I have just purchased a Corian fit for a three-wall tub surround (30×72).   Our cast iron tub does not have a tall lip, over which the rock laps, like you would find on a fiberglass unit that gets nailed to the studs.  Instead it has a really small, s-shaped rise of about ¼â€ at the outside edge.  The kit instructions said there must be at least a ½â€ gap between the tub and the bottom of the rock.  When you put the Corian over the rock, it only overlaps the lip of my tub by about 3/8″.   I had assumed that I would caulk between the Corian and the tub.  However, there was no mention in the kit instructions of a caulk joint between the tub and the Corian, which seemed to imply that the gap was necessary to allow any water vapor to escape, whether from the shower itself or from condensation behind the surround.  I am worried about water leaks if there is no caulk, especially since the lip is so short and there is a ½â€ gap between the rock and the tub lip.

Does this mean that I need to put in a new tub (!) with a tall lip in order to prevent bigtime leaks at that joint?  Or can I run the Corian down to 1/8^†onto the shims they provide, caulk it, and not worry?  Did I waste my money on the kit? 

Bicycle Bill.