Laying wooden floorboards over concrete - how?

[Lindyboo]

It must be covered somewhere in the forum, but I haven’t found it, so … I’ve been looking into the feasibility of laying wooden floorboards over the (uneven) concrete floors throughout the downstairs. We have low ceilings so can’t install joists. Having scoured the web for similar stories I find advice which says that it’s possible to have a floating floor laid on top of some sort of membrane, or alternatively, to fix battens to the floor and nail the boards on top of those.

In either of the above cases – assuming the advice is good - presumably some sort of breathable membrane would be necessary to avoid damp, but would the lack of an air-gap create problems? And if I'm correct about the membrane, what types are available? (House unlisted, btw).

 

[ljn]

I did this a couple of times and the result was a very solid feeling and sounding floor:

Level screed with wood battons embedded at 40 cm centers.
18 mm WBP ply or green chipbord screwed to the batons.
10 mm parquet pinned and glued to the sub-floor.

Any membrane would have to be beneath the screed

 

[shelli]

… I’ve been looking into the feasibility of laying wooden floorboards over the (uneven) concrete floors throughout the downstairs. We have low ceilings so can’t install joists. I find advice which says that it’s possible to have a floating floor laid on top of some sort of membrane, or alternatively, to fix battens to the floor and nail the boards on top of those.
In either of the above cases – assuming the advice is good - presumably some sort of breathable membrane would be necessary to avoid damp, but would the lack of an air-gap create problems? And if I'm correct about the membrane, what types are available? (House unlisted, btw).

I don't see how you could do that when the concrete is already in situ - Ljns advice would need battens embedded in the concrete and any membrane would need to be underneath the concrete and the hardcore/sand layers beneath

I would have thought that as your concrete is already down and head height is a factor a wooden floor wouldn't be possible - what about laying tiles ? then you would just need some self levelling compound if there are little wonky areas - or at worst you may have to break out any significantly raised bits of concrete and re-screed. This would eliminate damp issue -this is best if theres some insulation under your concrete or it will be very cold floor (possibly put underfloor heating under tiles ?)

the previous owners of our house had a concrete floor in their sitting room (pic attached ) - they decided on a lovely red carpet !! (but hey it was the 70's!)

 

[Lindyboo]

The flooring I’d envisaged, based on my own (inexperienced) assumptions and on stuff from the web, was to leave the existing concrete floor as is and then put down a suitable membrane which breathes in order to prevent moisture build-up beneath it. This latter point I assumed would be essential to avoid the moisture looking for other ways out – up the walls for example – but would not cause warping of the overlaid floor. On top of this membrane, affix the battens (25mm sq) – which would mean screwing through the membrane into the concrete – then laying the floorboards on top, much in the same way as a laminate floor. The floating floor would have an expansion gap, covered by skirting boards. Any dips in the floor I would hope to compensate for with some sort of noggins. I daresay I’m talking like a noggin, but I’d be glad of feedback on this grand plan.

Tiled floors are not an option, primarily because I don’t like ‘em. Underfloor heating (wet system off oil boiler) was examined as an option and we had a survey which concluded it could be done, though of course this would require substantial digging up of the floors. I was prepared to go that route but have ditched the hopeless builder who was setting up all this, and other extensive work here, so I’m back to square one. Reliable builders and other tradesmen are no more plentiful here than anywhere else, so the option I’ve outlined above is a second choice. Electric underfloor heating, as far as I can gather, is prohibitively expensive.

We’ve been waiting months for matters here to progress, but thanks to the builder sitting on his jacksy things are becoming urgent and we can’t wait any longer, so I’m looking at options which will achieve a decent look and feel underfoot, won’t break the bank and can be done DIY. Summer will soon be over and the heating system is on its last legs – it may last another couple of months – so we can’t hang about. The ex-builder’s faffing about has also meant that the full re-wiring, new windows and extensive plumbing has also become urgent. Should have been finished long ago and hasn’t even been started – never even got quotes … better stop now. Feel a bout of Tourettes coming on.

