Looking to pitch your sauna floor and install a drain? Consider the Trevor Trowel Fill Method

52 thoughts on “Looking to pitch your sauna floor and install a drain? Consider the Trevor Trowel Fill Method”

1. ok Glenn, i JUST left a comment on a different blag about DO I REALLY NEED A FLOOR drain? well, i think the time has come that i CHOOSE to trust your experience and wisdom with this stuff. my question is this, is the dura rock and vinyl cement repair flexible enough that i wont crack when the trailer flexes during travel? Should i consider using a epdm or rubber shower pan material over the dura rock instead of the vinyl cement repair method? if you say that the vinyl cement repair skim coat is the best way to go, can it be dyed black so it wont be too noticeable under my duckboards?

2. the above comment should also include: Im building a mobile sauna so weight is a consideration, the ‘sleepers with fill’ method would get pretty hefty. whats a comparable method that isnt too heavy?

3. Hi Barrett: For mobile, we have a very clear, optimal solution for hot room floor. No need to pitch our floor to the drain. Instead, we do this: we red guard our wood subfloor OR durarock/vinyl cement skim coat over our subfloor and punch in two drains: one in the back corner driver’s side and the other in the far corner passenger side.

   Ever so slightly, we pitch our trailer so water drains to the back, and Murphy’s Law works in our favor as the trailer will pitch left or right 98% of the time, and it need not be much.

   Mobile Non Level Activation works in our favor. 🙂

4. I like the durarock / vinyl cement repair team. I’m just a nut that way. It’s worked great for me for decades. In mobile, I’ve become quite open to the rubber floor mat solution (for weight and twisting and turning). So, you’ll do the right thing, i know it. Send me a pic, and let’s see how it rolls for you, Barrett!

5. I am currently under construction on a basement sauna. I have plumbed in the drain for shower in the changing/relax room and a drain in the hot room floors. I am looking to slope the floor in the hot room as the floor is level. would you recommend the above example for sloping an already existing level cement floor? (I already purchased your book and it really only covered sloping wood floors in outdoor saunas) . How have you sloped an already level, cement floor in the past? Thanks

6. David: Yes, for sure. I have dealt with an existing cement floor, to create a slope for drain. Before advancing in any direction, I’d contact a cement expert, someone into more of the nuances than the typical square peg, square hole cement contractor. Because an expert may suggest “scoring” your exiting cement slab, to make it rough, and able to accept an overcoat or top coat that you’ll pour and trowel atop existing slab, to create the slope.

   And another expert may say “don’t bother, you’ll get dust everywhere” and will point you in the direction of a good cement patch product and say “them are good for sticking to existing concrete.”

   First thing i’d check is: is your existing basement cement floor really level? I’m betting that it’s not. There may be an existing slope away from your exterior wall, which may work in your favor. Not seen to the untrained eye, but very trained for water which always seeks its lowest level.

   Second thing to keep in mind is that that if you do need to slope your floor to drain, your run is very short, and so you’ll need very little build up along your walls. So, if this were my build and I needed to create a slope to drain, i’d:
   1. be using 2×4 green bottom plate for my hot room walls.
   2. run a bead of silicone underneath my bottom plate before cement screwing it to existing cement slab.
   3. screw in a drip edge (as detailed in my ebook) with a bead of silicone underneath.
   4. run another bead of silicone along inside edge of drip edge, where it meets the existing slab.
   4. mix up some vinyl cement or recommended cement patch product and trowel it nicely along the perimeter of my hot room, with gentle slope to my new drain.
   5. rinse off my tools and crack a beer.

7. Glenn-

   The floor is surprisingly level. I put a 4′ level to it and dumped a little water on it. it doesn’t run, just sits there.

   You mentioned you have applied a slope to an existing cement slab in the past. How did you go about this and were you satisfied with the results? Thanks!
   -Dave

8. David: Yes, i’ve created a pitch to an existing flat slab a couple times. I’m pretty sure I used vinyl cement repair. 40# bag, which was more than plenty. I troweled it over the slab and up against the drip edge around the perimeter. I’m trying to remember if I did anything fancy to the existing slab, like acid wash the paint or score it in some way to accept the vinyl cement. My take is to ask a cement guy what he’d use, so that the new pour adheres to the existing slab.

   It’s easy work, i mean, it should only take you about 20 mins. once you figure out the best product to use and the prep involved.

