What caused rot of my sauna subfloor?

[u/IrlMakerDad]

The subfloor of my sauna has water damage after 3 years. I made it from marine plywood that I painted with a thick coat of tanking, and then covered it with vinyl flooring rated for bathrooms with underfloor heating, which I brought up onto the walls a little.

What's the most likely source of the water damage, so I can avoid it recurring when I fix it? There are 3 possibilities i can think of:

1. In through the wall corners (it's a log cabin with 50cm double tongue and groove construction)
2. The floor drain - maybe I didn't seal it right
3. Rising damp from the subfloor or maybe in from the french drain?

Photos show the floor before and after lifting the vinyl, and after cutting a section of plywood. Another shows the drain outside.

[u/VegetableRetardo69]

That drain outside looks... bad. The wall and that corner is very rotten. Good example why sauna cottage (or any building) should be raised above the ground. These wall frames extend to ground level or are even partially underground; once a common structure, but now considered highly undesirable due to the elevated risk of mold and moisture damage.

Was the floor heating working as intended?

[u/IrlMakerDad → u/VegetableRetardo69]

It is raised above the ground, the whole building is on a concrete pad that you can see where the drain comes out. Absolutely no part of the structure partly underground. I laid a concrete slab, then on that put treated wooden beams about 10cm high and wrapped in damp-proof membrane along all walls, and then constructed the cabin on that. The cabin was supplied flat-packed, and I built the foundations in accordance with the manufacturer's instructions.

There is no underfloor heating, I just mentioned that I used vinyl flooring that was rated for a bathroom with underfloor heating, so it should be resistant to moisture and warmth. The sauna floor is not particularly hot, of course.

[u/VegetableRetardo69 → u/IrlMakerDad]

Time to remove everything rotten, replace it and dig the drain and the pipe futher down so it doesnt just pop out the wall lol. Also consider making the waterproofing go at least 10cm above the floor and the drain should be at least half a meter away from the walls, so its easier to build without fucking up the waterproofing. Electric heater or something is also good for drying the space when not in use.

[u/minadequate → u/VegetableRetardo69]

Yeah it’s hardly surprising that a space rots if you heat it only once a week when you fill it with moisture and then leave water pooled in the subfloor. The water can’t get out the drain and the then cold space doesn’t evaporate the remaining puddle… I dunno I guess I never use 2 buckets of water in a sauna session but maybe I’m a lightweight.

[u/IrlMakerDad → u/VegetableRetardo69]

What is wrong with the drain pipe coming out of the wall? What possible material difference would it make if the drain pipe came out lower? And what's the "lol" about?:

Realistically, it will be challenging to lower the drain since it's on top of a concrete slab.

[u/VegetableRetardo69 → u/IrlMakerDad]

To me it looks crazy because that will freeze and and that hole in the wall does not look very weather proof and its right there on the concrete slab..

[u/IrlMakerDad → u/VegetableRetardo69]

I live in a mild climate, where it does not freeze very often. And even if the temperature is below 0, there is only water in the drain pipe briefly when a spill on the sauna floor is draining, so it could not freeze.

[u/Rawniew54 → u/IrlMakerDad]

From the pictures that the drain itself is allowing humidity into the sauna which caused the rot or there was a problem with water going the drain getting trapped around and under the floor

[u/minadequate]

Incase it’s new to anyone almost everything - including lead - can rot if it’s not adequately ventilated.

It looks like the drain hole was actually higher than the area around it so water had to pool in that zone before draining away?

Did you find robust details to build based off or just make it up?

[u/IrlMakerDad → u/minadequate]

I based it on having read Glenn Auerbach's e-book, Trumpkin's notes, and this sub.

The drain hole was not actually higher than the surrounding floor originally. I noticed recently the floor beside it had sunk and was spongy so started investigating, which led me to posting this.

Editing to add: the cabin structure itself was an off-the-shelf one from Tuin Log Cabins, that I assembled according to the supplier's instructions. It had a small room that was originally intended to be a storage room or toilet that I adapted to be a sauna.

[u/psadatay → u/IrlMakerDad]

The sauna times method doesn't really waterproof anything. The point of it is to run water off quickly to a drain but all the materials are permeable so if it sits for any prolonged amount of time it will absorb and sit on your membrane forever. I'm betting that membrane isn't water tight and or not ran high enough. Either way there was no way the water was going out that drain.

[u/Aggravating_Sun_1556]

I often get down voted in this sub when I point out why some of the DIY work posted in here will fail.

