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Preparing a plywood floor for tiling

BarraOG

I wish to tile the floor in a bathroom. The plywood on the floor is 24mm and I have added lots of screws to make it extra solid. I placed a bowel filled with water at various points in the room and it doesn't spill when I hop on the floor.

I've filled gaps between the plywood and walls with sillicon.

There is a deviation of 10mm on the floor, from highest point to lowest. I believe it is better to use levelling compound to address this than extra adhesive.

I should use flexible adhesive on plywood.

Can I paint the floor with PVA to seal it, then put levelling compound on that where required, and then flexible adhesive on that?

Thanks for your help!

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Rows Grower

There are more suitable primers than PVA and loads of different brands available. Is it a steady slope from 10mm to nothing or are there dips in the floor? How big is the area?

Economics101

https://www.boards.ie/discussion/2058365313/preparing-a-plywood-floor-for-tiling

A bowel filled with water?

Surely in the toilet, not on the floor? 🙃

BarraOG

https://www.boards.ie/discussion/comment/122525097#Comment_122525097

Thanks for your answer. I will research primers instead of pva. The room is 8 foot by 9 foot. The slope occurs across three joists, so a gradual drop of 10mm over 2 foot.

UPDATE: Sorry it's only 5mm. There is a 10mm difference at the opposite ends of the room. But there is no foot traffic in a 40% of the room due to a sloped roof.

BarraOG

https://www.boards.ie/discussion/comment/122525114#Comment_122525114

Haha, sorry bowl! :)

Rows Grower

https://www.boards.ie/discussion/comment/122525175#Comment_122525175

Would you be able to unscrew the plywood and add height to the low joists?

Padre_Pio

I would be thinking some form of decoupling mat between the wood and the tiles.

BarraOG

https://www.boards.ie/discussion/comment/122525217#Comment_122525217

Not a hope at this stage. There's too many screws. I would have to take up the whole floor. It's actually only a 5mm drop across the two joists. The extra 5mm comes in the parallel direction. Because the roof is sloped, there will be no foot traffic in the lowest corner so maybe it's less of an issue.

[Deleted User]

Pack it with tile adhesive. Will be fine.

BarraOG

https://www.boards.ie/discussion/comment/122526380#Comment_122526380

So would you recommend primer, then tile adhesive, then tiles.

[Deleted User]

Add a layer of cement board to get things perfectly level.

Grassy Knoll

use small floor tiles, odds are there will be some movement in the timber base, large profile tiles will be at high risk of cracking over time, particularly where floor traffic is highest. The smallest flex and you are in bother with large tiles. You could also add cement board to your ply, but if there is flex there already it might not be worth it.

10-10-20

Have a look at decoupling membranes and see if they fit into the budget.

https://www.goodwins.ie/decoupling-movement-mats/c-6322.html

Don't use PVA anywhere, just follow the guidance from the mat manufacturer - if you go that route.

In terms of the dip - it all depends on how the difference manifests itself - just like @Rows Grower said.

BarraOG

https://www.boards.ie/discussion/comment/122526529#Comment_122526529

The room is 8 foot by 9 foot. The image shows the number of mm difference across the floor. The orange is where there will be no foot traffic due to door location and sloped roof. There are no dramatic dips just slopes. How do you think that should be fixed if I go the mat route or not? Thanks for your help!

Rows Grower

You could lay 6mm plywood from the the three white 10's to halfway between the white 8's and 5's and then make up the rest of the imperfections with Flexi adhesive just making sure there is a solid bed under each tile.

Use the proper primer as PVA will have a chemical reaction with the adhesive over time and the tiles and grout will eventually become loose.

To make life easier I'd recommend you use a tile levelling system of clips and wedges. You can buy a complete set including pliers for about 25 euro online.

10-10-20

3mm in 2m is the general rule for flatness, but that's not to say that more than 3mm will actually cause an issue other than a visual problem or tile alignment. Being honest, I'd probably float-out that furthest section where you have a drop of around 5mm in 1.3m with a leveling compound approved for use on wooden floors.

The suggestion of using a decoupling membrane doesn't relate to the flatness of the floor as the mat won't allow for adjustments in height, it's just an additional barrier against leaks, but also mainly from cracking due to sheer from sub-floor movement. Have a google on that topic and see if it's what you want, but it's pretty much the default in any wet-rooms/bathrooms these days as far as I know.

Also, have a gander at this:

https://www.tiles.org.uk/tiling-guide/