Parquet on rough concrete slab

[Carb120]

I am going to help some relatives lay parquet flooring. I tried to take a look at the subfloor before the work is done. The subfloor is a concrete slab, but it looks bumpy. Nothing has been done with the floor since the house was built in the early 70s. I'm considering the subfloor because of the bumps, whether it can damage the new underlayment. If that's the case: What is recommended to do? Sand, self-leveling compound, extra underlayment?

 

[Stefan N]

How bumpy? Is it even straight if you lay out a straightedge? Sounds like a self-leveling compound might be in order.

 

[Carb120]

How bumpy? Is it even straight if you lay out a straightedge? Sounds like a self-leveling compound might be in order.

Unfortunately, I don't know what it looks like overall since the floor can't be torn up until just before laying. But from what we could see: like a textured wallpaper.

 

[BirgitS]

What kind of flooring is it today?

 

[Carb120]

What type of floor is it today?

The parquet floor that was installed when the house was built, solid stuff with about 18 mm middle and bottom layers. As a base, a thin plastic film, like a thin garbage bag.

 

[BirgitS]

Then maybe a Masonite board or something similar as a base can work to raise the floor surface to the same level as today, so it doesn't look strange with thresholds and door frames, in addition to the plastic foil and foam that the floor manufacturer recommends.

 

[Henrik.S]

If the current parquet is directly on the floor, it shouldn't be that uneven. A slightly thicker underlay foam like this one can even out minor unevenness. Masonite or something similar works if you need to build a bit more height.

https://www.byggmax.se/underlagsskiva

 

[Carb120]

We don't need to build height, on the contrary, there is a level difference now from adjacent rooms.

I'm just wondering what it would entail if the concrete bumps (which are naturally quite sharp compared to a textured wallpaper) perforate the underlay.

 

[BirgitS]

How much do you want the floor to be lowered?
There is 3 mm thick masonite.

Foam is intended to absorb small irregularities and they usually become quite flat over the years where you walk. But if the plastic that is there now has managed to avoid being perforated, then the new plastic should be able to handle it as well.

 

[Carb120]

That's true, but it's not something we know until we've pulled up the floor. And I want to be prepared for any possible extra work since the job needs to be completed within a certain, allocated time.

 

[BirgitS]

It gets a bit expensive to stock up on spackel or masonite if it turns out it's not needed.

Isn't it possible to remove some of the flooring in advance to check?

 

[Carb120]

It does get a bit costly to fill with putty or masonite if it turns out not to be necessary.

Isn't it possible to lift a bit of the floor in advance to check?

Exactly so.
Well, it's a fully furnished living room. The floor demolition will only be possible shortly before the installation.

 

[Carb120]

I haven't googled this, but is double foam/padding a big no, no? I was wondering if it might risk trapping moisture in some way.

 

[BirgitS]

Double foam makes it too soft, i.e., the floor gives way, and maybe it increases the risk of the boards separating. However, one can combine foam with hard underlays like masonite and lumppapp.

 

[Carb120]

Double foam makes it too soft, i.e., the floor becomes springy, and it might increase the risk of the boards separating. However, you can combine foam with hard underlays like masonite and felt paper.

Then perhaps felt paper could be the most flexible solution along with underlay if it seems necessary once we've torn up the floor.