Slate flagstone over heated floor-questions..

[bigbrother3269]

Ive been to look at a project for a customer today,part of which is to lay approx 120 sq mts of Stonemarket slate style stone slabs internally-something ive never done before,i got a few concern's regarding the following:- The sub floor is suspended timber incorperating wet underfloor heating(spreader plates and moisture resistant floor boarding),we are going to need a flexable adesive but will these take the weight of say a large 900x900 flagstone?,if laying outside it would be easy on sand/cement-but this aint going to work indoors over timber cause it wont key to the floor even with primer coat will it-and it wont be flexable when the heating comes on full.has anyone else done a job similar to this?-any recomendations anyone?,oh and thats before i get to the grout issue-as they are a porus stone wet grout is out due to staining....i had considered sealing them all as you would with slate tiles but i cant see this having much effect-again any ideas?