hi new to forum,im going to be laying 10mm calibrated 600x300 brazilian slate in 17sqm kitchen problem is the thickest ply i can use is 9mm ,this is on to sturdy floor boards,my question is am i better to use granfix fast set 1 part flexi which i can get for £15 for 20kg or am i better with granfix ultimate flex ? any comment,s gratefully recieved.

Slate is a dense heavy material and the extra load imparted onto your floor joists will be considerable. The reason floors are over boarded (generally) is to add strength to combat deflection. 9mm ply is not usually sufficient for this unless you have a really good base to start with. 15mm is what is recommended in BS so in reality, 18mm is the way to go as 15mm is not available here.

You will get some guys posting that it is "ok" to use 6mm Hardiebacker board instead but as yet I am not convinced that it imparts much additional strength to a floor.

As for the adhesive question, I personally have only ever used the Ultimateflex adhesive and it is very good, so I can not comment on the other one.

Ditto Grumpy

because of height issues ie washing machine fridge ect 9mm is the max do you think i have any way of getting away with it ,if i reinforce floor board,s ???????????????

Originally Posted by wullmc

because of height issues ie washing machine fridge ect 9mm is the max do you think i have any way of getting away with it ,if i reinforce floor board,s ???????????????

That is what the thick ply does!! Only real alternative is to lift the floor and reinforce the joists and fit noggins then relay the floor and just use 18mm ply straight down.