Is there a better way to seal our sauna floor?

I’ve done it every which way ’til Tuesday. Rubber mats, Red Guard, etc. etc.

But I like this method the best. The three “F’s” of skim coating:

FRUGAL: $50 all in. Durarock is relatively inexpensive, easy to cut and lay down. $8.00 – $9.00 for a 3’x5′ sheet. A bag of vinyl cement is well under $20.00.

FORGIVING: Well mixed vinyl cement is easy to trowel and works its way into cracks and mistakes like nobody’s business.

FAST: All this takes about 20 minutes (and is actually kind of fun!).

Here’s how we prep our cement:

Here’s how we prep our floor:

Here’s how we lay it down:

Here it is “Finnished.”

This has worked great for me for about 10 sauna builds. My cabin sauna is 23 years old. I found a crack in my floor and simply mixed up a little vinyl cement English milkshake consistency, poured it on, and filled the crack. And that was 7 years (and 348 saunas) ago.

Once skim coated, our floor is ready for duckboards or a suspended wood step.

All good on this Western front.

If there’s a better way to Finnish off our hot room and changing room floors, i’d like to know about it!

Stepping inside Michael’s German backyard sauna

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42 thoughts on “Is there a better way to seal our sauna floor?”

  1. Love the idea!
    I’m curious if you use water in your sauna or is it electric? Wondering how this would hold up to water being on it.

  2. I use a lot of water in both my sauna hot rooms that each have this floor solution. I’m a big fan of how it seals off and makes waterproof.

  3. Hello Glenn,
    The skim job looks great! Easy too. I have 2 questions post reading your ebook a few times now.
    1. My build is coming along and I was just introduced to Extreme Green sheeting. It’s a magnesium oxide board that works like dura rock but is smooth , I bought all the needed 4×8 sheets, did the trever square tapered sleeper s and am perplexed on how to seal it. Sorta new product with not much inside tips yet. Question is do you think The vinyl crete coat seal job above mentioned will work on the Mag/oxide board.
    2. Bigger question is My Kuuma stove is on the way, it has an 8″ throat which will be fed from changing room, I already built pedestal in hot room for stove being in the corner (with heat shields) and it will be 11″ off the wood framed wall, with 2 layers of the extreme green board (1″ air gap between) for a final distance off finished wall of 7-9″ per Kuumas’ details. Problem is the through wall for throat which is now wood, soon to be metal studs with Extreme green (fire proof) on both sides and throat hole of 18.25″ wide. Im told we need 16″ non combustible around throat but all other dimensions including pad were built around the closer numbers. Is there a creative solution to not have to eat up more hot room floor by moving the stove away from side wall. As it is I can easily get 11″ inch offset from wood wall but the 16″ is gonna make for a bunch of tear out and redo and worse than all it will eat up hot room bench space. Sorry for the mouthful, hoping you have a creative solution. Thanks!

  4. Timothy:

    1. i’m not familiar with extreme green. I’d say test out a remnant piece with the skim coat. i’m an old dog with the durarock, and just love how it sucks up the vinyl cement, so hopefully you’ll have same results with your product.

    2. I hear you on the rip up, but I want to say, it’s ok to go one step forward and one back and then another forward in another direction. it happens all the time with everything and may as well happen with our sauna builds. You are needing to have clearances to non combustibles big time if you’re going to go with the outside feed/throat extension. It’s critical you do it right. And you’re going to feel tons better when you get your stove rocking and you can whisper ‘lampomassa’ three times as you toss water on the rocks.

  5. Greetings, Glenn. I hope you and yours are enjoying a wonderful sauna season. I have a question about the floor method you describe above, using the combination of concrete board and the vinyl cement patch product. Just to be sure–Does this create a final surface that is waterproof? Lastly, does water “bead up” on this floor surface and if does not, then is the water soaking into the flooring below? Or should it be treated with a concrete sealer product?
    Many thanks for all you do for the world’s sauna traditions!

