Solid oak floor on a dry concrete slab.

[sliperooni]

Hello all,
I live in a very old house and - mainly thanks to this site - am very damp-aware, and usually go against all the advice about installing DPMs everywhere, and the rest if it, and my walls and floors are dry! But, in one of the rooms there's a modern concrete slab (looks fairly recent, but don't know for certain as I've only been here a year) covered with the dreaded blue plastic, carpet and underlay. As I said, the slab under that lot is dry but a third of the room is old quarry tiles, laid on ash I suspect. They're surprisingly dry too - probably because the walls are lime plastered, so everything's breathing. Sorry to be so long-winded, but my questions are 1) can I lay the boards onto a 10mm membrane of sheeps' wool and screw them down into the concrete, or, 2) glue and nail them directly onto the concrete using a breathable adhesive - if there is such a thing. As I said, the slab's dry, so probably don't need breathable materials, but the quarry tiles will. Any comments and advice would be greatly appreciated. Thank you

 

[Cubist]

Let me be clear in my understanding please; you have a blue plastic membrane of some kind which has been laid over a part concrete and part quarry tile floor and over the blue plastic membrane underlay and carpets have then been laid. Can you confirm this please?

Assuming this is correct then you need to re-consider your plan for this floor. The blue plastic sheet you have found was almost certainly laid as Damp Proof Membrane to protect the underlay and carpet - it can serve no other purpose. If you were to turn any part of this sheet over you are likely to find that some parts of its underside are damp or even wet. However, if you were to find that it is thoroughly dry like the concrete and quarries you've checked that, sadly, is not conclusive proof that the underlying floor is absolutely free from rising damp - much depends on the topology of the surroundings and the effectiveness of local natural and human constructed drainage. You would be unwise to assume that there is no future risk of damp rising through the floor.

Which brings me to your plan for a solid oak floor. Using solid oak boards for this purpose would be at the very least risky and, in my opinion, little short of a criminal travesty. Please re-consider. Solid oak boards can if exposed to significant damp bend and warp dramatically and cause such a floor to, in effect, tear itself apart. If you are wedded to the idea of an oak plank finish in that room I would urge you to consider using an Engineered Oak Tongue and Groove Planks which have a minimum 6mm veneer of natural oak glued to a minimum 14mm thickness of plywood. These share all the advantages in terms of appearance, function and performance as solid boards but the risk of significant warping in the future is thoroughly minimised. Either way, solid oak or engineered planks, DO NOT NAIL these through the plastic membrane and into the concrete. Doing so would destroy the integrity of the plastic membrane and the nails would then provide a direct path for any damp present in the concrete to migrate into the planks to destroy them. The correct approach is to lay the planks directly over the membrane to construct what is known as a 'floating floor'. No mechanical fixings or adhesives are needed for such as the interlocking tongues and grooves across its full length and width together with the collective weight of boards will hold them rigidly in place.

I have two such engineered oak floating floors here that have happily survived unscathed for 15 years - one of which avoided any material damage following a minor flood in part of the room concerned.

 

[Cubist]

By the way - this isn't the floor you have discussed in your other thread here is it?

 

[sliperooni]

Let me be clear in my understanding please; you have a blue plastic membrane of some kind which has been laid over a part concrete and part quarry tile floor and over the blue plastic membrane underlay and carpets have then been laid. Can you confirm this please?

Assuming this is correct then you need to re-consider your plan for this floor. The blue plastic sheet you have found was almost certainly laid as Damp Proof Membrane to protect the underlay and carpet - it can serve no other purpose. If you were to turn any part of this sheet over you are likely to find that some parts of its underside are damp or even wet. However, if you were to find that it is thoroughly dry like the concrete and quarries you've checked that, sadly, is not conclusive proof that the underlying floor is absolutely free from rising damp - much depends on the topology of the surroundings and the effectiveness of local natural and human constructed drainage. You would be unwise to assume that there is no future risk of damp rising through the floor.

Which brings me to your plan for a solid oak floor. Using solid oak boards for this purpose would be at the very least risky and, in my opinion, little short of a criminal travesty. Please re-consider. Solid oak boards can if exposed to significant damp bend and warp dramatically and cause such a floor to, in effect, tear itself apart. If you are wedded to the idea of an oak plank finish in that room I would urge you to consider using an Engineered Oak Tongue and Groove Planks which have a minimum 6mm veneer of natural oak glued to a minimum 14mm thickness of plywood. These share all the advantages in terms of appearance, function and performance as solid boards but the risk of significant warping in the future is thoroughly minimised. Either way, solid oak or engineered planks, DO NOT NAIL these through the plastic membrane and into the concrete. Doing so would destroy the integrity of the plastic membrane and the nails would then provide a direct path for any damp present in the concrete to migrate into the planks to destroy them. The correct approach is to lay the planks directly over the membrane to construct what is known as a 'floating floor'. No mechanical fixings or adhesives are needed for such as the interlocking tongues and grooves across its full length and width together with the collective weight of boards will hold them rigidly in place.

I have two such engineered oak floating floors here that have happily survived unscathed for 15 years - one of which avoided any material damage following a minor flood in part of the room concerned.