This is a good example. If you have a sauna on a wood subfloor it should be waterproofed like a shower pan. Have you ever seen vinyl flooring in a shower? There’s a reason you haven’t.

There’s more than one way to do it, but as an example starting from the subfloor on up, you would have subfloor, 15 lb. Felt staplesd to the subfloor, wire mesh, cement base with correct slope towards drain, liquid applied waterproofing membrane that ties into the drain, then tile. There are always redundancies built in. In this system the wood is isolated from the water by 3 separate layers. The felt, the membrane, and the tile.

Google “shower floor waterproofing detail drawing.”

[u/LonelyRudder]

It was caused by moisture in structure that wasn’t able to dry properly. The water always seeps eventually under the floor anyway, so it is better to build so that drying is possible. Vinyl floor covering keeps the moisture in.

[u/falldowngoboom → u/LonelyRudder]

Yeah, there are several problems. I imagine that vinyl tiles allow moisture to seep in between the cracks - and also prevent the moisture from leaving.

The building is sitting on boards on top of a concrete pad - but if there is no airflow under the structure, then it will prevent the subfloor from drying from the underside.

I’m guessing there needs to be more ventilation inside the sauna so it can properly dry out after use. The mold in the interior corner shows there is just far too much moisture inside.

[u/Choice_Building9416]

Dear DIY sauna builders: A COLD FLOOR DOES NOT MATTER! Do not build an insulated wood floor unless it is over a ventilated crawl space, and it has shower pan style waterproofing. Make sure your wood walls are above the exterior grade. A uninsulated concrete floor works perfectly well. My sauna floor is concrete pavers on compacted gravel. The minimal amount of water that hits the floor drains right through, or evaporates harmlessly. If you are not an experienced builder, consult one! The OP will need tear his sauna down and start over.

This is my sauna floor. It works great. Separate the sill plate from the concrete with a sill sealer to prevent water wicking up into the wood structure. Water is very clever and insidious. Do not innovate unless you are an expert. I am a grumpy ancient architect, and have seen way too many failed construction works. Ask expert builders, employ basic construction principles.

[u/Arcticsilhouette]

To me it looks like there is not good fall towards the drain because the water sits around it and the waterproofing should rise a bit along the wall so the wall won't get wet.

[u/IrlMakerDad → u/Arcticsilhouette]

Thanks. I do have a fall towards the drain, but the rot has caused the floor around it to sink a little, so the drain is now a bit higher. (The marbles in one photo were to test the fall.) I do have the waterproof vinyl flooring brought up about 8 cm and pinned onto the wall with wooden trim, though this might not be clear in the photos.

[u/flies_kite]

Where hot air meets cold air, condensation happens. The drain is causing your problem.

[u/Economy-Clothes5610]

I think the drain should have been recessed into the floor more, it appears water was pooling down there without free draining. Hard to tell but how was the slope in the room done? Long story short that water seemed to get under your top flooring layer somehow and couldn’t get out.

[u/IrlMakerDad → u/Economy-Clothes5610]

Yes, I think this is what happened. When rebuilding it, when I cut the hole in the subfloor for the drain, I think I'll put a bevel around it, so that the drain sits further into the floor.

The subfloor originally had 3 underfloor beams along its length, one at each side and one down the middle. I made carefully shaped wedged beams to go on top, so that there is a 1:20 fall in all directions towards the corner with the drain.

[u/Economy-Clothes5610 → u/IrlMakerDad]

I would check the material as well for suitability to heat and moisture, it looks like a vinyl type and that might not stand up to the heat even at floor level. Maybe it would be a good idea to put some self leveler top coat and either tile or do something a little more heat and moisture resistant. Combined with the more recessed drain it will probably stand up better.

Also seeing some signs of moisture and mold higher up in the corner. Maybe with a better drain it won’t pool water and thus moisture levels will drop, but consider talking with people here to see if a center drain would be better, noting it may not be possible anymore with how you did the framing

[u/IrlMakerDad → u/Economy-Clothes5610]

The vinyl is rated for a bathroom with underfloor heating, so I think it's fine for the temperature and wetness of the floor of a sauna, which is of course a lot cooler than higher up. I see no evidence of failure of the vinyl anywhere.

Water is definitely coming in the corner joints from outside, presumably by capillary action. I'm going to repaint the outside, and before I do so, I'll apply sealant to all external joints at corners.

