How flexible is flexible addy

[Dumbo]

Hello again. I decided to lift tray and screed underneath. The screed was fully under the tray so I'm baffled as how it wasn't set properly. Can anyone recommend how to bed it in now. I'm thinking maybe a wet sand and cement mix then screw it in when S&C sets. I will be installing no more ply around this and I only have about 3 inch around the tray where it'll be new screed. How long do I wait before laying the no more ply or can it go down few days after tray installed.

Edit. Would I get away with using fondu under wet tray?

What is fondu

 

[WetSaw]

Isn't it that Swiss thing with the hot cheese that you dip stuff in?

 

[Handyeire]

What is fondu

Cement fondu. Mix with sand and it hardens quicker and harder.

 

[Dumbo]

Cement fondu. Mix with sand and it hardens quicker and harder.

Just Googled it . I would just use single flexible adhesive