Laying floorboards on DPM over concrete

[Lindyboo]

Does anyone have experience/opinions on laying 'floating' t&g floorboards over a DPM on top of concrete floors? Is it feasible? It’s presumably no more risky than laying laminate flooring in the same way on a concrete floor. Thoughts?

 

[Tony.]

Never did it direct onto concrete, but onto purpose timbers like 2x2 with a foam strip on one side.
These were laid in the concrete floor and the floor nailed to the 2x2.
The gap allowed pipes and cables to be run under the floorboards

 

[Andys]

If you're laying boards without ventilation underneath then it really needs to be an engineered floor to stop it warping, on top of some special underlay to take out a few mm of unevenness across the floor

Because it's basically plywood with a decorative wood layer on top, the engineered floor won't curl up unlike solid wood boards which almost certainly will. Also if there's more than a few mm per metre of unevenness then it'll really pay off to do some careful levelling of the concrete floor with self-levelling compound or similar to avoid the curse of a bouncy, creaking floor later :evil:

 

[88v8]

.... the curse of a bouncy, creaking floor

You mean a 'period' floor?

Level... flat... smooth... arrrrgh.

Ivor

 

[Lindyboo]

May have to go down the engineered flooring route because we have very low ceilings here, but hoped to avoid it because it looks so - well - engineered. Anyone know what would be the minimum air gap needed beneath t&g boards?

 

[88v8]

If the sub-floor is dry, I think the depth of gap is of little importance. Twenty-five years ago I laid an oak (solid oak) tg floor on a concrete slab, by skew-nailing it to battens.
The battens were screwed to the slab. No screed. I think the battens were 2x1" laid on their side. The floor never moved, but I was starting with oak that was maybe 80 years old.
There was a dpm within the slab.

The deciding factor was the minimum thickness of batten needed to make a good fixing. If one uses seasoned flooring on good softwood batten, then 2x1" sideways, if rubbish softwood &/or new flooring, then something thicker. If hardwood, one could certainly drop to 1", but might be hard to nail into. I predrilled all the nail holes so as not to split the tongues.

I'm not clear whether your floor already has a dpm. or whether you plan to lay the dpm over the top. If the latter, it will need to be carefully tucked & mortared into the wall, and I would leave a ventilation gap across the whole width of each end of the floor, just in case.

I don't recall the details of your house. If there is no dpc in the walls, there is always the risk of pushing damp into the walls and making them, well, damp!
Concrete is not very breathable, but concrete with no dpm does have a small degree of breathability and by sitting a dpm on top that would be completely shut down.

Ivor

 

[CliffordPope]

Isn't traditional parquet flooring laid by pouring hot pitch onto the solid base and setting the blocks while it's still tacky? I assumed the pitch was both a glue and for damp proofing.

 

[Toby Newell]

Yes.

If the room is dry and stable and the slab is not damp then there should be no problem floating T&G floorboards.

Gluing is obviously preferable for wide board oak. Engineered would be safer if floating is the only option.

I apply a liquid DPM and would advise a compatible adhesive (they come in matched pairs as a system) for a solid wide board oak.

For a floating floor I would advise a high quality metalised rubber underlay which has sound proofing, levelling and thermal insulation properties all in one.

If you want the solid floor look with an engineered floor seek out an unfinished square edged board without the microbevel, they normally cost as much or more than the microbevelled pre finished variety but when you finish them onsite they are indistinguishable from a solid floor only with less movement. Go for a high quality 20mm + board with a 6mm wear layer which is only 1.5 to 2mm above the tongue and so to all intents and purposes can be sanded as much as a solid board. Avoid the 15mm boards with 3mm wear layers, they move more and don't last so long.

Always pay more for high quality European FSC certified flooring, it's worth it, oak normally comes in three grades, country, select and prime, I always opt for the cheapest country grade option as this has the most character, pips, knots and swirly grain. If you get a chance to view some of the timber try and check for excessive sapwood (creamy white stripes at the edge of boards) even in a country grade you don't want too much sapwood.