   And you’ll most likely have a cedar duck board atop your hot room floor, so any trowel marks, etc. will be hidden.

9. Thanks Glenn. I think I will do as you recommend and just use the vinyl cement. Will definitely use a bonding agent between the 2 pieces of cement. The floor is larger side (8×7) so I will probably put down some pressure treated kneeling strips to keep me in the proper slope.
   Thanks again.

10. I am building an outdoor sauna and it has a perimeter foundation but is not built up from the ground. I am not going to be able to put a drain in. I am wondering how to deal with the ground. My plan was a couple inches of gravel and then the cedar flooring on that. I am wondering if I should put vapor barrier over the ground first, or let it be able to drain any moisture that might go though the flooring. I do not anticipate any of the scenarios that are outlined above, i.e. bathing in sauna, water fights, hosing inside of sauna.
    Any thoughts?
    Thanks

11. I’m planning on building a backyard sauna, and I’m curious where people are taking the drain plumbing. Bringing the waste line back to the house seems like an awful amount of digging (Like 200ft in my case) for a line that should rarely carry any considerable amount of water. Especially if I have to dig below the frost line! Here, in northern Vermont temperatures dip into negative territory for 3 months of the year. I also picked a location that’s fairly far away from the house.

    I’m planning on putting in a 6″ deep 14 ‘x 14 gravel slab, and building a 12’x12’ cottage. I was wondering if I should dig a dry well a few feet from the structure, or just utilize a french drain. I thought about making a complete off-grid shed with solar electric/underground rainwater storage. The freeze proof rain water collection pump system, just seems too complicated when I can easily just bring water from the house. Probably will still go for a small solar system. Being far from utilities is really becoming a royal pain.

12. French drains work great. And we are bathing in our hot rooms with free range organic soap, if any at all, so i’m very comfy with French drain solution, and have this in my saunas and consider no harm, no foul to this approach.

    Also, with the clean rinse outside, and outdoor showers and cold plunges, even in Winter, very little water ends up down the drain.

13. Hi yes, I have a couple thoughts.

    I don’t use much water in the hot room, as “rinse offs are strictly an outside affair” says a fellow sauna enthusiast. That said, I appreciate having a drain for when I do a thorough cleaning/hose down couple times a year. And those times my wife joins or others, it’s a good feeling to allow others to dump water over their head and know the water gets out of there easily.

    So, in your case, i’d consider a post hole digger and just getting down a few feet and back filling with white 2″ or so PVC, and sticking a drain in there. Also, I happen to really like my method of skim coating sloped durarock. All this messing around may take a day, but you’ll benefit for the rest of your sauna life.

    This is just me, and I hope this help, g.

14. Hi Glenn, thanks for this, I’d been researching for a long time trying to work out the best way before I came across this!

    I am wondering how thick the sleepers and cement should be with the spokes method? I was thinking of having it vary from 10mm where the drain is to 20mm at the highest point… but am not sure if 10mm cement is too thin and likely to crack and move? and if that amount of fall will be enough to ensure the cement sheeting all sits flat at the desired pitch and ensures the water drains nicely?

    context : I am turning a 10ft shipping container into a sauna – the internal dimensions of the part I heat will be 2100mm by 2200mm and the lengths of the sleeper “spokes” will range from 750mm to 1700mm.

    I am also thinking to paint a fairly thin layer of epoxy over the skim coat as the final layer to ensure i get a finish that has the perfect grip, texture and non porosity. any guesses whether this will work well?

    Thanks again, any advice or thoughts would be much appreciated,
    Carl

15. you don’t need much pitch, Carl. I’d run sleepers from 3/4″ at perimeter down to the drain. search “ben square method” as this may be better for you with shipping container floor. I’ve done two shipping container projects, and it’s awesome. you can search “shipping container” on saunatimes and see a couple possible inspirations.

    vinyl cement and cracking: I have done this method in a few mobile saunas and no cracking. But here’s an entire left turn: Just put two drains in each corner of your shipping container hot room. Pitch the container such that water will run off into these corners. This is what i’m doing now in mobile saunas i’m building and it’s slick method. Water wants to run and we can encourage it, in the mobile domain.

    Epoxy: never done this, but if you do it, please email me pics and lemme know how it worked for you. I happen to like to come off the skim coated floor with cedar decking or the “3rd bench” so we’re walking on wood. Just me, and open to what you’ve got in mind.