  6. Hi Dustin:

    When mixed and applied properly, the vinyl cement is waterproof. The trick is to trowel it along the face of durarock that is sloped to the drain.

    You’re welcome! Keeping the tradition alive.

  7. Glenn,

    I am considering going up the sides of my hot room 6-12 inches of cement board and sloping the floor from that and bypassing the drip edge. I would then skim coat after that. Is this reasonable?

  8. Hi Glenn:

    Totally reasonable to run your cement board up the wall a bit. One thought is to skim coat your cement board before screwing to your wall, keeping gravity on your side.

    You can always use more vinyl cement or sealant as a second coat or patch job after it’s applied to the walls. I’ve done this several times and it works great. Even a car wash sponge to rub on the cement, then trowel after for that smooth clean finish.

  9. Hi there, am considering a sauna build, I have a 6×7 shed built on a concrete slab that I hope to use. This shed is rotting out and I will have to replace some siding and the treated wood sill plates at the bottom. This is in large part due to the slab—water runs under the shed, the wood soaks it up etc. From what I can tell unless I jack the shed up on bricks or something there is no way around the issue.

    My question is—is a drain and sealed floor necessary if water is going to come and go under the sill plates regardless? I’ve thought about grading the floor towards a corner and drilling a hole through to the gravel pad. My worry is if I seal the floor water will get trapped under the sill plate anyway. I’ve read that caulking/sealing along the sill plate is worth a try but I figure water will still end up getting through. Would love to hear your thoughts!

  10. Here’s a crazy thought. Maybe jack up the building and replace the sill plate/bottom plate with a double green 2x with gaps along the perimeter. Fill the gaps with good wire to keep rodents out, and you’ve created a breathable hot room. “That which blocks our path creates a new path?”

    This may not be your answer, Andy, but it may help bring you to the answer.

  11. That is a great idea and similar to one of the solutions I was thinking of. Perhaps I am confused—is a ‘green’ a #2 PRIME PRESSURE TREATED WEATHERSHIELD per the item list? Thank you for feedback!

  12. Garth: Yes, exactly. Seal everything on the floor up to the drip edge. This has worked great for me. The floor of my cabin sauna has held up for 24 years now and counting (and with heavy use) as well as my backyard sauna, 17 yrs now and also counting, with also heavy use.

  13. I have been working on my sauna for the last two years. It’s in the corner of our 32′ x 40′ steel shed just outside Pella, WI. Used white cedar from blow-downs harvested in our cedar swamp. 880 board feet sawn for just $80 locally; then milled/ T&G for $237. What a bargain! I’ve used Glenn’s E-book all through the process. I used stringers on my sauna floor in a pattern starting with 3/4″ against the wall down to 1/16″ at the drain. Nailed and glued. Then dura rock with vinyl cement patch. After that dried, I bought a Rust-Oleum epoxy product at Menards ( a little pricey, but not too bad) with copper-metallic color and anti-slip additives. It is spectacular! And…….even more waterproof than just vinyl cement patch. Not sure how or if I can post pictures with this comment. If someone knows how…..I’d appreciate the help.

  14. Hi Glenn,
    Just finished roofing the sauna “shed” building, and moving inside to the sauna part. Finally starting to get real!

    A question about how much Quikcrete Vinyl Cement I need. In these pandemic times of intense building supply demand, I can’t find a 40 lb bag of that product Quikcrete nearby (there is one about 60 miles away at a hardware store) . There are 10 lb pails of it available locally (or course, much more expensive)
    My sauna hot room floor is about about 47 square feet. I am planning on using your Durock/Quikcrete /sleeper/skim coat method of creating the hot room floor.
    I’m guessing I’ll be moving on to that phase of construction in a week or so, but trying to access my materials now. I’ve got the Durock/wafer screws and 2X for sleepers no problem.

    Can you give me a guesstimate on how much Quikcrete mix I”ll need for this if possible?

