How to seal large uneven gap between tub and surround?

Question

I need advice on how to proceed with re-caulking the joint where the tub meets the wall. The previous owners installed a plastic surround around the tub. I have reason to suspect there is tile behind it. It was in rough shape when I first moved in, so I removed the old caulk, cleaned the area, let it dry, and re-caulked it using silicone. Backing rods were stuffed into the larger gaps. I filled the tub and allowed longest cure time on package instructions before using it.

Less than 2 yrs later, it's cracking and separating from the surround. Apparently the surround isn't flush with the wall behind it. It shifts when touched in certain areas, creating unsealed areas. Water is getting stuck behind it and mildewing.

The surround is not in great condition, but we can't replace the whole thing right now. We also don't have the ability currently to open the can of worms that would be taking it down to investigate what's behind it.

• The bottom is chipping off in some places causing large gaps. I used backer rods previously but they weren't big enough to stop the shifting back and forth against the wall.

I've scraped everything out. Disinfected, treated with mold killer, and let dry for a couple days.

• How do I seal this so water doesn't get in? Between the broken areas and the gap between the surround and the wall I end up with more caulk behind the surround than in the joint.

• Should I use caulk tape first to seal the gap and then caulk that in? Just caulk tape?

• It seems like there is a tile piece behind the surround running the perimeter of the tub. Should I not caulk it at all?

Answer 1

The best solution would be to rip it out and check what's behind it.

If you can't do that now, here's a temporary solution:

The idea is that silicone caulk will not remain attached to your surround unless it is fixed in place. Right now as you say in the question, if you push on it, it moves, which probably tears the joints. So you need to put something behind it to prevent it from moving.

Obviously, spray foam.

It works much better with a foam pistol than straight out of the can, but well. The idea is to inject foam behind the surround to prevent it from moving and tearing the joint. In order to do this you need to add a bit of plastic pipe at the end of the spray foam nozzle, and this bit of pipe should be very thin walled so it can squeeze into the crack to fill it with foam from the inside. For example you can use a plastic straw, or heat shrink.

Spray foam tends to be messy, so it would be a good idea to tape some plastic to the tub to make sure none of it touches the tub. Don't spray too much, or it will push the surround away from the wall.

Then cut off the excess.

Now the thing should be secured in place, at least the bottom part.

The trick to make a good silicone caulk joint is first, cleaning, then not to fill the crack and remove the excess, but (as the drawing above shows) to have the joint thick enough that it sticks to both flat surfaces. In other words you need to use enough silicone, and finish it with a tool like this:

You'll probably need to use the largest corner. The crack should be completely covered and then several millimeters of flat on both sides also, so the silicone can stick to the surface you were able to clean. Also these caulks have a maximum amount of stretch above which they break, so a thick joint can stretch more than a thin joint.

I recommend water based silicone for ease of cleaning.

This temporary solution is only to be considered if no water gets behind the surround from the top of the sides. If water does get in, then the caulk will prevent it from draining, and the wall will rot. And you should really rip it out.