Grouting to use over marine ply

[chattykathy]

Is there a special grouting we should use in our shower? We used marine ply to even out the walls and to suit the size of the cubicle but now we feel this is causing the grout to crack. Is there a special type of grout or sealent we should have used?
TIA

[buildersmate]

Use a flexible additive to the grout mix such as Bal Admix GT1. It is mixed into the water that goes into the grout powder and makes the grout less liable to crack on slight movement.

[PeelerPal]

Ok there are several things that you should have done when fitting marine ply in a wet area. Firstly, once the ply has been fitted, you should still seal it with an additive to prevent the ply drying out the adhesive so a PVA or SBR should be applied with a brush or sponge directly to the surface, this admix should have been watered down to 5 parts water to one part additive. If you have done that then you must be sure that you have used the right adhesive for the tiles, I.E, if you are using tiles larger than 300mm by 300mm you cannot use ready mixed adhesive as the surface area will be too large to hold the weight.
Finally, it is important to remember that when you grout your shower, it actually is advisible to not use the shower for 2 weeks.

if you have followed all of this then either the ply is too thin or not fixed securly enough. hope this helps.

[AlBags]

Yes ... Waterproof grout with a polymer added .. improves flexibility.
What thickness ply?
Surprised you did not use WBP and coat it.

[chattykathy]

Thanks for your answers and advice. I shall ask OH what he did in the first place and report back. Hopefully we will be able to cure the problem using buildersmate's advice. Don't think we waited two weeks after tiling PeelerPAl! :-(
Thanks again.