Outdoor sauna: cement or wood foundation?

My 8×12 sauna plan assumes a wood, not concrete floor.

  • 2×6 green rim joists.
  • 2×6 green studs at 16″ or 24″ on center.
  • 3/4″ subfloor,  et voila.

Stephen, I know your sauna has a cement slab, and i’ve built a couple saunas with a cement slab base (which could be argued is the “A” job) yet I find with a wood base to your backyard sauna it can be:

  1. built quicker.
  2. leveled easily, even down the road.
  3. moved if you move, of if your partner gets wiggy.
  4. called a ‘temporary structure’ for frowning building code inspectors.
  5. extended easily as a header for a deck (yet I prefer a slate patio with an outdoor sauna, so as to reintroduce the stone medium from sauna rocks to your feet whilst between sauna rounds).

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13 thoughts on “Outdoor sauna: cement or wood foundation?”

  1. What about a gravel foundation? the foundation seems to be the hardest point to decide on… especially when considering the changing room. I would be interested to hear your thoughts on this….

  2. Steve:

    GREAT question. I am ALL about a gravel foundation. Dig out the top couple inches of top soil. Then lay down class 5 gravel, building up from grade, THEN build a 2×6 green perimeter base, 16″ OC floor joists.

    Gravel is super easy to prep. It’s easy to level. What’s best is that it sheds run off. I suggest at least 1′ roof overhangs all the way around, and gravel extending a few inches more than that along the perimeter, tapering down to grade.

    For an 8×12 structure, one could certainly mule train gravel in a vehicle, yet I highly suggest getting a load dumped in the driveway from a landscape company. It’s cheap and best 5-7″ thick vs scrimping with too thin of a base. If too thin, your structure will sink after a year or two and you’re in for more work after the fact, not fun.

    Ain’t nothin’ better in my book. Well done Steve.

  3. when using treated lumber, be aware that there are different ‘grades’. some is only rated for above ground use, like for handrails on a deck. treated lumber rated for ‘ground-contact’ is what should be used for any type of structure that will be in direct contact with the ground. this includes grass, concrete, gravel, dirt, etc.

  4. One last question. In Belarus, we dump water over our heads and actually bathe. Is there a “better” floor for that? There they have duckboards over dirt in most of the outdoor saunas (Banyas). so I was considering the base you had shown with gravel & pavers, but wasn’t sure about a changing room with that.

    In your floor above, standard 3″ floor drain piped away from the structure?

    And thanks so much for your replies… I have been planning/dreaming about this ever since my first experience in Belarus in early 2000’s.

  5. I want to try to build a sauna in Mn. I need photos to understand the foundations. I need to plan for supplies. It will be by a marsh wetlands so would need to be built to code and legal to be by wetlands. It would need to seat 6 people. I do have access to rough timber and like rustic

  6. Margaret: Check with your township regarding legal wetland setbacks, etc. Get the ebook. If it doesn’t get you going, i’ll give you your donation back.

  7. We are getting a rectangular sauna delivered and need to know what would be the best foundations for it. We live in a terraced house and space is very limited in our garden.
    The manufacturer has said there need to be 6 supports (it is 3 by 2 metres). Would you recommend using gravel with concrete slabs on top or is this too unstable? Any advice gratefully received.

  8. Stephen:

    Yes, for sure. Options are detailed in my ebook, and you summarize the options pretty well, as directed from your manufacturer. You can go with 6 supports in terms of sonotubes (cardboard cylinder tubes that you put in a hole and backfill with cement) or deck blocks, or cinder blocks. This depends on your soil condition. Clay sucks. Good drainage area is best.

    For a small structure, fairly level, many lay their free standing structure on 4″ of class 5 gravel, raised above grade for drainage.

    More here: https://www.saunatimes.com/building-a-sauna/the-best-base-for-our-sauna-building-a-pour-some-piers-or-just-a-bunch-of-gravel/

  9. One option, if soil conditions allow, would be screw piles. Easy to transport to site, easy to ”install” and provide a solid foundation.

  10. Olli, thank you for your note re the screw piles. I am in the planning process stage of building my sauna here in S Minneapolis and have been wrestling with foundation options. It appears the screw piles would save a load of time and money. Has anyone else used such a foundation option here?

