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I'm building a traditional outdoor wood-fired sauna cabin. The traditional Finnish ways to build drainage is either gaps in the wood floor to allow direct drainage to the ground, or sloped floors into a linear drain. As shown in these two build log blog posts:

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I currently have an open floor with exposed flat/level floor joists. I need to slope the floor as I install the floorboards. I'm ok raising the floor above the level of the current joists to set the pitch.

This is just a simple "2D" slope, not a complex slope to a single drain like you would see in a shower pan.

How should I support my floorboards to create the drainage elevation? Some kind of shims on top of each joist? Cut down the existing joists? Attach sistered and protruding joists, cut to a specific angle?

This would be similar, perhaps, to building a flat roof or deck with a slight drainage pitch. Or a curbless shower with linear drain.

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Your drain is installed parallel to the joists. That means you have an easy solution available: Simply increase the height of each joist going away from the drain channel in a progressive manner.

Assuming two joists outside the pair flanking the drain, rip 1/4" and 1/2" slats and tack them on the second and third joists, and so on. I don't even consider it important to slant them. The floor boards will rest on the edges just fine.

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  • I would like to slant the shims for a more solid floor and for my own sense of craftsmanship. Any thoughts on how to get those angles/cuts correct?
    – pkamb
    Commented May 31 at 18:41
  • It's just a bit of trigonometry depending on your joist spread and height variation. Find an online angle calculator and put in those two dimensions, then set your saw to the angle. It'll be very small, like 1 or 2 degrees.
    – isherwood
    Commented May 31 at 18:55

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