Depth of the ro
Following from the two very good answers above, here are some further thoughts.
The measurements of a rodan 炉壇 you can purchase from most dōgu shops are (sorry/not sorry for metric):
Under the 305mm depth from the floor, a further space is usually built in to chashitsu, making the total depth of the hearth around 450mm:
In the past, I have built a chashitsu with a raised floor in an existing room. The ceiling came to 6′ (1800mm) high with the raised floor. The bottom of the rodan came to only 50mm (2″) above the existing carpet floor. I solved the heat problem by placing a heat-proof tile under the rodan and it worked very well.
I assume along with John-Leon that you’re building this into an existing room. In the US, this could fortunately mean you have a good height to work with.
If you are limited, however, you could try and work with an okiro (a hearth placed atop tatami). You could incorporate the okiro into the lower floor structure, perhaps. The okiro would mean that you would probably have to forgo a genuine sumi and ash arrangement, but you would still be able to achieve a sumidemae to a certain degree. You would also need a lower kama, too.
Back to the height of the chasitsu space, if you are building a space under 4.5 mats, I think you could get away with a low roof. Under 5′ (1500mm) may be a bit extreme, though. If you’re going with 4.5 mats or more, I think it would be best to make the height 6′ (1800mm) at the least. The bigger space means guests and host will need to walk more and there will likely be people that are 6′ or taller.
For the building of all the parts, robuchi, rodan, etc, JAANUS is gung-ho:
In the article on the robuchi there is a link to rodan:
One of my students built a rodan from copper and it is working very well. Under the house (in Sydney) was copious amounts of sand and he uses that for the ashes. It works a treat.
Finally, for making the rodan for use with both denki and real sumi, I did this in a previous chashitsu. When I was using the electric element, I removed the ash, washed the rodan and the tripod and placed the tripod back in the clean rodan. I then sat the electric element on top of the tripod for the real sumi, adjusting the height with tiles resting on top of the bottom tripod.
If you want to use sumi one day, you should make it deep. I suggest a depth inside the Ro of ~ -35cm from +/- zero (this is the floor level under the tatami). Tatami is 5.5cm. If this is impossible, it can’t be helped. If the room is big, the ceiling should be high. If you have a limit of the ceiling, you have to make a decision. The point is: the gotoku should rest in the ash. It is no Problem if there is more ash under it. It is a problem if the ro is too shallow. If the Ro is shallow you have to use a suitable kama. The other problem is safety. If the ro is shallow you don’t have enough heat-insulation (because of a thin layer of ash) at the bottom. This could cause a fire.
It depends on a number of factors.
First how high do you want your ceiling and how much space do you have to play with? I assume this space is within an existing structure.
A standard metal ro is about one foot deep so you need a little more than 14″ below the surface of the tatami. If you do not have enough room you can have a more shallow ro or even substitute an okiro below the mats. Keep in mind that the regular robuchi is 1/8″ thicker than the tatami. You probably will be most comfortable with a 7′ ceiling but you can survive with less. There are rodan made of sheet metal that can be used for either electric or charcoal fires. You might start with their dimensions if you have enough head room.
There are other considerations. As an example, for my space I wanted the tatami high enough to use an existing window to enter from the garden and I wanted to use the space under the tatami for storage, crawl space.
There is no universally correct answer but there will be a correct answer for you and your space.
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