My old boss swore by using a 1/4 inch spacer for every shower door install, but I think it's a waste of time.
He drilled it into my head for years that you always need that gap for silicone and to prevent stress. So I did it, on hundreds of jobs. Then last year, I was doing a big remodel in Portland and the tile guy left me with a perfectly plumb and level opening, the kind you dream about. I decided to just set the frame right against the tile, no spacer, and caulk the seam. It's been over 8 months now and the client just sent me a picture, still perfect, no cracks, no issues. The spacer just adds an extra step and a bigger bead of caulk that can look messy. I'm starting to think it's an old habit from when materials and installs weren't as precise. Has anyone else moved away from using spacers on a solid substrate?