Grout Cracking on Tiled Floor

[TDan]

We have recently (last week) had a new bathroom fitted with floor tiles to match those on the walls, however the grout on these tiles has cracked and become loose.

The fitter removed the original grout and re grouted thinking it may be a bad batch or similar, however it has happened again.

The floor tiles have been layed on a flexible adhesive on a 5mm layer of hardboard on top of the floorboards and we have come to the conclusion that the floor is obviously flexing and causing the grout to crack.

Question is - what is the best solution to stop any flex and ensure the tiles stay solid?

I've read that circa 1/2 inch ply with cement backer board on top is best but looking for some opinions!

Thanks

[TDan]

Thanks for the responses all, the fitter was a family friend who is a plumber by trade so no formal contract agreed - i fully understood what i was getting into by doing this but some of the frankly outrageous quotes we had forced our hand.

I know it needs ripping up and starting again so happy to do so (area is circa 2sqm), just wanted to see what the consensus was on the floor preparation.

It seems the best option is to remove the tiles and thin hardboard, leaving the floorboards down and over boarding with either ply or concrete hardiebacker.

Ideally the solution would be as thin as possible so the floor in the bathroom isnt significantly higher than that in the hall - so shall i go for the 6mm or 12mm hardiebacker or 1/4 or 1/2 inch ply?

[TDan]

I think i'll be going down the route of ripping up the tiles and hardboard and replacing with the 6mm hardiebacker, secured with adhesive and screws to the floorboards (which i'll make sure are screwed down solid!) - fingers crossed, any more tips?

The adhesive was flexible trade stuff, not pre mixed and the same with the grout, i.e. suitable for floor use.

Re quotes, we had - £4,300 / £4,500 / £5,500 - this is for a 1.8m x 2.2m room fully tiled with shower over bath, moving of toilet and vanity position and new towel rail.

At the moment we're at £2,783 including all labour, materials and even the towels and fittings so a considerable saving - although hard to put a price on the additional hassle of the floor!