 

[MdB]

I'm not an expert on this but you seem to be saying that you want the membrane to both allow the moisture to escape from the floor and to stop the moisture from reaching the wood. Its is a bit difficult to do both.

 

[Lindyboo]

Exactly, which shows how little I know and why I ask questions on this forum. I was making an uninformed assumption that some moisture had to be allowed through to prevent ‘sweating’ of the concrete and thereby building up moisture that would find its way out elsewhere in the house, but that a suitable membrane would not let through enough to rot the boards. Crazy, but that’s inexperience for you.

I’ve been reading further on this since my last posting and found a new seam of sites which talk about a product called Combilay which apparently does the job of a DPC. According to B&Q (I can hear you all shudder): 3mm Combilay and tape. Suitable for all sub-floors that require a damp proof membrane. Combilay has combined cushioning, sound proofing and damp proofing, meaning you only have to lay 1 layer rather than 2. Apparently this stuff can be laid directly over concrete and then a floating real wood floor goes on top. Various sites mention this business about laying down battens first, but many others say it’s not necessary – this Combilay stuff is considered adequate, and any unevenness in the concrete can be compensated for by skimming it level.

Other sites talk about putting down a 2mm thick layer of polythene on top of the concrete, but that still leaves me with the question, where does the moisture go? If it’s not getting through the polythene etc, won’t it start climbing the walls?

 

[MdB]

How much moisture do you have coming out of the concrete currently?
There is a trade off, put down plastic and you will protect the wood but may get damp pushed up the walls (this is what happened in my house). Don't put down plastic and there will be some evaporation from the floor (but not as much as with it bare) but you risk the wood rotting.

One possible compromise is to put down battens and floor as you suggest but leave air vents at the sides of the room to allow air movement under the floor. It won't be much but it might be enough.

 

[chuckey]

Lindyboo, you say "uneven" concrete, roughly how much? inches ? mm? Also has the concrete a DPC underneath it? And finally what sort of floor are you after?
I would say the thinest floor is first a self leveling screed (can only work if general floor is horizontal - else it runs down to one corner ), then lay carpets. the next thinnest is a new screed, 12mm thick - floor need not be horizontal, then lay carpet. My personal favorite "thin floor", carve up 15 X 50 mm battens so they level out major dips and bumps, stick down with "gripfix", using glue thickness to level out holes less the 6mm deep. Use 18mm P5 green chipboard to cover, carefully selecting screw length/position so not to go right through battens. Leave small air gap at ends of boards (under skirting). Very labour intensive but very solid feel when finished.
Frank

 

[biffvernon]

Unless your concrete is very ancient, it probably already has a dpc under it. Unless there are very obvious damp issues I'd be tempted to lay battens and then the boards without bothering with membranes.

 

[MdB]

Unless your concrete is very ancient, it probably already has a dpc under it.

My 1970's floor doesn't but it is only going to exist for the next couple of weeks thankfully.

 

[Moo]

If your floorboards will be anything like the ones we have in our kitchen, surely the inevitable gaps between them will provide sufficient ventilation?
We inherited a concrete floor in a modern extension and as a temporary measure we battened and boarded; a decade on and they're still ok.

 

[Craig]

Hi - we did exactly this. We laid a screed over the concrete to level it, and then used Sikabond to "glue" the solid planks down. The planks are laid over a membrane which has slots cut into it into which the adhesive is laid, so that the planks are glued directly to the concrete.

Its most likely that the concrete already has a DPC fitted. The membrane provides acoustic protection for people underneath (if this isnt ground/cellar) and also keeps the boards a litlte warmer if the concrete is cold. However, I would imagine you could do without it.

Here is what our spec said:

M45 WOOD FLOORING

- Substrate: Clear of covering and as much adhesive as possible. Skim with smoothing compound to give smooth, even surface.
- Supplier: Reeve Flooring
- Product reference: European Oak Country 1800mm
- Underlay: Sika AcouBond System
- Adhesive: SikaBond®-T53
- Method: Boards to be glued to floor using SikaBond System T53 as per manufacturers guidelines.

 

[Lindyboo]

Thank you all for your valuable input. I’ve sorry to have given a lengthy response below, but I hope it helps.