16. Question: If I’m pitching my floor towards a drain using the sleeper/Durock method, will the Durock bend enough to “mold” itself into the shape I need? Or will I have to cut it into a dozen pie-slice pieces to fit?

    Also, is the vinyl cement repair kit sufficient to seal the gaps where each piece of Durock meets its neighbor?

    Thanks! Great book by the way.

17. Hi Sean:

    Glad the book is helping you.

    re: your floor: it’s best to not try to bend the Durock.

    You’ll want to cut a bunch of sleepers to slope to the drain – long thin triangle like – and you can backfill with the Trevor Trowel method, if you like that. Also, there is a more orthodox sect of sauna builders, reported to be of German descent, who are having success with the Ben Square method, as a guy can set his table saw to run very precise and accurate thinner and thinner sleepers and lay them down on the subfloor this way:
    https://www.saunatimes.com/building-a-sauna/slope-your-hot-room-floor-with-the-ben-square-sleeper-method-and-enjoy-a-water-fight-in-your-sauna-hot-room/

    And no matter which way we go, the vinyl cement skim coat is really a great system. When you’re done, I am sure you’ll agree that the products (vinyl cement and Durock) work well together, in two part harmony, kind of like that David Bowie and Bing Crosby Christmas tune… two that you think wouldn’t necessarily jive well together but end up doing so, and very well at that.

    Ebook bonus track here:
    https://www.saunatimes.com/building-a-sauna/is-there-a-better-way-to-seal-our-sauna-floor/

18. Can you tell me what drain to use in a concrete sauna floor that we are hoping to seal with epoxy

19. So, drains in concrete floor. I go to big box and purchase the standard shower drain that accepts, I believe 2″ PVC. It may be 3″ PVC, I can’t quite remember. This drain has a removable metal mesh, with 2 screws. I like to blue tape the metal mesh as I set the drain, flush with the cement, then run a wet sponge around to clear off most of the cement around the drain. Then when everything is dry, peel off the tape.

20. What do you recommend using to cover a plywood floor. We used the sleeper method for the drain and covered it with plywood. Would an epoxy coating be our best option?

21. Staci:

    Durarock (cement board) is the answer. It’s in my ebook, and please search “durarock” or “vinyl cement” for more.

22. New to sauna building here, my husband is sort of going at full speed right now to get ours done by winter. We like the idea of a slate floor – flooring with texture. We prefer not to have wood to deal with the mold and mildew and slimyness. Any suggestions on slate? Is ceramic better? Is any tile just a horrible idea? Having a hard time finding info so any advice or where I should look to learn more would be super helpful. Thanks from Northern WI!

23. Hi Vicky:

    Slate is great and I did it in my changing room in our S. Mpls. backyard sauna, but here’s the thing: it’s cold. And it gets colder as the winter gets colder and when feet are cold it’s a buzz kill to good sauna, as rounds get short because feet are cold.

    And here’s another thing. I brought down some really awesome slate rock from our island cabin and set them as a really awesome slate patio and it was a really awesome idea until it got below freezing. Then it became an iceberg, and I built a cedar deck over the slate rock.

    So, all i’m saying here is if you’re going with a slate floor, and this is a new build, I’d be isolating the slab or surround with 2″ rigid insulation, and have a way to try to keep the slate floor from being an iceberg. It is possible to add a cedar deck on top of your slate floor, which I ended up doing after one too many icy feet between rounds.

    As far as hot room goes, i’m a huge fan of the “3rd bench” or “raised floor”. 43-44″ ceiling to top bench (not 42″ as noted on the graph here). https://www.saunatimes.com/building-a-sauna/lets-talk-about-sauna-ceiling-height-bench-height-venting-along-sauna-door-down-to-the-inch-and-centimeter/

24. I’m building a 7′ x 5′ outdoor sauna in southern California where temperature never gets below 40 Fahrenheit. The floor will rest on cement blocks about 4″ off the ground. Instead of sloping the floor to a drain, I’m thinking about using 2×2 cedar with 1/16″ gaps and one or two 2×4 beneath for support. The entire floor will be vented/drained to the gravel/dirt below. I intend to have an electric heater and one small vent near the top of a wall that can be opened or closed. Heat rises, the top vent will usually be closed, and it never gets really cold outside. Do you think there is anything wrong with this? Will I regret something, or is it no problem?