  15. Hi Maureen,

    I’ve done this project more than a dozen times, so my guesstimate for you should be close. For a 6×8 hot room, 48 sf., i’d be using about 3/4 of a 40# bag. Vinyl Cement Repair. It may be called something else in your neighborhood, but this is what you want.

    Please take note of how it is detailed in Sauna Build Start to FinnishSauna Build Ebook, which includes some critical instructions including:
    1. Sponge wet your cement board.
    2. Mix well, like an English milkshake. (I like the double bucket back and forth method, as the good stuff will want to settle to the bottom).
    3. Work into the corners, and along your drip edge, and trowel with the cement board as your guide.

    Hope this helps,

  16. Thanks for the quick reply, Glenn! I managed to get two pails of Quikrete Vinyl Cement repair, totalling 40 lbs. Next step is electrical and insulation, but after that’s completed, its on to the hot room floor. In good sauna building tradition, have just finished closing in the shed exterior today, and the fine weather has ended today. Looking forward to having first sauna around…..mid December….hopefully!

  17. Maureen:

    That’s so great! I can totally relate. As I think about it, I’ve undertaken most all of my many sauna builds during this same fall time of year. In the spirit of good sauna building tradition, it’s such a warm feeling to have the building completed, then the weather outside can become frightful, as we focus on the interior.

    I particularly like how after insulation and foil vapor barrier, our saunas can actually be warmed by a light bulb. Precursor of great things to come.

  18. Is there a concern when nailing your T&G cedar that the holes in the vapor barrier behind the boards will allow moisture into the walls?

  19. Brett,

    I know, right?

    Moisture is lazy. It’s like a teenage son getting ready for school. But given the right motivation, it’s out the door fast. What this means is that opening the door from a steamy hot room to a cold dry winter’s night, woosh, moisture movement like crazy.

    Yet tiny pin holes leading straight into wall joists..well, there’s really no way, and nowhere to go. Moisture will only permeate if there’s an easy path. And there’s no path. Moisture doesn’t much get behind our paneling. And the little that does can’t get through tiny nail piercing. If moisture could travel through the nail piercings, it can’t go anywhere as foil is sandwiched tightly to wall joists.

    So, that’s how I sleep at night. Teenage son dashing out the door, but not sneaking around.

    That said, there is the sauna design concept of insulation -> foil -> firing strips (air gap) -> t&g. This method gets away from foil nail piercings. There’s a lot of argument pro and con air gap, more questioning if foil is doing its job without an air gap than nail piercings. And all this is valid and comes at the minor expense of a tad smaller hot room, and how to deal with “cleats” or supports for our benches.

  20. Hi Glenn,

    This seems to be a rather successful method, but I am still wary of using it in the sauna that I’m building because I have a shower and spigot plumbed into my steam room. Is the amount of water this method can handle enough to run a shower onto?

    If I can use it, with some board running up the walls, do I tuck my foil vapor barrier over the top of the backer board to allow for smooth water flow, or do I install the backer over the barrier which runs to the floor.

    I have loved reading your humorous posts as I give life to my first sauna.


  21. Jonah:

    Glad saunatimes is working for you.

    Regarding considering your situation, I think it a good idea to think like water. And think like a contractor building a shower. In this case, contractors install a “pan” and “red guard” the floor. It’s a bigger hammer than the skim coat cement board method, and may be the right step up for your situation, where a shit ton of water is going to flow, with a shower in your hot room.

    Also, generally, foil wants to be behind everything, interior wise, but on top of your studs. Kind of like lettuce to your sandwich. This is the kind of thermal moisture break we’re looking for.

    Good luck on your project, Jonah! We’re with you in steam and spirit.

  22. Hello:
    enjoy the site. What can I use for a to protect sill plate from rot in my indoor sauna placed on concrete. I do not want to use pressure treated as I’m concerned about off-gassing. (Or I’d PT ok?)

