Raised engineered wood on a concrete floor

[kcr]

I am reflooring a room in an early 1980s Wimpey home with engineered wood. The floor has to match the height of the wooden flooring in an adjoining corridor.

I've stripped the carpet, and the concrete floor is covered in thin, hard, dark brown tiles glued to the concrete floor with black adhesive. The tiles are well glued down and in good condition, apart from where carpet grippers were nailed through, and cracked the edges.
I am aware that there is a chance these old floor tiles might contain asbestos, so I plan to leave them in place and just cover them. I have removed the broken pieces around the edge.
To get the floor to the correct height, I plan to lay 22mm tongue and groove OSB, appropriate underlay/insulation and floating engineered wood.

I had a couple of questions, if anyone can advise, please:

1. The tiles are in good condition, well bonded and there is no sign of moisture on the floor. I assume there's no need for a vapour barrier/dpm and I can just lay the OSB directly on top of the tiles?

2. Could I just float the OSB on the tiles? I would like to avoid having to start screwing down into the tiles and concrete floor, so I wondered if heavy T&G OSB would be stable enough if I just lay it on the floor. Alternatively, I've seen some examples on the internet of people using two thinner layers of OSB cross laid and screwed to each other to lock everything.

Thanks

 

[chopnhack]

1. The tiles are in good condition, well bonded and there is no sign of moisture on the floor. I assume there's no need for a vapour barrier/dpm and I can just lay the OSB directly on top of the tiles?

No, there will always be moisture migrating from the slab up into the house - it will vary with seasons and local. Assuming England, where Wimpey is hq'd - you get quite a bit of moisture Engineered wood floors typically always need a vapor barrier, consult with the manufacturer's installation sheet for specifics.

2. Could I just float the OSB on the tiles? I would like to avoid having to start screwing down into the tiles and concrete floor, so I wondered if heavy T&G OSB would be stable enough if I just lay it on the floor. Alternatively, I've seen some examples on the internet of people using two thinner layers of OSB cross laid and screwed to each other to lock everything.

The OSB is going to take in moisture from the floor and cup and curl. Without screwing it down, there may be significant movement in your floor. Why are you adverse to screwing into the concrete? - It can be done easily with the right tools.

If the tile is in good shape, you might be better off putting down a skim coat of a thinset adhesive with bonding agent and then cementing down some cementious backer board. I would still put a few screws in each sheet - even with some wet cement between. This will allow you to control your height and have a solid, rot free subfloor to work off of. Next comes your vapor barrier/pad for the wood per the manufacturer and then you can layout the floor.

 

[kcr]

Thanks for the reply.

Apologies if I was a bit ambiguous; I do intend to fit an appropriate underlay/vapour barrier above the OSB and under the engineered wood itself, but I wondered if I needed another DPM layer at the bottom of the stack to protect the OSB, given that there is the tile layer on the floor already? I don't think there is any real moisture coming through the carpet, or we would have had soggy carpets! I understand there could be some vapour, but would moisture resistant OSB directly on the tile be OK?

I had another look at fasteners, and I guess I could just use self drilling concrete screws?

 

[chopnhack]

but I wondered if I needed another DPM layer at the bottom of the stack to protect the OSB, given that there is the tile layer on the floor already?

Yes, if you decide to go this route.

I understand there could be some vapour, but would moisture resistant OSB directly on the tile be OK?

There is always some type of exchange of vapors so I wouldn't rely on wood, go with backer board instead.

I had another look at fasteners, and I guess I could just use self drilling concrete screws?

I am not familiar with self drilling concrete screws - self tapping yes, but not self drilling. The difference is that you need to drill a hole first and then the self tapping bit will cut its own threads.