Grout depth - prefilling needed?

[mick1970]

Hi all
I am replacing the grouting on my tiled bathroom floor - it had starting crumbling only a few months after being installed (not by me) and I've finally bitten the bullet to try to replace myself - first time doing any grouting.

I have taken out the existing grout using a multi tool (cut through it like butter), but the underlying cement lifted as well, so in some spots the gap between the tiles goes all the way down to the floorboard liner, a depth of 7-8mm.

My question is - could/should I put grout in directly to fill that space, or does the gap need to be prefilled to reduce the depth of grout needed? And if so with what?

If relevant the specific grout is Mapei Anti-mould Flexible grout
https://www.wickes.co.uk/Mapei-Anti-Mould-Flexible-Coloured-Tile-Grout-Beige-5kg/p/112720

Thanks in advance
R.

 

[plastic_peanut]

You can do but check how much movement you have in the tiles. If the sub-floor has movement in it you will be fighting a loosing battle with cracking grout. Ask me how I know.

Was tempted to try a silicone type product or caulk for better flexibility but moved house before it got that serious

 

[mick1970]

Thanks. Yes there is a little bit of movement (you can hear the floorboards creak if you really put your weight on a tile), but I’d hoped the flexible grout would have handled that. I guess the question is how flexible is flexible?

 

[opps]

Flexible grout will fall out the first time anyone vacuums the floor if the tiles are rocking.

On a couple of occasions I have used silicone but told the client that it was a temporary fix (read: bodge) and that tiles needed to be lifted.