    Thanks for the advice

25. Brad… Nothing wrong with this. Unless you’ve got some real estate constraints, if you’re thinking of going 7’x5′, you may think about going 7’x6′ instead.

    And this is one of those rare instances where I’m thinking you may want to nudge up your square footage a tad. Typically, i’m more in a position to encourage a smaller hot room, because people are thinking dimensions like 8’x10′ etc.

    And unless you’re thinking a bare bones electric heater, in your case the 7’x6′ may work better as you will have the footprint to raise up the floor 6″ and allow for this step up, which will give you better heat with 3 levels of living.. 44″ ceiling to top bench. 16-18″ top bench to lower bench, 16-18″ lower bench to raised floor. 6″ raised floor to grade, where stove sits.

    3 level living (large).

26. Glenn,
    How do we solve the elevation problem between hot room and the changing room referring to the floor highs. Duckboard floor added an 1 inch approximately above the finished floor for the deck and changing room. We do not want to raise the existing floor height of the deck/changing room

27. The elevation between hot room and changing room isn’t really a problem, but an opportunity to get our feet off the cold floor.

    I’ve been adapting from the duckboard concept to what I call the “3rd bench”: a good 3-5 inch step up from the changing room, to a raised floor/duckboard system, which allows us to get up, up, and away, closer to the ceiling (two fists above one’s head) and feet closer to the level of the rocks on the sauna stove.

    In Finland, this is called “the Law of Loyly.” While sitting on the top bench, if one’s feet are below the level of the sauna rocks, it calls for 30 lashes from the vihta wisk, and a major scorn.

    Here in US, we have to balance the “climbing up the high school bleacher effect” with the “Law of Loyly” such that we want good flow and good heat and make things happen the best we can. All this is a future post, now that you got me going, and hope this helps for your design.

28. I’m wondering the same thing as Sean was, which is how to install the Durock over the sleepers. I’m confused, as the book shows square pieces of Durock, whereas you say it won’t bend. If you can’t bend it, how are you installing it without at least 4 compound angled triangular pieces, if not more?

29. Lucas, you can search “durarock” on saunatimes, and see how we apply it to floors. The Ben Square Method, bicycle spoke, and i’ve uploaded video tutorials on skim coating… triple bucket rotational.

30. I’ve pretty exhaustively looked through the posts that have floor info, they all seem to have photos of square pieces of Durarock, which doesn’t make sense with the no bend philosophy.

31. correct, the durarock doesn’t bend. It lays flat on hot room floor that is sloped from decreasing widths of sleepers.

32. Glenn,

    I’m moving an existing basement sauna to a new location in the same basement. Sauna floor will be concrete (under duckboard) with no floor drain. How much consideration to I have to give to water on the floor. The existing floor is not particularly level which means any water will end up in one corner. I am inclined to level the floor with underlayment self leveling concrete with some type of rubber or membrane fishing. Seems like overkill since the old sauna was just sitting on the old cracked concrete floor.

    Are there any moisture concerns on the floor that is not poured on or spilled by the sauna user. We don’t really any water except to pour on the rocks.

    Thanks

33. The one thought I have is to consider a drain in that low corner. It’s a big project and dusty and involves saws and pipes and cement, but you’d do this one time and benefit forever.

34. Hey glen, what’s the drain I use on a small mobile build using the sleeper on ply, trowel cement set up? Is it small drain just exiting out the bottom into a pipe or something that drips to the street? Doesn’t it create an updraft of cold air into the hot room? I understand the theory of the floor pitch drawing moisture out, im just missing a piece of how it all comes together

35. Hi Jack:

    Drain system for a mobile sauna. This is easy.

    I suggest two drains, one in each corner at the bow or stern of your hot room. As you set your sauna for deployment, a slight pitch to bow or stern will be completely unnoticeable to the sauna bather, but water on your floor will notice for sure, and be drawn to one corner or the other and down the drain it’ll go. No pipe or anything fancy, just a couple Round Snap-In Stainless Steel Drain Covers.

