★waterproofing and wetrooms

one of the most simple concepts yet more difficult concepts to grasp about japanese tubs is that you do not really need to worry about the plumbing.

The best configuration is to have the ofuro simply sitting on a waterproof floor, sloped towards a floor drain and just let it drain onto the floor, without a fixed piped connection. Think of the ofuro as a large bassinette.


1. you do not need to raise the ofuro and create a space below it to mount the pipe, the syphon etc.
2. it is easy to move the ofuro over for cleaning and maintenance.
3. you can flip orientation or position so it looks at the sunrise at sunrise and sunset at sunset!
4. you can even move the ofuro to the terrace/garden and fill it up with a hose or other fun things like that.
5. you can let the ofuro overflow from the top rim, as in a japanese hot spring, giving a sense of abundance and infinity.

Objections & Counter-arguments:

6. the drain is requested for filing to the building department

I understand that regulations may have scarce flexibility. That said, custom made wooden tubs are not certified sanitary equipment (NSF, UL, CSA, ETL etc…) anyway. An ofuro is a soaking tub therefore is a fitness/wellbeing equipment like a home sauna or a tanning bed. Be creative, think out of the box. Your shower is a sanitary equipment, your ofuro is not.

7. We need a drain to connect to the overflow

Again, if you have a wet room, you do not really need an overflow. Think again about it!
An overflow is mounted some 50mm or more (if the ofuro has a top frame) under the top rim.
This means that you are losing 50mm or more of depth. Also you cannot enjoy the feeling of the “vanishing edge” and relaxing sound of water gently overflowing from the top.
Another problem with the overflow is that you will need a ledge or lining to hide the corrugated pipe of the overflow.
See the following pages to see examples:

8. We cannot install a wetroom

In some cases, we see ofuros are installed in bedrooms, or in situations where is not possible to recess or built-up a wetroom curb.
Actually, even in a bedroom, it is possible for instance to have a section with a waterproof floor (recessed or built-up) covered with a floor grating platform. See this smart example from a client here:

9. We do not know why but we want to connect the ofuro to the drainpipe anyway you can even move the ofuro to the terrace/garden and fill it up with a hose or other fun things like that.

No problem!! you can use a 1 1/2″ pipe (or 32mm pipe) with male G thread which matches our long socket ring.
Please check our drain details page here: and click on the links to download detailed photos and drawings.

and now, here is the main course of this article. Please read it if the previous argumentation convinced you.

OK. I do want a wetroom for my ofuro. How can I build it?

For the sake of bullet lists, I can think of three main categories.

a) use a large size shower pan/shower tray.

It may not be big enough for XL ofuros but there are sizes to fit up to our M-7 (L=1500-1600mm) size.
Cheap and easy to install, you can just buy one from amazon and have it installed.
Many variants are available: low rise or with curb, black, white, or even gray or purple! corner drain or center drain, etc.
Just internet search “shower tray large” or something like that.

b) use a wetroom system.
Some have already the drainpipe built inside the panels for a slimmer and smarter installation.
Here are some popular ones:!/wetrooms-waterproofing-glass-screens/linear-waste-trays-kits

c) DIY.
There are infinite ways to create a waterproof layer using PVC liner, asphalt mats, FRP, urethane paint coating etc.
Similarly, there is a wide choice of floor drains, linear drains etc.

We are available for liaising directly with your plumber or contractor (via zoom, WhatsApp call, or email) to answer all their questions, also at the time of installation.
On the other hand, please do not ask us which system is better or how much should be the gradient of the floor.
As you understand, these are all environmental variables, and only the people on site can evaluate the peculiarities and requirements of each project and decide which system makes more sense for your project \(^o^)/

★water heaters

one of the most common questions we receive is “can Bartok design supply tubs with a system to keep the water warm all the time?”

The short answer is: NO.
We are makers of Japanese wooden bathtubs.
If we were selling how water or spa system we would just be resellers of some other company product.
Moreover, we would not be able to supply spare parts or maintenance services.

To keep the water warm, there are basically 4 options:

1) Add hot water

Simple as the title, you drain some lukewarm water and add some 60-70 C. hot water to raise the average temperature. Pay attention not to get scalded!

