Cement over the sauna hot room floor

As you recall, I had made my sauna floor waterproof by putting down a shower pan liner—a thick waterproof membrane on the sauna floor along with a drain. I planned to leave it exposed and just put duckboard over it. However, the city inspector would not allow it and told me that I need a non-combustible floor. Although it was more work, the inspector was right.

First, every time we fired up the sauna—you could see the membrane on the floor become more malleable and less rigid.

Second, no matter how carefully we used the sauna—I noticed ash marks on the membrane that could have burned through.

All three concerns (inspector included) would be solved by cementing over the entire floor of the hot room. There could also be a potential benefit (haven’t tested yet,) that the cement floor would radiate the heat better than a PVC liner. Now, one concern is the added weight. For my hot room of about 6’x12’, I ended up using 6 bags of the Sand Topping Mix cement from Quickrete.

Sand Topping Mix cement used for sauna hot room floor.

Each bag is 60lbs alone. Plus, you add water to activate it. So, from my quick calculations, 6 x 60lbs = 360lbs + water = (Roughly) 400lbs extra weight to the sauna. That’s like 2 guys. However, I think I should be OK because:

One: the cement is spread out over the entire floor—not concentrated in one area

Two: when building the sauna, I oversized things (smart, eh?) 6” x 6” instead of 4” x 4” posts, double 10” beams instead of 8” and so on.

Anyway, then came the hard part of actually doing the work. To begin, we got everything we could out of the hot room—the less obstacles the better.  We cleaned the existing floor as well as we could for the best outcome for the cement. I made sure that all the holes or openings were well sealed on the floor membrane. This was the last chance to truly make the sauna floor waterproof. I glued down the liner around the drain hole to make sure it was sealed. I also glued the actual drain to the liner as well. Things get dirty when working with cement, so I made sure and put down a tarp over the changing room floor so nothing got ruined. Finally, I gathered my tools so I was ready for work. I used 5 tools:

A mixing tub (about $6)

A shovel

A 16” x 4” Cement Trowel (about $14)

A 6” x 4” Cement Trowel (about $8)

A 2” x 5” Cement Trowel (about $4)

Tools used to form and shape the sauna hot room floor.

The work started by putting one cement bag, wrapper and all, in the tub. I then used the shovel to cut the bag open in the middle, then flip it over to empty it out in to the tub. I then poured water in to a bucket and added it to the cement in the tub in batches, mixed, and added more water as needed. The proportions of how much water to add are on the bag. Those are the best guidelines for you. However, I am too busy to read instructions ;) besides, I’ve used cement before. I just mixed it to a state slightly more watery than wet sand at the beach. If you get it too watery, add cement. If you overdid the cement, add more water. We used the shovel to mix the cement with water. There was no water left in the tub, the cement was pretty malleable and easy to spread yet it didn’t run down like water. Anyway—read the manual :)

Mixing cement with a shovel in a cement tub.

Now, cement is not like glue—so you have some time to work with it, but you still need to be aware that as it dries, it’s harder to work with (you got about 30-40 minutes). So, I started putting it down in the hard to reach areas, such as behind the sauna stove.

First batch of cement around and behind the sauna stove.

With the Sand Topping Mix cement, there’s no aggregate (pebbles) just sand and mortar. Technically, it can be spread as thin as ¼” and still do its job. I was aiming for about a ½” thickness throughout the floor. Now remember, we have already previously given the floor a slope towards the drain from all angles of the sauna hot room. Our goal with putting down the cement is to preserve that slope. This can easily be checked with a long level after you have finished putting down a section of the cement on the floor. The cement mix is pretty thick and should hold the level tool just fine.

When forming the cement on the floor, use the longer trowel as much as you can to give the cement a nice uniform smooth look. If the big trowel doesn’t fit, switch to the smaller one. Only use the smallest trowel for detailed work and not to smooth over an area. Remember, you can always add or remove cement from an area and smooth it over again. Do that until you’ve achieved the look you want. Finally, once you confirmed that the slope still exists, and the cement is smooth and looking good, you can move on to the next hard to reach area.

Next hard to reach area of the sauna hot room floor.

Farthest section of the sauna hot room floor is done with cement.

Final section of the sauna hot room floor being cemented.

Last piece of the sauna hot room floor being cemented.

Once you have all the hard to reach areas completed, go ahead and finish off the last part. Remember to feather the cement down towards the drain right around the drain—use the small trowel. Once done, clean up and let the cement cure. Spray the tools, the tub, and the area where you mixed the cement with water to keep everything clean for next time. Next day, you can walk on your new cement hot room floor to admire your work, but give it about a week to let it fully cure before really using it.

 

Cemented sauna hot room floor 24 hours later.

First sauna firing after the cement floor was completed.

Well, I am more than pleased with how the floor turned out and what it does for the hot room. I think others agree with me as well. Not only does it look good, it's functional. We can now stack wood near the stove while we take a sauna and not worry about it ripping the waterproof liner. The cement was cool to walk on barefoot even though we had the sauna up to about 190F. The light color of the cement actually seems to make the room brighter. Overall, I am very happy with the results. We will soon see if the inspector agrees with me ;)

Making of a waterproof sauna floor.

