I love the idea of the luxurious texture of wood flooring in a shower. Does this add significant maintenance/cleaning concerns to the shower?
There are woods used in wet environments, mostly those that are fairly impervious to rot - teak, cedar, redwood, mahogony (less so).
Untreated: Sometimes cedar, redwood and teak are left natural. In almost all these cases, they are not exposed to prolonged soaking. Even in saunas they are misted and then dried, not soaked the way a shower soaks. On Boats they get spashed, they dry. When they are left untreated, the wood weathers, loses its color and eventually becomes somewhat rough. Cedar and redwood are fairly soft, and on a floor will wear out, probably quickly. Teak will last longer but will roughen up and wear out eventually. Also untreated wood tends to hold water and is more prone to developing mold or organic growths, such as bacteria, especially in a shower that is not exposed to sunlight.
Treated: These woods can be finished with a penetrating sealer or a hard marine finish, like spar varnish or the harder polyurethanes, or even an epoxy finish. Even then, these finishes need to be redone often, sometimes every year, depending on the amount of soaking and other wear.
Finished or sealed Wood can be used in bathrooms where there is just an occasional spash or where the wet is quickly mopped up. I have hard pine floors finished with several coats of poly in my bathroom that works fine. My shower is a porcelain tub.
On the whole, it would be very questionable to put wood in a shower floor.
Disclaimer: There are some newer exotics used on outdoor decks such as ipe. I am not familiar with their waterproof characteristics, but even in deck use, the soaking is probably not as complete and frequent as a shower.
Sorry I didn't check back and see your question re: maintenance and cleaning, etc. I used ipe decking in my master bath shower, raised off the tile floor with rubber feet. I rinse the ipe off soap scum after every shower, this takes about 5-10 seconds. Every regular cleaning the wood is lifted out, tile cleaned normally. I have a hard brush to scrub off any residual soap scum on the ipe, there usually isn't any. Two adults use this shower daily, but the wood does dry out completely over the course of the day/night. It is not continuously wet.
I've had the ipe for more than two years without issue, no mold or other concerns. Ipe is heavy, however, and removing it every week or two to clean the tile may be difficult.
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