I’m afraid I must further display my ignorance when I say that I don’t know if a DPC already exists – how can I find out? We have no detailed ‘history’ on what was done to the house or when, but we suspect that the concrete floors were laid sometime after 1960, based on what our neighbours have told us about the sporadic occupancy of the place since then.

There are seven obvious damp areas, but apart from one place, I feel fairly confident of the cause and the remedy – though advice is of course, welcome. Damp area one is on the chimney breast of the main room (which is back-to-back with another fireplace). It has a bloom which I’m told are salt deposits emerging through the plaster and paint on the right front of the inglenook. The deposits start at the bottom and extend up for about 2 feet. It may be, from what I’ve read on this board, that mea culpa in this respect, because to temporarily get rid of the dark red paint we inherited I slapped on a coat of Dulux white emulsion. The plan is to knock off the plaster (if it is plaster) and expose the brickwork.

Damp areas 2 and 3: the fireplace in the room behind the inglenook is an unknown quantity as yet because it’s been clad in – well, god knows what – plasterboard perhaps, but down the sides in bits of hardboard, formica and polystyrene. A wet area appeared on one side of the fireplace last year – again perhaps after I’d emulsioned over to relieve the awfulness. I hacked away at what I presumed was plaster but found it was 2-inch thick polyfilla. It’s now dry. In the alcove at the other side is a large wet patch on the wall, starting from behind the skirting and extending up about 2 feet, spreading about 18 inches. Peeling back the paper I find that the wall has been partly finished in sand and cement. The plan here also is to expose the brickwork

Damp area 4 is a small wet patch emerging above the skirting board in this same room on the wall which adjoins the shower room. I strongly suspect that this is due to a leaking shower tray positioned against this wall.

Damp area 5 is above the skirting board next to a door frame in the main room – the wall has gradually been going black here over the last few months. This is the one area I’m not sure about.

Damp area 6 is a vestibule area in which it appears that parquet flooring has been laid in part on (presumably) a concrete floor and in part on a raised area of bricks. The lower part (on concrete) isn’t lifting, but the raised part – which was raised to accommodate drainage from the shower room which is accessed from this ‘platform’ – must be too damp as the parquet is lifting into ‘v’ shapes along the edge furthest from the shower room.

Damp areas 7 are small sections where the wallpaper is lifting away for about a foot above the skirting in two places (exterior walls). One of these patches is discoloured – ie, a stain showing through the emulsion beneath the paper.

In short then, at this stage, it seems that there are no damp issues on the floors as a whole, apart from the ‘brick platform’ with the parquet. None of the carpets feel damp and apart from the room with the dodgy fireplace cladding, there is no smell of damp. The walls seem to be the affected areas.

As to the unevenness of the floors, the main vestibule (not the one with the parquet) has a distinct and uneven slope along it’s length and slopes away on its width. I guesstimate that to make it level would require hacking it down by about 3 inches at the higher end and making a step or ramp up to the main room which it serves. The external ground and interior floor levels are virtually the same, so there’s no room to play near the front door. The room with the dodgy fireplace has a bit of a slope from the doorway and undulates slightly throughout – I guesstimate that the undulations amount to no more than an inch, but to get rid of the slope might mean digging away about 4 inches and creating another step/ramp. The other alternative appears to be to level the rest of the room up to this, but that would mean steps into the two other rooms served from this one and would shorten the room height too much.

I hope some of this makes sense!

 

[ljn]

It sound like the damp issues you have are coming from above and can be sorted out.

If there is no barrier for damp under the concrete, there will be a certain amount of damp. If the concrete floor is exposed, any damp would evaporate and you lay a wood floor on top you will get an accumulation of damp under the floor and this will lead to rot.

To see if there is damp rising through the concrete, you put a sheet of plastic down for a couple of weeks and see if any damp collects. If there is no damp, there must be a membrane under the concrete and you could go ahead and install the wood floor.

An alternative to the embedded batons is to use self leveling screed to make the floor level without the extra thickness, and screw the subfloor directly onto concrete using wall plugs. You would need a big strong drill to make all those holes drilled every 30 cm.