    Thank you

  23. Spencer:

    Sill plate for indoor sauna: I’d use pressure treated. The sill plate is buried by foil vapor barrier. And matter of fact, I recommend 2x pressure treated stock for drip edge. I know it’s janky to be thinking this for a sauna hot room, but 1) it’s way down low and doesn’t get warm there, and 2) very little is exposed.

    In my book you’ll see how I recommend treating where wall meets floor, and to me the benefits of pressure treated for this one spot is well worth the very minimal jankiness. And my wife and I are very free range organic types.

    If above doesn’t give you the total confidence, and I respect if you’re still not on the reservation, well, you could use 2×4 cedar as bottom plate.

  24. Hi Spencer,

    Another option, if you can find a mill that has it, is to use black locust. It’s more rot resistant than PT, and it grows prolifically, but not many mills work with it. Still, if you start calling local independent sawmills you may well find someone who has a bit around, and you don’t need much for just the bottom plates.

  25. Wondering if any one has used waterproofing mortar from rapid set to seal their floor? Thanks

  26. Hi Chris:

    No, i wouldn’t vinyl cement skim coat over plywood. It’s not the right medium mix. Cement board and vinyl cement work really well together, like a baguette and a good French brie.

  27. Hi Glenn

    Could you clarify your typical cement to water mixing ratio for the english milkshake? …about 6:1? … or 3:1?

    have yo applied the milkshake with a squigee?

    + any issues with resulting weakness in using thinner mix ?
    (per quikrete’s spec “…to much water (less than 7:1 ratio) will produce a weaker product…”)

    + As Quikrete’ vinyl concrete patcher does not seem to be available in my region, have you used any other suppliers’ products with same success as the vinyl cement? … Sakrete’s Top N’ Bond or Flo-Coat concrete resurfacer or Quikrete Self-Leveling floor fast setting resurfacer? … any recommendations?
    (a bit mystified by the many options! … with or without polymer modifiers?)

    Thanks for your work in supporting DIYselfers!

  28. Patrick:

    1. English Milkshake mix.
    It’s all feel. I’ve become that same guy I used to know: grey hair, garage stereo, whining about the price of things, no measuring devices whatsoever. What I do know is that the vinyl cement mix is thin. And The three bucket method is really the best. As well as you mix your cement, it settles. The way to counteract is to pour from bucket A over to bucket B, then back to A, then maybe B again, and back to A, then pour some onto the cement board.

    2. Squigee.
    Yes, matter of fact. And the squigee does too good of a job (it pulls all the cement away from the cement board). The trowel is the perfect tool. Use that.

    3. Finding vinyl cement.
    Find it. You can do it. It’s the perfect product. All that self leveling jazz is a different animal. As noted above, I’ve become one of those grey hair stubborn mules that locks into something that works really well and doesn’t want to try to fix it.
    This link brought to you by Ace Hardware.. Ace, for all your DIY Sauna Floor Skim Coating needs!

    Patrick: This skim coating business. It really goes fast after you set yourself up for it. Tape the drip edge. Tape your drain. Fill gaps where cement board butts. Wet the cement board (so it’s thirsty for more, not saturated.). Start in the far corner. Trowel fast. Do the B job, not the A job. Some trowel marks help wave the flag of freedom.

    You’re welcome. Happy you are tackling your own sauna build. I enjoy helping fellow DIY’ers! Hashtag: empowering! (it’s all in the book).

  29. Hi Glenn, we are building a portable sauna in Scotland but struggling to find Durarock, do you know of any other alternatives?

    Many thanks

  30. Durarock is a brand name. Like Kleenex. Or in your country: Hoover.

    Cement board is the generic term for it. It is exactly this. Cement in a board form. 3’x5’ but not sure the metric conversion. And there is webbing inside the cement board that holds it all together. Hope this helps!