  11. Hello Glenn;

    Dave Shattler here, time to update you on my progress here in Central Michigan. I bought your book in early December 2018, (I think) and actually started construction just before Christmas, when we had a break in the weather. (35-40 degrees for a couple of days) I am posting here as the first part of my project is of course, the foundation.

    First some basics, My design started out as an 8 X 14 structure, divided into a 8 X 6 hot room, a 8 X 4 Changing room, and a 8 X 4 front porch, (with roof) for summer time cool downs. I decided on 2 X 6 inch walls for the hot room for ease of heating and wood conservation. With the loss of 2 inches on each wall because of the difference between 2 X 4, and 2 X 6 inch walls in the hot room, I decided to add an additional 1 ft. to the overall building length making it 8 X 15, and increasing the hot room to 8 X 7, This helped greatly with spacing for stove clearances. (Using a Karhu wood burning sauna stove 16CK With Glass Door).

    Now for the foundation choice. Several friends advised me to have a cement pad poured to simplify the process. I considered the gravel type foundation, but it did not seem to be as permanent as I wanted.

    After pricing having a slab poured in Michigan in December, I chose to go with a post and beam type foundation to save money. I laid out the outline of the building with builder’s string and then dug 6, 10-12 inch holes 42 inches deep. I spaced them 12 inches in from the sides and 12 inches in from the ends. That made the holes approx. 6.5 feet apart length wise and 6 ft. apart width wise. Now for the tricky part of pouring cement in Michigan in the winter!

    Everything I had read advised against pouring cement unless the temp was above 40 deg. F for at least 24 hours. That is hard to do in Michigan in December. After “belling out” the bottom of the holes and assuring that my 6 ft. 4 X 6 inch posts fit properly, I put a shop light with a 60 watt bulb in each hole and covered with some old insulation and blankets and left the holes covered for 24 hours. After checking the bottom of the holes with a remote reading meat thermometer with a probe, I found the soil temp. at the bottom of the holes and 4 inches deep in the soil to be 58 deg.! I kept the holes covered and mixed one 50 Lb. bag of Quickcrete fast setting cement mix I put the mixed cement in each hole and dropped the lightbulb just above the wet cement and recovered with the old insulation and blankets. I left them in place with the meat thermometer in one of the center holes to monitor the temp. inside the hole as the cement cured The remote reading temp. probe never went below 60 degrees even though the overnight temp. went down to 20 deg. F outside!

    After 24 hours, the cement had stayed warm and set-up nicely. The single bag of cement had filled about 12 inches of the bottom of the hole. I then placed my 5 ft. 4 X 6 inch posts in each hole, and filled the hole with 2 fifty lb. bags of mixed fast setting Quickcrete, leveled all the post vertically. I then covered each post with a 32 gal. plastic garbage can, slipped a lightbulb under each and covered with old blankets, (2 each), The temperature under each garbage can stayed at 48 degrees or higher for over 36 hours. The posts set nicely and stayed plumb.

    After I removed the cans, I used a chain saw to cut off each post 15 inches above ground level and each cut was level with all the others.
    I then cut a 1 and 1/2 inch by 9 and 1/4 inch section off from the top of each post on one side so my 2 X 10 inch 15 ft. beam could be bolted to the posts.

    I then covered the entire area with 2 layers of 3 mill plastic sheeting and covered that with 4 inches of pea gravel.

    I then attached my 15 ft. beams to the posts with 2, 1/2 inch by 8 inch hot dipped galvanized bolts with two standard washers and a lock washer on each bolt. These 2 X 10 inch beams run length wise. When these were secured, I then nailed 2 X 6 joists across the beams every 16 inch on center, using galvanized rafter ties between the beams and the joists. With this completed, I nailed rim joist all around, filled the cavities between the joists with 2 inch foam insulation, and then added 3/4 in treated plywood over the joists. (All wood used so far is ground contact treated lumber.)

    Also before every cut or board nailing, I checked for level and correct length. So far, anywhere you lay the level on the structure, the bubble is DEAD CENTER! (I am not that good, but damned lucky!!!)

    At this point, I covered the project with big plastic tarps and we went to Florida for a cruise!

  12. Sauna is waiting for a door so a blanket and plywood is makeshift but the concrete floor is ice cold even when the electric heater is warmed up. How do you handle the extreme cold concrete versus the higher level heat contrast?
    Susan Svensson
    White Salmon, WA

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