sand and concrete screed on on top of a plywood floor?

[The Groutch]

Hi, i'm having limestone tiles (400x400x17mm) put down as a floor in my kitchen. Half the floor is a very old concrete and the other half has had 20mm plywood nailed down over the joists.
My problem is that a floor layer has been brought in and he wants to lay a sand and cement screed right across the floor, concrete and ply. He says it will level the floor and he can tile straight onto it without adhesive. I think because of the plywood etc, the floor needs to flex, so a flexible adhesive should be used? I'm worried about tiles coming up if/when the screed starts to crack on the plywood side of the room. Am i right? i've got to admit i'm a novice when it comes to flooring, but this could be an expensive mistake to fix later on.
Cheers.

 

[Deluks]

The floor must not flex at all. After all, your tiles won't. The 20mm ply should be rigid enough provided they are all supported and well screwed down, if the floor layer is saying screed it then take his advice.
oh, make sure the ply is wbp.

 

[mattysupra]

Do they mean a flexible levelling compound? Not sand and cement/screed! But you wood still need adhesive . Have a read of the stickys at the top the forum.