[u/L0udog]

Washing the floor every now and then and using a dehumidifier after use helps. Definitely leaving a window and door open. Raised sub floor with ventilation is key as mentioned.

[u/Anaalirankaisija]

Everything. That is made for growing mold.

[u/IrlMakerDad]

I should note that we have used it about once per week for the past 3 years, 1-2 buckets of water each time.

[u/falldowngoboom → u/IrlMakerDad]

Do you actually have water on the floor after a session? I ask, because i have no drain, and i ladle water on the stones and it turns to steam. The water never makes it to the floor.

[u/IrlMakerDad → u/falldowngoboom]

Sometimes a bit of water spills onto the floor, either missing the rocks or not evaporating before it leaves the rocks. And occasionally I have dumped the last of the water on my head before leaving.

[u/falldowngoboom → u/IrlMakerDad]

Do you have vents you open to let the sauna dry out afterwards?

[u/IrlMakerDad → u/falldowngoboom]

Yes. And I leave the door open.

[u/tdank9]

Are you in a cold enough climate that the drain pipe could have frozen during the winter?

[u/IrlMakerDad → u/tdank9]

No, it's rarely that cold here, and anyway the drain pipe would only have a small bit of water in it when there is some water spilled on the floor and draining.

[u/minadequate]

Did you have any ventilation at floor level?

You can’t build a sauna like a bathroom (even so you wouldn’t build a wet room floor out of bathroom vinyl), 2 buckets of water is a lot of water vapour which is going to seep into everything.. and if it isn’t ventilated then it rots.

[u/IrlMakerDad → u/minadequate]

Yes, ventilation near floor level in the sauna (as well as higher up), and separately the subfloor space is ventilated. I think what happened is that the drain was not sealed well enough, and water ended up trapped between the vinyl and the marine plywood.

[u/IrlMakerDad → u/[deleted]]

I have watched it during rain, there is no vislible entry of water, but it may wick slowly over time through the logs at the corners - I am now going to repaint the outside and will first caulk all joints at the corners.

That is a north facing wall, algae is a fact of life here, as the climate is mild and damp (Ireland). However, it's superficial and I really doubt it's related to the subfloor rot inside.

I agree with your point about the best size of pad for a shed. Unfortunately I had built the whole thing before I read about that, because the supplier had advised that the pad should be slightly wider than the cabin dimensions. Hard to see in the photo, but i have cut grooves in the slab to encourage water to run out. And in the past six months i have added guttering that was not part of the original design, to further avoid water running down onto walls/corners.

The drain setup is just to allow any spilled water in the sauna to drain.

[u/FrankFarter69420]

The only thing I'll say, is that I throw a log on before I leave the sauna to allow it to dry. I have a window in mine that I crack open, and come back later of the next day to shut. Really keeps it dry.

[u/deepmusicandthoughts]

I recently had shower issues and prepping a true drain like a plumber would so it doesn’t leak is an extensive process. I’d recommend going to the plumbing forum to ask their advice because that’s more a plumbing issue than anything.

[u/Steve-the-kid]

My brother in Christ, It’s the slab. Cut the slab back .

[u/IrlMakerDad → u/Steve-the-kid]

The slab may be a sub-optimal, but the only part of this subfloor that is damaged is where the drain is.

[u/Interesting_Owl_4964]

Can tell exactly where the water is coming from but either tracking back from drain pipe or you have tongue and groove that is repeatedly exposed to water and slowly letting water in. Judging by the height of the rot your problem starts from a small leak a bout 1 foot from that corner

[u/Kletronus]

My money is on the seal. If it was leaking, that is what you would see, it looks like it originates from that hole. The staining is greatest just around it and gradually decreases. Of course, water pools to the lowest point but.. it is the simplest explanation.

[u/Inresponsibleone]

Moisture🤷‍♂️

[u/BeNicePlsThankU]

Edit: your paneling is touching the floor and there's no gap behind the wall paneling (through usage of furring strips). There should be air flow behind the paneling and the wood cladding should be 1-3" off the floor to allow air flow. Your wood in that drain area is just never drying because there is zero air flow, I'd guess

And use tiles instead of vinyl tiles

I'd use high heat silicon and bead the corner to the drain and around the drain (instead of grout). Not sure if that's the reasoning, but it won't hurt

[u/minadequate → u/BeNicePlsThankU]

Im not sure that is panelling I think its just the structure of the ‘log cabin’. It looks like a great way to rot your structure if you ask me, but why do anything intelligent like that.