Decoupling mat over self-levelling plywood floor

[myristate]

I am going to be tiling my upstairs bathroom that has a wooden floor. It originally had wooden floorboards 22mm thick. We have removed all the tongue and groove floor boards as they were rotten and uneven. The joists were solid and in good nick.

It has now been sheeted with 22mm Ply and screwed every 4-6 inches along every joist.

However, the entire floor is not perfectly flat the room slopes to the corner of the room. From the highest to the lowest point in the room is about 11mm. In hindsight we really wish we had checked the level of the joists more accurately before screwing it down. However its not possible to lift it up with everything in place. We are going to use a fibre based self levelling compound (Bal Level-Max) and level the floor.

My question at this point is do we need to put down a decoupling mat on top of the self levelling compound when it has set or can it be direct tiled. These are long narrow plank tiles that emulate wooden floors. Also should I use an S1 adhesive or step up to a S2 adhesive?

 

[J K Tiling Cornwall]

Although you can self-level over plywood I'd never personally self-level over a substrate I wouldn't tile directly onto. If height is not too much of an issue I would fit 6mm cement backer board to the floor area (bedded on adhesive and screwed down). Then build up the low point with self levelling compound. Will then be ready to tile. S1 flexible adhesive will be fine.

 

[Dave]

I am going to be tiling my upstairs bathroom that has a wooden floor. It originally had wooden floorboards 22mm thick. We have removed all the tongue and groove floor boards as they were rotten and uneven. The joists were solid and in good nick.

It has now been sheeted with 22mm Ply and screwed every 4-6 inches along every joist.

However, the entire floor is not perfectly flat the room slopes to the corner of the room. From the highest to the lowest point in the room is about 11mm. In hindsight we really wish we had checked the level of the joists more accurately before screwing it down. However its not possible to lift it up with everything in place. We are going to use a fibre based self levelling compound (Bal Level-Max) and level the floor.

My question at this point is do we need to put down a decoupling mat on top of the self levelling compound when it has set or can it be direct tiled. These are long narrow plank tiles that emulate wooden floors. Also should I use an S1 adhesive or step up to a S2 adhesive?

Hi. Did you out noggings at the sheet joints with them not being T&G joints. ?
And as above i too would overboard on plywood all the time. Then slc if required to get it flat.