  31. I just ordered a Kuuma small, and while waiting for it to arrive am building my sauna. It will sit on a pressure treated frame, open to the outside underneath and screened to keep rodents out. I am in cold Vermont, what should I do for a floor system on my 16″ o.c. framing? I am building the frame rugged enough so that if I wanted to move the sauna, I could drag the whole thing onto a trailer and relocate it. Should I use a vapor barrier on the floor?

  32. A system I like is 2″ rigid foam, ripped on table saw at 14 1/2″ so as to set between floor joist framing. You can run some 2×2’s 2″ down inside the floor joists so that the rigid foam sets ‘flush up’ to your plywood.

    This is a great system for creating an insulative barrier for your sauna building.

    It doesn’t cost too much, and we pay for it one time. Then with a good ventilation system that draws fresh air into your hot room, your changing room floor can be that much warmer for hanging out between rounds on cold Vermont evenings.

    I suppose you could use vapor barrier on the floor.. I’ve never thought about that. hmmm..

  33. Hello

    My Sauna is being built on a concrete slab base.
    I am wondering if I can simply place bathroom type tiles onto the concrete?
    Would there be a need for insulation the floor with rockwool etc. Do you think this would make a massive difference? Obvioulsy, adding insulation under means there is a need to have a totally waterproof solution that would be placed on top of that. I don’t have the budget right now for a complicated floor solution.

    Any thoughts?

  34. Glen,

    Got my shed built and dried in. Next the floor. Planning on using the vinyl cement method from your book.

    Do you first use thinset under the cement board (on the plywood subfloor), on cement board seams, etc? Or do you just screw it down and coat it with the vinyl cement?

    And do you first put a layer of vinyl cement under the paver block stove platform? Or do you just glue them in place onto the plywood and then coat the top?


  35. Hi Glen, essentially the same question as Erik above; I will not be using sleepers as my floor is already slightly sloped towards a drain. therefore, the cement boards will be screwed directly on the plywood. Do I need to use thinset under the cement board (on the plywood subfloor) ? I’m not tiling over and just need a splashproof surface so not sure if it’s really worth it. Let me know what you think. Thanks.

  36. Hello Glenn,
    I just purchased your book and looking forward to checking it out.
    We are about to start construction on our second wood fired sauna. Step one was to purchase a Kumma stove.
    It will be here in about 6-7 weeks. We live in Maine so the timing is good.
    This sauna will be different in that we are building it into a hill and will be covered with earth. All concrete. Think root cellar with a chimney. The sauna room will be 9×9 and the ante room 6×9. A stick framed wall between. The front door area will be stone facade with a pergola over. All white cedar inside.
    It will have a concrete floor w drain. 1. I’d like your view on insulation 2. Cedar grating sections for floor.
    Thanks. I’m grateful for your insight. James Buchanan

  37. Hi James:

    I get a handle on your project. When you say root cellar with a chimney, it is clear to me. We want to be careful here. Stone is an excellent conductor of heat, which sounds nice, but it is a buzz kill when considering about how it will be sucking infinite amounts of heat from the Kuuma into the walls, where it will be lost forever into the earth.

    If it’s not too late, I would be thinking of applying one of two major construction design elements to this project:
    1. 2″ rigid outside the concrete and under the concrete floor. or
    2. Firing out your concrete with 2×2’s, filling joist cavities with rigid insulation, then polyiso over the entire area, then firing strips, then paneling.

    With either #1 or #2 what is critical is a thermal envelope separating the stone from the hot room.

    #1 is less desirable as all that stone has to get hot before the sauna will get hot.
    #2 sounds like a drag, but this is how we do basement saunas with an exterior wall.

    Think of a cold Maine winter’s day and how this stone will want to stay as cold as the earth around it. Even with a Kuuma, you are dealing with a little engine that can’t.

    We have to insulate the stone.

    This sounds like a wonderful project with great outcome, but please talk to others about this before advancing.

    Talk with cement contractors who lay down in floor radiant heat. This is the avenue you’ll be going down. Isolating the concrete.

    Hope this helps James!

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