"Could the cracking of the grout be from moisture from the opening of the concrete floor under the shower? i" You pretty much hit it on the head there in that it is moisture related. For the most part, it could be from one of three things: 1) It could be moisture or moisture vapor coming though the unplugged hole, and wicking outwards to the curb. But if that was the case, in terms of there being significant moisture, you'd probably see signs of water presence or water damage at the base of the curb. 2) It could be the wood curb simply wicking moisture through the slab below. The thing it, as best as I can read the photos, it appears that the majority of the swelling is on the top of your curb. The tile on the outside curb facing could have the bottom edge locked in place by the floor tile, in that case it could be the floor tile that is preventing the bottom edge of the curb tile from blowing out. But to me it looks like the curb tile sits on top of the floor tile, so if the bottom of the curb tile wanted to blow out, it would be free to move. Correct? Incorrect? Based on that assumption? I'd guess that: 3) In the first two photos, it looks like there is a metal track or trim on top of the curb. If that track/trim has been fastened through into the top of the curb, it's possible that moisture is getting to the curb through the fastener holes. The top of the wood curb got wet, then expanded, and it's that expansion that is blowing the curb apart. I don't know how the membrane was detailed at the curb. It should be wrapped up the inside face, over the top of the curb, and stapled on the outside face of the curb. Any fasteners through the top of the curb could have penetrated the membrane and allow moisture to get to the wood. I've seen a lot of curbs where the membrane never completely made it over the top of the curb. In that case, it could be as simple as water intrusion through the grout lines on top of the curb. Again, that's based upon my assumption (cough, cough) that the floor tile outside the shower is not locking the curb tile in place and the bottom edge of the curb tile is free to move if it had to.

Hi, Thanks for the reply. Had I known limestone was such a nightmare I never would have chosen it but the bathroom designer said it was fine as long as sealed properly. With regards to the installation/construction the only thing I can tell you us that it involved a wooden frame on a concrete (old concrete) floor, they used some sort of waterproof boards and a paint on blue rubberised/waterproofing membrane. The tiles are sealed with Lithofin. The pitch of the floor is good, ie all the water drains to the centre well. Husband thinks we should just let it dry out and then seal all the grout lines and the broken tile with silicone. The installers are confident that the waterproofing underneath is tight but surely can't be good to gave water sitting under the tiles all the time? Installers on holiday until 16th - have already emailed them with photos. I guess my questions are: 1) Is it actually doing any harm - ie could we carry on as is? 2) Would the drying out and siliconing work? Currently only the changes of plane and the axis radiating out from the drain are siliconed - the rest is normal grout (which has cracked). 3) Would it gave been better NOT to seal the tiles at all (so water could dry out through them)? 4) If the installers were to rip up the floor and replace how horrific would this be? Would all he wall tiles be wrecked too? Could the glass shower frame be removed without too much damage? I am pretty sure they said ripping up the floor would break the waterproofing. 5) Should we re-tile the floor with something other than limestone, eg marble or ceramic mosaic? I can't tell you how much I wish I had chosen another material. :-(

It's 8 years old and I have re-grouted twice underneath the shower. The grout closer the the shower is definitely thinner from scrubbing/cleaning. Also the pebbles themselves are starting to chip away probably from the scrubbing/cleaning. I'm not ready to completely gut the thing and redo it, so any patch ideas to get the longest and easiest cleaning. It's a pain to clean it weekly as water/soap scum collects in those crevices where grout is missing. Here is a pic of the issue along with the original Wedi boards, it'll be a costly and time consuming process to to redo the whole floor.

@millworkman, you mean where shower glass meets shower floor there should be silicone caulking? I believe the GC mounted the glass directly (deeply?) into the grout. I remember asking another bathroom contractor about this and he said it's Old School but works. No? Here are more photos, two of master shower and two of my son's shower. Note the crack along the grout of master shower grout. GC said he'll come and fix once we let him (or anyone) into our home isolation. Thanks for chiming in! I was going to ask about this next. Son's shower Master shower (with grout crack, which we keep dry during showers):