QUESTIONSeveral years ago we had a new bathtub installed and paid to have a tub surround built out of beige porcelain tiles. For some reason, the tile guy brought the tile down to within about a 1/4 inch of the very top edge of the tub "lip." Then he used the same beige sanded grout to cover the seam between the tub and the tiles with a flush edge. Within a short time with the natural flex of the tub, it cracked out. We lived with it for years now but it was getting some moldy discoloration so I started flecking it off carefully expecting to find the tub lip continued up behind all the adjacent tiles or at least some kind of backer board. Instead, I found open air cracks.. I stopped flecking it off but now have several inches that are exposed both horizontally at the margin as well as opened up vertically a couple of inches. I can send you a photo if you give me an email address. I need to finish removing the grout that is cracked on the perimeter and figure out how to seal out moisture. I thought I would regrout the caves the open craters that have opened up but can you apply grout where there is nothing but open space behind it? And can we fill in those spots with new product such as epoxy group when the other appears to be old fashioned grout? I doubt I will be able to match the beige color. If we push backer rod in there, there is nothing to press it up against...won't it just push through or protrude towards the front and make the caulk we try to apply over it a mess? Help!
ANSWERANSWER - The transition joint between the top of the bathtub flange and the wall tile is not supposed to be filled with a cementitious grout. It should be filled with a resilient ASTM C920 sealant that is either a 100% silicone sealant or a polyurethane sealant.
Per the plumbing building code the top edge of the tub should have a 1" tall vertical flange where the moisture paper and/or backer board or mortar overlaps the flange so water cannot get into the wall cavity where it could develop microbial growth.
Hopefully there is some sort of moisture barrier behind the wall tile. You should remove all of the cementitious grout out of the horizontal transition joint from the top of the bathtub to the wall tile. Since there is not backing and only an open joint after you remove the grout, I would get a closed-cell foam sealant backer rod. It is round and comes in a roll. Get the size where the diameter of the backer rod is larger than the height of the transition joint, so when you push it in it will stay in place. Make sure you leave enough room so you can apply at least 1/4" thick sealant into the joint. Use painters tape on each side of the joint tightly attached to the wall before applying the sealant as this type of sealant is hard to clean up. After applying the sealant and tooling the joint to a slight concave shape then immediately pull of the tape. If you don't pull it off right away it could pull the sealant of out the joint. If you do it right, there is not mess and it gives you a nice clean caulked joint.