Thanks you so much for your sound advice - makes absolute sense. I was just hoping that 22mm thick oak planks would withstand a little potential damp, but from what you say, my hope was very misguided; I didn't realise that oak would warp so significantly on what seems like a dry floor. So, I've cancelled my original order and ordered engineered wood at the thicknesses you recommend - and I will ensure it's laid as a 'floating' floor. Could I just ask please, do you recommend a plastic membrane? I only ask because plastic, where's there's damp, is anathema to me.
It's good to know that you have used engineered oak successfully, and I wish you many more years of enjoyment of it.

 

[sliperooni]

By the way - this isn't the floor you have discussed in your other thread here is it?

No, it's the room next door, where there are some of those same quarry tiles, but, as I said, only a third of the room is made up of them, the rest is concrete, so I can't salvage that floor unfortunately.

 

[Flyfisher]

I was just hoping that 22mm thick oak planks would withstand a little potential damp, but from what you say, my hope was very misguided; I didn't realise that oak would warp so significantly on what seems like a dry floor.

I'd suggest that a big part of the warping risk is the different conditions on both sides of the oak planks. Even "a little potential damp" on the underside combined with the topside enjoying, presumably, the warmer and drier environment of the living room, will create good conditions for warping.

I used 28mm oak planks to replace a suspended floor over a cellar and, being aware of the likely warping risk, I biscuit-jointed the planks as well as screwing them down to the new joists. Three years on, the planks haven't noticeably moved. However, within the floor there is a hatch to the cellar of about 5x3 feet. I made this out of the same 28mm oak planks, also biscuit jointed, and braced underneath with additional 28mm oak planks, screwed and glued. It is so heavy and solid that I had to fit two gas struts to enable it to be easily lifted for access to the cellar steps, yet it has warped slightly and the centre line of the hatch now sits about 5mm above the floor level. Not a big deal, especially with a rug on top, but an indication of how much an apparently rock-solid piece of oak can move under certain conditions.

I would not have expected an engineered oak hatch to warp but being G2* I wanted to get as close to a like-for-like replacement of the old floor that was in a fairly dangerous condition after a few hundred years of exposure to the damp cellar. At least the hatch seems to have stabilised into its current gentle curve and doesn't seem to move with the seasons, so I'm pretty confident it's now good for another few hundred years

Thread and Photo here

 

[Cubist]

Could I just ask please, do you recommend a plastic membrane? I only ask because plastic, where's there's damp, is anathema to me.

Most suppliers of hardwood and engineered wood planks will often also supply DPM like membranes or underlayments designed to protect the wood. I regard such as an insurance policy insofar that whilst a solid concrete or other floor may seem to be thoroughly dry, that condition can rapidly change for a variety of reasons. Yes, there is a trade off that needs to be considered insofar that any damp that may then rise to meet the underside of the membrane could then be pushed toward the perimeter walls but this problem can also be effectively managed there.

Belt and braces perhaps but well worth the investment.

@Flyfisher is spot on, as always and for additional information the warping results from the internal pressures induced in the natural grain structure of the individual planks and every one of them is thoroughly unique. Engineered boards, by virtue of their internal structure resist those internal pressures much more effectively.

 

[sliperooni]

I'd suggest that a big part of the warping risk is the different conditions on both sides of the oak planks. Even "a little potential damp" on the underside combined with the topside enjoying, presumably, the warmer and drier environment of the living room, will create good conditions for warping.

I used 28mm oak planks to replace a suspended floor over a cellar and, being aware of the likely warping risk, I biscuit-jointed the planks as well as screwing them down to the new joists. Three years on, the planks haven't noticeably moved. However, within the floor there is a hatch to the cellar of about 5x3 feet. I made this out of the same 28mm oak planks, also biscuit jointed, and braced underneath with additional 28mm oak planks, screwed and glued. It is so heavy and solid that I had to fit two gas struts to enable it to be easily lifted for access to the cellar steps, yet it has warped slightly and the centre line of the hatch now sits about 5mm above the floor level. Not a big deal, especially with a rug on top, but an indication of how much an apparently rock-solid piece of oak can move under certain conditions.

I would not have expected an engineered oak hatch to warp but being G2* I wanted to get as close to a like-for-like replacement of the old floor that was in a fairly dangerous condition after a few hundred years of exposure to the damp cellar. At least the hatch seems to have stabilised into its current gentle curve and doesn't seem to move with the seasons, so I'm pretty confident it's now good for another few hundred years

Thread and Photo here

Looks good! Thanks for this. I am amazed though, having lifted up the carpet and the plastic dpm, at how intact and free of rot the Edwardian, one inch thick, wainscotting around the room is, and it goes right down to (earth?) floor level! This baffles me, as it doesn't look like hardwood, but some sort of pine, I think. This house seems to have an amazing respiratory system!

 

[Cubist]

Sorry about this but a couple of afterthoughts/questions struck me;
Has the contractor explained that any doors opening into this room will require some surgery? And also, has he explained how the transition into any neighbouring rooms would be handled? Your new floor would be circa 25-30mm higher and could therefore pose a trip hazard.