted a nice Oak floor ASAP and after a few weeks it was starting to come up (wide boards bowing) and he was gutted - £1000 worth of fire wood.

Do the right thing and sort the problem before it is too late as Matt says.

 

[lorbight]

One final thought (and forgive me if I sound dumb)...

The DPM behind the skirting will finish higher than the finished floor level?

 

[Matt Green]

That's right. You want the water to evaporate into the void behind the skirting rather than into the edge of the floor or under it.


Matt