    What is a Round Snap-In Stainless Steel Drain Cover?
    Home Depot sells it: “4-1/4 in. Round Snap-In Stainless Steel Drain Cover.” Buy two of them and:

    1. Locate and avoid your trailer supports / floor joists.
    2. Mark location for your two drains.
    3. With appropriate size hole saw (3″ or 4″ I forgot), cut out floor. Your cut out will be about 1/4″ less diameter than your drain.
    4. Set drain.
    5. Trace the circle of your drain onto your vinyl flooring.
    6. With a utility knife, cut out the vinyl floor around where your drain will sit.
    7. Run a bead of silicone around the exposed ends of your subfloor (to seal the ends of your plywood) and around where drain will sit.
    8 Set drain cover in place.
    9. Rest an unopened 16 oz. IPA or Session Ale on top of drain overnight, until silicone dries (don’t use a shitty beer, your sauna will know).

    Above will take you 20 mins. start to Finnish (not including the trip to big box, or the 10 Wim Hof breaths necessary to get psyched up to navigate through the big box).

    BONUS: the depth of your vinyl flooring atop your 3/4″ marine grade trailer plywood is exactly the depth of the Round Snap-In Stainless Steel Drain Cover.

    It’s as if they made this product for us.

    And the above instructions, I hope are easy for you to follow.

    Oh, and as far as updraft of cold air into your hot room: The secondary benefit of the two drain mobile sauna system is exactly this.

    As you know, ventilation is critical to good sauna. The air flow created by the two drain system seems to be working perfect in the mobile saunas that I’ve had my hand in. These two drains double as air input for good ventilation. These two drains, err..intake vents, coupled with two vents up high, controllable by vent cover/chutes are doing an amazing job of oxygenizing.

36. Hello, I’m considering a concrete tile floor. The layers would be plywood, hardie board, thin set and then tile. Am I missing anything? Do I need a waterproof plastic barrier anywhere in this equation?

    Also, I noticed in the picture that you put osb in the inside of the sauna. I haven’t seen that yet. I would’ve thought insulation and then vapor barrier.

37. Great drain ideas, exactly what I was looking for. I have a question about the OSB walls. What’s the advantage of using OSB or perhaps plywood before the paneling? Does it help with expansion/contraction, or does it provide a more secure surface to attach the paneling to?

38. concerned that water will seep in on sides of floor drain with the type of drain and glued in installation shown in ebook pictures.

    have you used different kinds of shower drains or installation such as the Schluter type of drain system?

    https://www.schluter.ca/schluter-ca/en_CA/Shower-System/Drains/Schluter%C2%AE-KERDI-DRAIN-A/p/KERDI_DRAIN-AR

    or is this overkill?

    or would any floor drains with 1/4″ height (to be flush with 1/4″ cement board) work for such a sauna floor application?
    how to adjust floor drain installation with added tile installation (to insure floor drain will be flush with higher finished floor)?

    Thanks

39. Patrick:

    Water seeping in on sides of floor drain:
    Fair concern. I run silicone around the ends of the exposed plywood before setting the drain. And I seal it really well.

    Different drain systems:
    I’ve had best luck with “less is more.” Just the simple shower drain cap, set and sealed flush onto subfloor, then trowel around it.

40. WHEN FILLING IN BETWEEN SLEEPERS IS IT BEST TO USE MORTAR TYPE N, PORTLAND CEMENT ETC. ? SHOULD A BONDING AGENT BE USED ON 3/4 PLYWOOD PRIOR TO APPLICATION? AFTER FILLING IN BETWEEN SLEEPERS, I PLAN ON USING A LAYER OF THINSET APPLIED WITH A 1/4 ” TROWEL THEN SCREW DOWN THE DUROCK TO THE SLEEPERS.
    THANKS,
    JOHN

41. Hi Glenn,

    Just a quick question here as I’m about to embark on getting my pitched floor done. If using the Trevor Trowel method, which I plan to do to create a solid floor, is there still a need for the Durarock or could you apply the vinyl cement repair or something like an epoxy coating right to the troweled concrete with sleeper guides?

    Let me know whenever you’re able! Thanks.

    Sam

42. Hi Sam:

    I recommend the Durarock base over your sleepers. This creates one medium upon which you can skim coat your vinyl. And rough side out, as detailed in my e-book, as this way you can run it thin (like an English milkshake) and uniform (like a German security guard) which being one medium will avoid cracking (like a politician caught with hand in cookie jar).

