Laying plywood on concrete

[Deadeye]

A while ago I took some more of the excellent advice here (thread: post1227766.html#p1227766)

I concluded that overlapped plywood sheets might be the best bet.
Three questions:
1. Do I need studwork underneath or can I lay the ply directly onto the concrete. the concrete is dry - does it need to breathe?
2. If it needs to breathe, can I achieve this by drilling small holes through rather than a network of studding under?
3. Is there any difference other than cost in using 9mm+9mm rather than, say, 6mm + 12mm?

Thanks!

 

[That would work]

Hi
The concrete appears to be 'dry' because moisture is constantly evaporating off of it. once its covered it will retain that moisture that is being drawn up from below. For my money it would require a good DPM, perhaps a liquid one.

 

[Deadeye]

Hmmm... So should I put some heavy polythene sheet down? Would that cause other problems?
I didn't want to lay a grid of battens because it'll get rather thick and I don't want trouble round the door etc.

 

[Zeddedhed]

You could try this....
https://www.everbuild.co.uk/product/sik ... ood-floor/

I haven't read the tech data download, so you should check that it's ok on concrete. As has been said i would probably apply a paint on DPM first though.

 

[MikeG.]

Hmmm... So should I put some heavy polythene sheet down? Would that cause other problems?.......

No you shouldn't, because yes it will (unless this is an old floor that you know for certain is on a DPM).

 

[Deadeye]

Hmmm... So should I put some heavy polythene sheet down? Would that cause other problems?.......

No you shouldn't, because yes it will (unless this is an old floor that you know for certain is on a DPM).

Not on a DPM I think.
So no alternative to the battens?
If I go back to plastic jigsaw tiles will there be the same issues?

 

[Zeddedhed]

I'm not for one moment questioning the wisdom of MikeG, but I don't understand how it would cause a problem installing a DPM onto the concrete if there was not already one there.

I don't need to understand this, but my natural curiosity made me ask (aka the voices in my head)

 

[MikeG.]

If you've got damp coming through the concrete, or the concrete isn't yet fully cured, then a DPM will simply trap water on top of the concrete and below the plastic. This will lead to the growth of mould, and to rotting skirtings etc.

Of course, the obvious way to test it is to put a piece of DPM down, say 2 foot square, and weight it down with something substantial. Leave it a week or two, and then lift it. If the concrete is a different colour underneath then it's damp.

 

[Zeddedhed]

If you've got damp coming through the concrete, or the concrete isn't yet fully cured, then a DPM will simply trap water on top of the concrete and below the plastic. This will lead to the growth of mould, and to rotting skirtings etc.

And if the new DPM was properly lapped up the walls and under the existing DPC? Would that still pose a problem?

 

[MikeG.]

What are the circumstances we're talking about here?

I can't see how one can properly run DPM up an existing wall, then lap it under an existing DPC. What's going to cover the plastic? DPM laid on concrete will fail quite quickly where the surface undulations push through the plastic as the area is subject to traffic. Exposed vertical plastic will last 5 minutes, and trying to get a proper lap underneath an existing DPC with the weight of a building on top of it is optimistic at best.

 

[Deadeye]

What are the circumstances we're talking about here?

It's the garage floor - roughish concrete that's been laid at two different times by the look of it - one side is relatively smooth, the other has tamping ridges galore.
I won't be laying the floor all the way to the walls for various reasons but want a floor that is smoother and warmer than what I've got without paying toooo much!

 

[MikeG.]

Then I refer to my previous: plastic is pointless on rough concrete, as the pressure from above will wear it through in no time.

 

[peter-harrison]

I did this in my workshop about 6 years ago. It was an old builders store with the roughest of rough-cast floors. I went all over it with a diamond cup disc on my angle grinder, to take off the sharp bits, then covered the whole floor with sand to even out the many dips. then covered with black plastic DPM, then flooring chipboard. No problems so far........

 

[MikeG.]

I did this in my workshop about 6 years ago. It was an old builders store with the roughest of rough-cast floors. I went all over it with a diamond cup disc on my angle grinder, to take off the sharp bits, then covered the whole floor with sand to even out the many dips. then covered with black plastic DPM, then flooring chipboard. No problems so far........

You seem to have done that job as well as possible.........but how do you know there are no problems? There could easily be a pool of water under the plastic, and black mould everywhere below the floor, for all you can see from the outside.

 

[AJB Temple]

To add a bit of perspective. The house I am renovating had a "gym" according to the estate agents in partitioned off part of a barn. This had a concrete floor, with ply laid over it then pine planks. Between the concrete and the pine was a thick sheet of plastic dpm.

I gutted the barn (which will be a kitchen) and ripped out the floor. The dpm layer had failed in multiple places, was soaking wet and smelly beneath, and a lot of the ply was black and rotten. The top floor looked dry.

I think if you are going to go to the trouble of laying a floor, do it properly.

 

[Lazurus]

I had an earth floor in my workshop and laid a new concrete one with correct dpm, and reinforcing mesh, it wasn't hard or expensive and made a world of difference but I wouldn't consider covering with anything else for the reasons as above. Why not use a quality self leveling compound and build it up to ice rink smoothness.