 

[Lindyboo]

Damp coming from above – ay caramba! I thought we’d got round that one when we had a nice new roof put on last year. What should I look for to determine if it’s ‘falling’ damp?

 

[Andrew Bodenham]

As others have suggested your concrete floor probably has a dpc under it. You could drill / dig out a trial hole about 6" square to check if this is the case. Our concrete was only about 3" - 4" thick, and I was pleased to find a dpc in place. You can patch up your test hole with a bit of concrete with a waterproofing additive mixed in.

We wanted a solid oak floor in our kitchen, over the existing concrete, but didn't have enough height in the doorways to use battens and a subfloor. What we did was to level out the worst of the lumps with leveling compound (Sika N5). Then we laid a 3mm neoprene underlay to give a little spring, a little insulation, and to deal with local variations in the concrete surface. Over this I fixed 12mm ply, screwing direct into the concrete with multi-monti screws, which are essentially self-tapping screws for masonry - they need no plugs so you can just drill through, hoover out the hole, and drive them in. Once the ply floor was down I laid 10mm thick solid oak t&g overlay flooring, glued to the ply and pinned through the tongues - a belt and braces job! The finished floor looks great, and has been down 18 months now without any problems. The overall thickness gain was only 25mm.

Hope this gives you one possible way forward.

Screwfix are a good source for underlay, fixings etc.

 

[Lindyboo]

Thank you, Andrew. I like the sound of that. I’m coming to the conclusion, based on what I’ve read on this thread and elsewhere on the web, that maybe I was being given a bum steer originally from the (now discarded) builder. He insisted from the outset that in it was necessary to dig out as far as Australia and then pour in tons of stuff before a new wooden floor could be laid … no wonder he’s a millionaire …

Cheers.

 

[ljn]

I was thinking that if you have damp in specific places, it might be from specific sources. The damp around the chimney breast, might have come down the chimney rather than up through the floor, or the damp around the shower drain might have something to do with the shower drain. But I'm no damp expert.
My damp floor with no DPC has a pretty uniform distribution of damp.

 

[Andrew Bodenham]

Vitally important when you finally get to laying your floor to acclimatise the flooring material to your house before laying. By which I mean remove all packaging etc. from the boards and stack in the room where they will be laid, ideally with spacers between to allow good air circulation. Leave for as long as possible before laying (I'd say a couple of weeks for 10mm thick overlay flooring, twice that for 18 or 20mm boards)

The purpose of this process is to allow the wood to absorb / expel moisture and shrink / contract before you fix it down. If you don't do this then you increase the potential for future problems. Only yesterday I was looking at a floor which had been laid with 10mm solid oak boards. The boards were laid sraight out of the packs, and had subsequently exanded in the slightly damp conditions of their new home. The result was a "mid-Atlantic ridge" effect in the middle of the room where the expansion had pushed the boards up off the sub-floor, forming a mountain range about 4" high. That's going to be tricky to fix, and expensive.

The opposite effect can also occur in a very dry house, with shrinkage cracks appearing, but I don't think you'll be suffering from that problem!

 

[Lindyboo]

Ljn – thank you. Would I be correct in my guess that any damp which is coming down from the chimney breast can be prevented by putting a layer of DPC around the top under the roof? I’ve read somewhere online that this can resolve the issue.

Andrew – thanks for that reminder. I’ll be sure to acclimatise the boards first.

By the by, the heating engineer has been this morning to size up for the new central heating. He reckons it’ll take him 3 weeks to get the radiators installed. The downstairs pipes will have to be laid in channels cut into the concrete and the logistics for getting pipes upstairs in our quirky little shack make things worse. I don’t mind the timescale if that’s what it takes, but I wonder if 3 weeks sounds overlong to anyone with experience of such installations? I’m not really doubting the lad’s word – he’s been the only reliable tradesman we’ve found since we came here a year ago – but the length of time to install surprised me. (Small cottage – 2 beds upstairs + tiny bathroom; 1 bed downstairs + 3 public rooms, kitchen, shower room + 1 hall + 1 vestibule).