The sauna floor is the one surface in the cabin that should not be the same timber as everything else, and it is the detail self builders most often get wrong. Water reaches it every session, it never sees the hottest air, and a beautiful cedar floor in a sauna is a rotting floor with a countdown on it.
Why not timber
The floor is the coldest surface in the cabin and the wettest, which is the combination timber handles worst: it stays damp long after the walls have dried. Cladding a floor in the same cedar as the walls looks right for two years and then darkens, softens and smells. Use a hard, non absorbent surface and put timber on top of it if you want the look.
What to use instead
Tile over a waterproofed substrate is the durable answer and the usual one in a built in sauna. Sealed concrete works and is common in a garage or basement. A commercial sheet vinyl rated for wet areas is the cheap option and is entirely serviceable. All three want a slight fall if there is a drain, and all three want to be non slip when wet.
Duckboards
A slatted timber duckboard or a perforated mat over the hard floor gives you the warm surface underfoot that people actually want, keeps feet out of any water and lifts out for cleaning and drying. This is how you get timber into a sauna floor honestly: as something removable on top, never as the floor itself.
Drains, and when you need one
A domestic dry sauna does not need a floor drain: the water that reaches the floor is a few ladles' worth and a duckboard over a sealed floor with ventilation handles it. A drain earns its place if you will hose the cabin out, if it is a commercial installation, or if a shower is in the same enclosure. An unused trap dries out and smells, so a drain you never use is worse than none.
Questions people ask about sauna floor
Can a sauna floor be wood?
Not as the floor itself. It is the coldest and wettest surface and timber stays damp. Use a hard surface with a removable duckboard on top.
Do I need a floor drain?
Not in a domestic dry sauna. A duckboard over a sealed floor with ventilation is enough, and an unused trap dries out and smells.
What is the cheapest workable floor?
Sheet vinyl rated for wet areas, or sealed concrete if the slab is already there. Both are fine with a duckboard over them.