43. Sounds good, thanks Glenn!

44. Hello Glenn,

    I bought your book a couple months ago and have been reading your blog while I build my backyard sauna. I have an engineering background but it is all in earthwork. I know enough about structures to get myself into trouble. I intended to use your sleeper method to form a drain pan to make things lighter than a conventional drain pan, but was somewhat hesitant to put any more weight than necessary on my floor joists. One day it occurred to me that I could slope my floor with rigid insulation and save weight while keeping things even warmer. I live in Alaska and use 60psi extruded insulation pretty regularly under roads and parking lots for various reasons. The stuff is designed to hold the weight of asphalt or concrete with cars and trucks on it, and can be submerged in water while doing it. It weighs nearly nothing. I called our local Insulfoam manufacturer and asked them if it was possible to form a drain pan if I gave them a drawing. The representative said he had never tried it, but construction is slow this time of year here so he’d give it a shot. They called me 2 days later and had everything ready. I picked up an 8’x8′ drain pan that came in 4 square pieces that I cut to fit onto my 7.5’x7.5′ floor and around the stones that the stove will sit on. I laid 1/2″ cement board on top of it which is flexible enough that when I screwed it to the floor through the insulation it formed right to the surface with perfect 1/4″ to 1′ slopes all around, draining right to the center of the room. I’m not too worried about the flammability of the insulation because it’s rated for 160 F and is covered with cement board anyway. I don’t think there will be any problem with cracking because everything seems very rigid. I intend to skim coat or grout/tile in the spring though because grout needs temperatures a little warmer than we have to cure. If you want send me an email I can send some pics.

    Dan

45. Hi Dan:

    Love this and thanks for chiming in! What a great solution. Sending you an email, and love to see your custom insulated floor/drain pan.

    This reminds me of a few things, specifically, (and stay with me) the Die-Hard battery commercial from the 1970s. The commercial showed a couple cars sitting overnight on a frozen lake in International Falls, MN. And how the batteries stayed strong enough to fire up the cars after sitting for nights in sub zero temps.

    What i’m getting at is how in places like Alaska and here in MN, where we know cold, we engineer (!!) build it once solutions for our temperature extremes.

    Designing really good sauna for cold environments brings us really good sauna principles for anywhere.

    You could live in Kansas and have a car with a Die-Hard battery, and it may never get to -20°f., but in the extremes is where we learn how to do it best.

46. If you ever had to take the floor up off the subfloor. How would you get the durarock off if the screws are coated in skim coat?

47. All, about to have my shed built for my first sauna. The flooring options I’m getting from the shed builder do not include plywood. It’s LP smartfloor or 1” TG pine with floor joists closer together than 16”. LP choice is much cheaper.

    Should I go with the LP, then cover with Durock and skim coat, then duckboard placed on the walking areas?
    Thanks!
    Jim in MI

48. Hi Jim:

    Yes, i’d go with the LP, then Durock skim coat. This system has worked great for me over decades. Where moisture can sit and become a problem, I like to come after it with a material that is water tight and long lasting.

    Yes, duckboard atop is a great thing.

49. I am wanting to build a outside sauna in an existing bbq shed. The building already has a nice cement floor and walls but does not have a drain in the floor. I am a huge wet sauna lover(think dumping and splashing pitchers of water). Is it worth it to add and go through the trouble of putting in a elevated floor with drain or will it dry enough?

50. Maybe what could be considered, Jenn, is to cut out a 12″ square from your slab and post hole dig a French drain. Noodle on that idea, and type more on here if you like.

51. Hi Glenn, first of all… I’m loving your eBook – thank you!
    Apologies for a rudimentary question… I’m struggling on the *best* material to fill in between the sleepers (prior to running construction adhesive and screwing down my Durock boards). I’ve seen cement, mortar, and Great Stuff mentioned in different comments. What’s the best?!?

    Should a bonding agent be used on my Kilz primed plywood floor, prior to filling with whatever you recommend as “best”?

    How do you feel about a layer of Thinset allied on top of the filler before securing the Durock?

    Apologies for all the questions… really want to get it right as there is no going back after I’ve filled between the sleepers with some of the products mentioned!

52. Hi Wayne,

    Glad my ebook is helping you.

    I can tell you’re after the “A job” so, as you’re going for the Trevor Trowel method to fill the gaps between sleepers between subloor and cement board, in my experience, most any material will do! The trick is to not walk on the cement board when wet as the material will ooze.

    Bonding agent: great idea, can’t hurt.. i’ve never used it.

    Thinset applied on top of the filler: again, this probably wouldn’t hurt.

    Questions are good, and glad you are asking on here, in comments section. (vs. inbox email yikes). Glad you’re rocking along, g.