Now that we had both the hot room and the changing room all insulated and cozy, it was time to start getting the hot room floor ready. The installation instructions for the Kuuma wood burning stove call for the ceiling to be protected with a non-combustible material having a 2 inch air gap. So, I bought ½ inch Durock solid cement board for that purpose. The plan was to have 2 sheets of this stuff sandwiched together with a 2 inch space separating them. We first took down measurements from the back wall to the chimney support box. We then transferred those measurements on to our Durock and marked the support box opening with a marker. For making the opening, the best tool I had was an angle grinder. Actually worked out really well.

Using an angle grinder to cut the Durock

  

We then transferred the mirror image of the cutout on to the 2^nd sheet. Make sure you have the unmarked side of the Durock board as the visible side when you hang it up on the ceiling. Now that we had two pieces all ready and cut, we needed to hang them. The first sheet goes up with just 2.5 inch non-rusting screws. Then, to make a 2 inch air space, we used scrap pieces of the Durock as spacers. 4 ½ inch pieces make up 2 inches of spacing. For those, we used 4 inch screws to attach the spacers to the joists in the ceiling. The final piece of Durock will then go over the spacers with 4 inch screws.

Durock over chimney support with spacers 1

Durock over chimney support with spacers 2

Next step was to build a non-combustible pad for our stove to stand on. The big box store has 12”x12” patio pavers that sit about 2” high. I figured 6 of those would be perfect for this project. At under $2 each—this was a great deal. A few things go in to laying the pad. You need to be sure the stove sits on it in such a way that there’s a some slack behind the stove, and that when you open the stove door, if any coals fall out, they fall on the pad—not your floor. Also, you need to make sure that the chimney lines up with the stove and the stove is center on the pad from left to right. Once all that was calculated, the pavers then get glued to the floor with a high heat liquid nails—PL400. One mistake I made is I didn’t apply any glue to the edges of the paver—only the bottom. Applying glue to the edges will prevent water from getting in the cracks between the pavers.

Because I intend to use the sauna to the fullest, I needed the floor to have a drain. I found the best location where I could make all the water run to and marked the spot. Next, I needed to give the floor a slope so that any water from any sides would run down to the drain. To do that, I learned from Glenn of SaunaTimes.com that I need to build sleepers. Sleepers are pieces of wood made in to triangles that start out for me at about 1.5 inch thick at the walls and go down to nothing by the drain. Those can be cut using a table saw. To decide how thick your sleepers should be, simply use a level. The point at which the bubble goes out of level is where you sleeper height can start. Make sure there’s not too much space between the sleepers so that the Durock that will be sitting on top of them wont crack when people walk on it. I did about 6-8 inch gaps. Once I laid all the sleepers out and they looked good, I put down some liquid nails on the bottom side of each sleeper and nailed it to the floor with brad nails.

Sleepers create slope towards the drain.

With all the sleepers attached we started putting down the Durock. This stuff comes in sheets of 3x5 feet. I used the ½ “ thick ones. They’re pretty strong yet still bend. Starting in one corner, I laid the first sheet down, pre-drilled the holes through the sleepers and using exterior screws, attached the sheet to the floor through the sleepers. 

Setting the first floor sheet of Durock.

I tested that the sheet can hold my weight without any soft spots. If it was good, I’d do the next sheet. One thing I learned here is they have special screws for Durock. The plus of those screws is they drive in flush with the board. That way no sharp heads stick out that can cut our waterproof rubber membrane that we’ll be putting on top later.

Floor slope completed. View from one side.

Floor slope completed. View from the opposite side.

Once all the Durock has been attached to the floor, you should be able to see with a level, that the floor slopes down toward the drain no matter where you put the level.

With the floor finally complete which took longer than I wanted to, it was now ready to be waterproofed. There are different techniques to do it. You could put RedGuard over the Durock. However, that needs temps of like 50F+ for it to cure for 72 hrs—it’s the middle of winter now. You could skim coat with cement, but that too requires certain temps and getting dirty. The option I chose is a 40 mil rubber membrane used for waterproofing shower pans. It sells at $5 per liner foot at 4 foot high pieces. So 4 feet x N-number of feet you need x $5. It’s more expensive than skim coating, but cleaner and quicker. Now the stuff you buy at the box store is PVC material. The other option is CPE material which is more appropriate for a sauna, but is only available online and is more expensive. If you have the money, that stuff is used for radiant floors so it withstands like 600F heat. At the money I’m saving, I’m willing to try the PVC stuff as I’ve seen some references to it being used in saunas online. For 25 feet I paid around $100.

The way I used this material is the same way I did it when I did my walk-in shower in the house basement. I started by rolling out my 12 foot roll the long way the length of the sauna making sure I had even overhang on both sides. Then, I took it up the wall about 6 inches on one of the long sides. Using my leftover roofing nails, I nailed 1 inch from the top of the membrane along the wall on every stud. So, in essence, I had half the floor covered with the 4 foot piece and the corner where the wall meets the floor was protected as well. Next I did the same thing on the opposite long wall. I left the 2 short walls unattached as well as the seam in the middle. That was done so that I had slack to install the drip edge along the floor perimeter. Stay tuned for more in the next post.

Waterproofing membrane going up the wall 6-8 inches.

Attaching the waterproofing membrane to the wall with roofing nails.

Waterproofing PVC membrane attached only on 2 opposite walls.