Tiler trying to tile directly on top of a 9mm OSB sheet, using standard adhesive. Is this a good/bad

cownorris

New Member
Recently started to renovate the flooring in the house - we had carpets and we want to get tiles. However as I read on the topic, I became a bit concerned about how the contractor is doing our flooring. The contractor is supplying all of the materials apart from the tiles themselves.

The old carpets were on top of a timber base, where some planks were loose. The tiler fixed the old planks into place, and added a substrate on top - a 9mm OSB, this one in particular: https://www.wickes.co.uk/Wickes-Gen...nd-Board-3-OSB-3---9-x-1200-x-2400mm/p/102972 They then bought some standard adhesive (but haven't used yet) https://www.wickes.co.uk/Mapei-Fast-Set-Ceramic-Tile-Adhesive-Grey-20kg/p/234986, and are planning to use them directly on the OSB subfloor. However, earlier today I saw in the product documentation they were "unsuitable" for wooden substrates, and another "flexible" one shoudl be used to lay on top of wood.

This concerned me a bit and so I checked through the rest of the set up. I found the majority of resources online are suggesting that laying tiles on OSB boards are a bad idea, and in any case the 9mm is too thin. Instead something like this should be laid instead to tile on top of: https://www.wickes.co.uk/NoMorePly-12mm-Fibre-Cement-Construction-Board-1200-x-800mm/p/225437

This did not look good to me of course and I am planning on brining this up with them tomorrow. As I'm not very familiar with flooring, I'm not 100% sure of my conclusions and so would really appreciate a second opinion on this! How big of a problem is this?
 
You have every right to be concerned, I'd let them take away the materials they haven't used so far and pay for the materials + labour to date.
Then tell them you're changed your mind, you now want carpets again.
Don't let them back in.
The OSB already laid won't be a problem, just go over the lot with 6mm ply, and find someone with a bit more of a clue about what they're doing to lay the tiles.
 
This is old now but replying in case someone reads it for a future project. It is absolutely not acceptable to tile on 9mm OSB. 9mm OSB is way too thin for subfloor under carpet too. Even at 450mm centers you will have a significant amount of bounce in the floor and with heavy furniture you absolutely risk going through the subfloor. You need 18mm or more of subfloor and that can be OSB or a structural grade plywood. If you want to tile on that, you then need to fix a tile backer board ( no ply, Hardie, etc. ) Typically flooring is installed on 12mm tilebacker and do not let someone convince you otherwise. You can try a decoupling membrane but you are absolutely wasting your time. I have never seen an installation that didn't fail over the years.
Make sure that tile backer is installed correctly, screwed into the floor with screw heads and joins taped and joined with tile adhesive. It is not enough to rely on the tile adhesive that's holding the tiles and this really doesn't take long as it doesn't need to be pretty.

Lastly, if this is a bathroom, you must tank/apply liquid DPM to the floor and bottom of the walls. Not only is it good practice but it's a BBS requirement. It is actually a terrible idea to cut corners on subfloor because repairs will be very disruptive and quite costly.
 
This is old now but replying in case someone reads it for a future project. It is absolutely not acceptable to tile on 9mm OSB. 9mm OSB is way too thin for subfloor under carpet too. Even at 450mm centers you will have a significant amount of bounce in the floor and with heavy furniture you absolutely risk going through the subfloor. You need 18mm or more of subfloor and that can be OSB or a structural grade plywood. If you want to tile on that, you then need to fix a tile backer board ( no ply, Hardie, etc. ) Typically flooring is installed on 12mm tilebacker and do not let someone convince you otherwise. You can try a decoupling membrane but you are absolutely wasting your time. I have never seen an installation that didn't fail over the years.
Make sure that tile backer is installed correctly, screwed into the floor with screw heads and joins taped and joined with tile adhesive. It is not enough to rely on the tile adhesive that's holding the tiles and this really doesn't take long as it doesn't need to be pretty.

Lastly, if this is a bathroom, you must tank/apply liquid DPM to the floor and bottom of the walls. Not only is it good practice but it's a BBS requirement. It is actually a terrible idea to cut corners on subfloor because repairs will be very disruptive and quite costly.
I don't think the OSB was for support as it had flooring planks beneath it?
As an aside I tiled my extension 9 years ago with 6mm treated ply screwed on 22mm chipboard and to date no issues Ditto sons bathrooms but on planking. Guess I am just lucky like all the others were before tile backers were invented.
 
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