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Crack in bathtub caulk comes and goes

QuestionWe bought our home in 2004 and discovered that the bathtub in the main part of the house separates from the tile surround up to 1/4 inch. I cleaned out the old caulk, put in new, and waited a week before using the tub. When I filled the tub and got in, I noticed the crack was back, even with my new caulk. Even more bizarre is that the crack went away when I emptied the tub.

I think that the tub flange was never attached to the backboard, drywall, or studs and therefore slips down when weight is put in the tub. I don't know how to fix this without disturbing the tile. Can you help me with this before a mold situation occurs? Attached are photos of the tub with and without water in it so you can see my problem more clearly.

AnswerThanks for the photos as they helped me understand your situation. You were also a good sport to stand in the tub while we talked on the phone as it helped us narrow down the problem.

It's true that there's a small flange on the foot, head, and side of a bathtub. However, the purpose of the flange is to give rigid support to the overall tub, to act as stop for a wall surface such as wallboard or cement board to rest on, and to serve as a surface to caulk up against. It is not used to fasten the tub to the wall, but the flange on an acrylic/fiberglass tub is a little higher so it could be used to fasten the tub to the wall studs using screws.

Your photos showed a porcelain finish, so I ruled out acrylic/fiberglass, but it was hard to tell if your tub is steel or cast iron. Your standing in the tub and seeing some flex in the tub's floor and around the drain area told me that you have a steel tub because cast iron doesn't flex at all. The fact that only the weight of your body forced the caulk to open tells me that there's little or no support under the tub's flange.

Cast iron tubs do not require additional support, but steel and acrylic/fiberglass tubs need adequate support fastened to the back wall, e.g., 1-by-4 or 2-by-4 material secured horizontally for the underside of the tub to rest on. Additionally, a 2-by-4 should be positioned vertically in each corner where the tub meets the wall. Also, a 2-by-4 might be required next to the bottom plate to properly support the 2-by-4 under each bathtub corner.

In your case, you'll need to cut an inspection hole into the back wall at both ends of the tub so you can see what's there. Because the horizontal support would have been put in first (once the tub was leveled) before the tub was installed, it will be a little more difficult for you, but not impossible, to install the support your tub needs. You will have to open the end walls as high as the tub and all the way to the back wall where the tub is resting. The opening should be as wide as the tub to allow room to work and to attach support on both ends. This project will require two people.

Once adequate support is in place, completely clean out the caulk on all three sides. Many plumbers believe that a tub should be filled with water prior to caulking. My experience has shown that if the tub is properly and completely supported, it will not normally move enough to exceed the caulk's expansion and adhesion.

When applying caulk, only do the back wall or the two end walls first—but never all three at the same time. If you do, your finger will disturb the caulk on the adjacent wall(s). Allow the caulk to set up overnight before applying caulk to any adjacent wall(s). Good luck with your project!

Copyright © 2005, 2006, & 2007 LAF/C.R.S., Inc. All rights reserved.
Question answered by Leon A. Frechette.

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C.R.S., Inc. · Spokane, Washington · USA

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