Wood floor over concrete subfloor

[robertoi]

Hi all,
This is my first post - so be kind to me!

I am about to start renovating our kitchen. It has a solid concrete floor, possibly around 100 years old, and so with no proper DPM. Currently the kitchen floor has a crappy bituminous levelling compound (which only covers about the 70% of the room which has floor covering, the areas under the kitchen cabinets have nothing on them). The floor seems to be pretty level, although there are the odd rough areas/ cracks.

We had various damp 'experts' and buidlers come and look at the floor as we thought it might be causing a slight damp problem - and were interested to see how much it would cost to get a new screed with integrated DPM put in. Given the conflicinting advice (and downright nonsense from some) we got - Ive decided my preferred course of action is to leave the concrete floor pretty much alone and install a wooden floor over the top. If there is moisture in the concrete floor, I woudl rather it was able to evaporate up and out of the floor than get trapped underneath .

I have been trying to work out therefore if it would be possible for me to lay battens down on the floor, and use T&G treated pine floorboards as my covering. So was wondering if anyone could advise me on whether this would work, and what the likely pitfalls might be. For example - if my foor is not currently completely level, what would the impact of that be - and whats the easiest way of sorting it out etc....

Muchos thanks in advance!
Rob

 

[mattysupra]

your subfloor still needs to have a working dpm. Is the floor wet or not? What is the RH moister reading of the floor?

 

[robertoi]

I havent measured the moisture content yet - when it was measured a while ago in the area where there is no impervious tiles covering it (ie under a kitceh cabinet) - the guy from the damp company told me there was negligible damp. When I lift a tile, the floor looks damp, but soon dries out, so Im assuming it is condensation getting trapped behind the tiles that are there at the moment.

Why do I need a DPM? I hear alot of people these days in older properties are deliberately installing no DPM, and using a lime mixed concrete when replacing solid floors - they then let the floor 'breathe' and moisture is allowed to evaporate out rather than collect behind a floor covering. I would go down the limecrete option myself but I cant afford it.

Thanks for your help.

 

[robertoi]

Forgot to mention - another reason I want to go for a battened floor is that curently my pipes all run along two of the walls and Id really like to get these moved under the floor - hence Id need to install new kitchen cabinets onto the wood floor.

 

[mattysupra]

A dpm is needed or otherwise the moister entering the bottom of the wood will be miles away from the moister content above the wood. The wood floor will start to take in the moister and grow and a different rate from top to bottom. The flooring will start to cup in no time at all and start to grow/expaned. You wil also have a problem with the floor starting to rot. But the wooden flooring will fail before it goes rotten to be fair.

the guy from the damp company told me there was negligible damp

What is negligible damp? What was there moister reading? To british standard you can not or should not fit any kind of floorcovering on a floor that reads more than 75% rh. For wood flooring it needs to be alot lower than this.

 

[robertoi]

I see your point - higher mooisture content below the floor presumably? Dunno what their actual reading was - can u buy a suitable damp meter from somewhere like screwfix?

Cheers

 

[mattysupra]

no you cant. You need a hygrometer floor tester. I dont know where you can hire one from either. The test takes between 24-72hrs to complete aswell. You could do with getting someone in that knows what they are talking about to do the test for you. If your anywhere near me (solihull) you could borrow one of my meters.

 

[robertoi]

ok, think ive found somewhere to hire one from. If the moisture content is say ~50 - 75% rh, what steps would you recommend?

The meter I could hire is a Vaisala HM134 is this the correct kind of meter?

 

[mattysupra]

If your floor has a working dpm it will read around 45% rh at a guess. You need a maximum reading of 60% rh to put down a wood floor without a surface or some other kind of moister barrier. If the floor reads above 60% rh you will need a vapour barrier ( if floating ) or a epoxy liquid dpm if thinking about gluing. If the floor reads above 75% rh then you will need to install a surface dpm ( epoxy ) for both methods of fitting.

As for your meter you have found, Well i have not used that meter before but it is simular to one i have. To measure concrete rh you need to drill holes in the subfloor and fit a special plug into the hole. You then leave for 72hrs and then dip the probe into the plug to get the correct moister reading. You need to find out if you can get these plugs for that meter.

The moister meter i mean is basically a hygrometer that is sealed into a box 10"x10" in size that you then glue to the subfloor. There is a air void below the meter that traps the moister over a set time ( 24-72 hrs ) and this then tells you the moister content of your subfloor. Try here http://www.f-ball.co.uk/product_results.asp?searchTrue=search&productSearch=hygrometer Think they are about £90-100 quid. don't know where to hire from tho. Still if you can get the plugs for the other meter i think you could just use that tho.

 

[robertoi]

Thanks for your advice

I have found that I can buy one of those metres locally for the same price it would cost to hire one!

I have been speaking to a damp expert (one I actually trust!) this morning about this, and he strongly recommended not putting anything such as a liquid DPM onto an old floor such as ours as it is likely to just force water up into the walls and potentially cause rising damp. I have been told this by other people in the past also. He reckons that the best way of dealing with our floor would be to get rid of it and put in a suspended floor instead. Obviously Im not going to do this as the time and expense would be ridiculous.

However, conceptually, a suspended floor lying say 40cm above ground level below is not that different to a battened floor of soft/ hard wood planks resting 5cm above the ground level. With a sealed floor such as laminate or engineered wood I can see why moisture would be a problem as there is essentially no-where for it to go once it leaves the concrete. However, with a 5cm cavity below it, and gaps between the boards (if I didnt use T&G) - I cant see how the moisture gradient between the back of the board and the top would be that great.

 

[mattysupra]

you will need to install air bricks so air will flow under the void. Other wise there will be big problems. Good luck with your project.

 

[gazzaharwood]

you will need to install air bricks so air will flow under the void. Other wise there will be big problems. Good luck with your project.

sorry for jumping on to someone elses post...but im about to lay a battened floor onto concrete in the cellar...
concrete floor...DPM....loose battons...floorboards
do i need to vent this void???? if so how? and why dont voids need to be vented in ceilings (ie upstairs and downstairs)

many thanx
Gazza!!!

 

[mattysupra]

the air bricks are to give air movement where there is a build up of moist air. You dont need them in between floors as there is no wet air either side. In other words the humidity should be almost equal below and above. There still is air flow between floors anyway. As for your cellar, as long as you dpm and tank the walls to above ground level there is no problem.

 

[gazzaharwood]

the air bricks are to give air movement where there is a build up of moist air. You dont need them in between floors as there is no wet air either side. In other words the humidity should be almost equal below and above. There still is air flow between floors anyway. As for your cellar, as long as you dpm and tank the walls to above ground level there is no problem.

phew, thanx m8!!!