Scott,
I have no experience with the newer systems. The last shower I put in was just a simple multi piece fiberglass one for guests. But I have a couple observations I hope are useful. First, the fiberglass one went in to replace a mud pan shower that failed because they used a galvanized steel pan. That doesn't say anything about the longevity of a copper or possibly a plastic pan. With a good pan, I think a mud bed can last indefinitely. The mortar will not fail if the house doesn't move but hairline cracks can create leaks. That is the job of the pan. If it does not corrode away, the shower won't leak. The right plastic is probably lasts indefinitely but buried in mortar isn't necessarily a good environment for plastic. The kind of leak that caused me to tear out the mud bed shower might not even have been noticed over a crawl space. It was pretty small, in other words. But when it drips onto the drywall ceiling of the kitchen it is a lot more noticable than a few drops into the crawl space.
You know your layout a lot better than me, obviously. But don't forget that you can add posts in the crawl space instead of sistering joists. Might be easier to put a beam or two across the area, put in some pads and posts.
Jim
I have no experience with the newer systems. The last shower I put in was just a simple multi piece fiberglass one for guests. But I have a couple observations I hope are useful. First, the fiberglass one went in to replace a mud pan shower that failed because they used a galvanized steel pan. That doesn't say anything about the longevity of a copper or possibly a plastic pan. With a good pan, I think a mud bed can last indefinitely. The mortar will not fail if the house doesn't move but hairline cracks can create leaks. That is the job of the pan. If it does not corrode away, the shower won't leak. The right plastic is probably lasts indefinitely but buried in mortar isn't necessarily a good environment for plastic. The kind of leak that caused me to tear out the mud bed shower might not even have been noticed over a crawl space. It was pretty small, in other words. But when it drips onto the drywall ceiling of the kitchen it is a lot more noticable than a few drops into the crawl space.
You know your layout a lot better than me, obviously. But don't forget that you can add posts in the crawl space instead of sistering joists. Might be easier to put a beam or two across the area, put in some pads and posts.
Jim