Bathroom subfloor prep for porcelain tiles

[Duchard]

So I've decided to finally rip up my 3 year old lino in the upstairs bathroom and lay some lovely looking wood effect porcelain tiles down. I have some experience with past tiling jobs but these were on a concrete floor.

Bathroom setup
Just over 4m2 area with 25mm wood ply/chipboard flooring from original build.
I have already cut out 6mm hardibacker which sits nicely on the existing wood floor (loose)
Plan is to lay 150x600mm porcelian tiles.

Questions
Is it better to lay the hardibacker in a thinset of tile adhesive as well as screw down?
If so would a standard mapei bag be ok for both hardi and tile application?
Looking at this http://www.screwfix.com/p/mapei-floor-wall-tile-adhesive-grey-20kg/4367d
I would use flexi grout but not sure if I need to use flexi on above also?
Is it ok to screw hardi into floor with 20mm screws as I am concerned any further will hit a pipe somewhere (already done).

Any advice on the above much appreciated. As learned from previous jobs its all about that prep work!

 

[Andy Allen]

6mm hardi and adhesive + 25mm sub base = at least 31mm so you can't hit anything with 30mm screws.

If it's chipboard it will only be 18mm
I would go 25mm screws max or you maybe hearing the dreaded hissing sound!!

Just re-read the op post..... And he dose say 25mm sub floor..... I would check that if I was you, if it's tongue and groove chipboard it comes in 18 and 22mm and I've yet to meet a house builder that uses 22mm.

 

[Bond]

If it's chipboard and it's nailed at 600mm centres then it should be 22mm. If 450mm centres then it might be 18mm. At least in this neck of the woods, that is normal sinario.

 

[Duchard]

Thanks for the help. I have used BAL SBR on the subfloor follows by a flex adhesive from top tiles. They put it through at trade price which was good of them so a 20kg bag was only £13.
Hardi backer now down and screwed in using 25mm reisser screws every 30cm pretty much.
I bought the plaster fibre tape thinking that would do for Hardi but soon learnt after this is no good for a cement mix. .
Is the tape a must have? Or can I just skim over the joins with adhesive?

Can send pics if better?

 

[Bond]

I know it's supposed to alkali resistant tape, but have used fibre tape, when stuck, seal board joins with SBR, when dry - tape and fill, should be alright.