Rebuilding the Surround Around a New Tub
The Substrate Is Not Ordinary Board
Standard wallboard in a tub surround absorbs water, swells and gives up. The replacement is cement board, a foam backer panel or a board with an integral waterproof membrane, each installed to its own instructions. This is the single most common corner cut in a tub replacement and it is invisible until the day it is not.
The Flange Detail
An alcove tub has an upstand along the wall edges. The backer board sits above that flange, not on top of it and not behind it, so any water running down the wall lands inside the tub rather than behind it. Getting this relationship wrong routes water into the wall cavity permanently, and no amount of sealant afterwards corrects it.
Waterproofing Over the Top
A liquid membrane or a sheet system applied over the backer, taken into the corners with the correct banding, is what actually keeps the wall dry. Corners and the joint at the tub edge are where failures start, so those are the places to watch during installation rather than the open field areas.
The Joint at the Tub Edge
That line is silicone, never grout. It has to move, because a tub with a person and water in it sits lower than an empty one. Filling it with a rigid material guarantees a crack, and a cracked joint at exactly that height feeds water straight into the wall.
Fill It Before Sealing
The correct sequence is to fill the tub with water, then apply the silicone joint, then let it cure and drain. Sealing an empty tub sets the joint in its raised position, and the first bath stretches it. It costs nothing to do in the right order and it is the difference between a seal that lasts years and one that fails within months.