2) Ask a local SPA or POOL equipment company

A re-circulation system consists of a small pump, a cartridge filter, a gas or heat pump operated post-water-heater and optionally a water sanitizing system (UV or Ozone. Chlorine is not good for the wood)

3) Ask Rinnai

Rinnai is one of the main gas domestic equipment makers in Japan and in recent years they started an aggressive expansion policy overseas. Rinnai offers “on-demand” gas-operated how water heaters.
“instant” or tankless “tankless” is just another way to call them.
Differently from other makers, Rinnai hot water heaters can be equipped with an option to also reheat the water with the very same equipment. This is the option (almost standard in japan) called OI-DAKI.

Try to Google search “Rinnai + your country” to find the best way to contact them.
They will propose one of these 2 diagrams (direct or indirect connection) :

Do not ask us which one is better. Ask your plumber or your contractor and discuss the topic directly with Rinnai.

What is more important from our side is to decide which type of sockets to use for the SUCTION and for the RETURN.

The tubs are made in solid wood so a hole can be drilled on-site almost anywhere to install these sockets.

On the other hand, you will need to advise the plumber on how to secure them because the tub walls are thicker (t=42mm) than an acrylic or PVC tub.
I think there are at least 3 possible approaches:

A) use a dedicated valve.

You may also ask Rinnai but your plumber might have to customize it.
Here below is a diagram and an image photo to explain what I am talking about.

Basically, there is a mushroom-shaped valve that sucks lukewarm water from the front and returns hotter water from the slits around the base.
This is a smart solution and also has a filtering function to protect the recirculation circuits from clogging etc.
Again, I am almost sure that the valves available on the market (but maybe you would have to import from Japan) are too short to bypass the thick wood wall so your plumber will have to customize /extend it.

B) use SPA connectors

There are many makers such as Waterways, Hayward, Certkin, Jacuzzi, Pentair etc. that are producing SUCTION and RETURN fittings. Here below are 2 photos for your reference.
Also in this case chances are your plumber will need to extend the threaded part but maybe this solution offers more flexibility and the parts are more readily available.

C) use the existing FILLER SPOUT

This is a little more difficult to explain but I am sure your plumber will grasp the concept.

If you use one of our wooden spouts, you can just feed the return pipe (from the Rinnai system) inside the spout and let it gently flow from the beak of the spout! You do not even need to have a dedicated spout for this.
You can just have 2 pipes feeding the spout: one with the hot water from the wall-mounted mixer or thermostat and one from the recirculation circuit and hit two birds with a stone!
In this case, you just need a suction socket but your plumber could even use the existing drain socket for this and mount a valve outside of the tub that can be opened to drain the water from the tub once finished.

It would be ideal if your plumber could do the sucking for the circulation from the drain socket and do the return inside the spout.
You would feed also a normal premixed water pipe into the spout so it can at the same time output re-circled water and can also be used as a filler.

4) install a chofu wood burning stove

This option applies to outdoor tubs only.
Chofu is another famous Japanese maker producing wood-burning stoves that are attached directly to the back of the hot tub. Not only for re-heating, it can be also the regular hot water supply for your outdoor ofuro.
With reference to the specs of the heater and ways to connect it, please ask Chofu.

Quite honestly, this is all we can say about our experience in re-heating or recirculating the water.

As said earlier in this article, the main problem we are concerned about is “HOW THE CONNECTION INSIDE THE TUB IS MADE.” Again, your plumber can drill holes as necessary almost anywhere as our tubs are made in solid wood.

We can also drill the holes for you if you prefer, but would definitely recommend you do it on site which gives your team much more flexibility.

One more tip: to overcome the problem of the thickness, you can either:

X) extend the fitting`s threaded connection


Y) make a trim around the hole towards the outside face (obviously) in order to reduce the thickness locally and be able to tighten the nut.
See an explanatory image below.

Yes, this photo is for an anchor bolt and not for a pipe, but you understand the concept.

I hope this article clarifies some issues and remain available for any question you may have.
Also, it is probably a good idea if you have your plumber or contractor calling me (whatsapp) or mailing me directly.

Back to top