First time poster on here. I hope you guys can help me out here.

Upstairs bathroom. 2.5M x 2.5M

Have just replaced my bathroom floor with 18mm exterior ply and screwed down every 6inches on EVERY joist. Put down 330mmx330mm ceramic tiles using flexible tile adhesive. 2mm gaps. Grouted using Mapei Ultra Colour Plus (mixed exactly as per instructions). Left the grout for 24hrs before walking on the floor.

After just 2 days of walking on the floor, a lot of the grout towards the centre of the bathroom (most floor movement) has cracked !

The description on the grout box does state the it is suitable for timber floors. I took this as meaning that it is flexible.

Any ideas what the problem coud be?

Thanks for looking in guys.

Keez, it is more than likely a deflection problem, ie the floor has some "bounce" in it. Did you put any noggins between the joists? To be honest, tiling directly onto ply isn't a good idea either imo, you should overboard the ply with a hardiebacker type board, this wont stop deflection,(only a completely solid floor will do that), but will provide a more suitable tiling surface.

Hi,

Did you put the ply straight on to the joists or onto the floorboards/chipboard that was already on the joists?
What adhesive did you use - ready mixed in a tub or powdered in a bag?
When you say most movement do you mean there was deflection in the middle of the room
that you could detect before tiling?

Definately not an issue with the grout Keez. As Doug and Macten have said, you have bounce in your floor. I suspect, not only are the joists not strong enough, you may well have not supported the edge of your ply sheets with noggins and they are moving as you walk over the area.

No easy solution by the sounds of it. Sorry.

Wow, some quick responses there, thanks guys.

I did put noggins in.

Ply was put straight onto the joists, old chipboard removed.

The adhesive I used was Nicobond Quickflex, powdered bagged.


Yes, there is deflection in the middle of the room. When I step out of the bath/shower, there is a slight bounce, approx 1/4inch. I thought this was quite normal, what flexible grout was designed for.



What are my options now? I really need to get this floor sealed and protected from water. I know it's not the done thing, but what about removing the grout and using either silicone or acrylic?

With that much bounce i don't think it would work sorry to say you need to fix the deflection in the floor first

Flexible grout (and adhesive) are not really designed to take up deflection, they are designed to help with expansion and contraction of the different materials - tiles will expand and contract at a different rate to timber for instance. Even if the grout could move 1/4 inch, your tiles certinaly wouldn't as they are rigid!

But the grout is moving 1/4inch vertically, along with the tile. It's not like one tile is staying still whilst it's neighbour is movinf 1/4inch.

This is so frustrating, I need to find a solution to this. Looks like silicone may be the only alternative, unlees I can find such a thing as a SUPER flexible grout.

Originally Posted by Keez

But the grout is moving 1/4inch vertically, along with the tile. It's not like one tile is staying still whilst it's neighbour is movinf 1/4inch.

This is so frustrating, I need to find a solution to this. Looks like silicone may be the only alternative, unlees I can find such a thing as a SUPER flexible grout.

...and that is why it has cracked! There is no product on the market that will take up that amount of movement, even silicone will struggle to stay in tact after a while. If the grout hadn't cracked to take up the stress imparted upon it, the tile would have.

The only real solution you have is to strengthen your floor.....or put vinyl down! Sorry to be the bringer of bad news but that is just the way it is I'm afraid.

This is the reason we all use 25mm wbp ply straight onto joists and not any thinner.
That extra thickness would remove any deflection.

with that much deflection keez, I'm surprised that the tiles haven't debonded yet....I'm sure they will though.

I have to agree with the guys above, Im afraid. ANY bounce in a floor will cause a nightmare to a tiler. The grout has cracked, the adhesive will not be able to stand that kind of deflection either and removing the grout and replacing with silicone will not achieve anything. As has been said above the tiles will come loose and crack, we have all seen this endless times and people always try filling the crack in with silicone to keep it water tight, not once have I ever seen it do anything but make a mess. The only option is to strengthen the floor. Sorry.

Originally Posted by Keez

Wow, some quick responses there, thanks guys.

I did put noggins in.

Ply was put straight onto the joists, old chipboard removed.

The adhesive I used was Nicobond Quickflex, powdered bagged.


Yes, there is deflection in the middle of the room. When I step out of the bath/shower, there is a slight bounce, approx 1/4inch. I thought this was quite normal, what flexible grout was designed for.



What are my options now? I really need to get this floor sealed and protected from water. I know it's not the done thing, but what about removing the grout and using either silicone or acrylic?

With the Nicobond Quickflex did you add a diluted Nicobond Enhancer 2002 for the ply, if you did'nt you should have done..

Standard Quickflex is ok for screed floors http://www.ncdirect.co.uk/pdf/quickflex.pdf

got the same problem in my bath anyone know if you inject a bit of gripfill into to the big grout gaps wait till dry and the grout over ,will it help ? anyone ?

a silicon acrylic caulk over the grout may provide for a quick fix

Originally Posted by tilerbob

a silicon acrylic caulk over the grout may provide for a quick fix

A very poor idea IMO

If the grout is moving due to this deflection then it wont be long before tiles start de bonding IMO.
As already mentioned did you use the liquid with the powder adhesive ?

Originally Posted by eddi

got the same problem in my bath anyone know if you inject a bit of gripfill into to the big grout gaps wait till dry and the grout over ,will it help ? anyone ?

if the 'cracks' are that big it sounds more like you've used too wet a mix and the grouts dried out. if you put pictures up we should be able to help you more