What are the alternatives to a big, complicated hot tub that uses so many chemicals and so much energy? I just wanted a small hot tub for myself and my partner that was efficient and easy to own. I really didn’t want to spend $5k+ on a manufactured depreciating asset that I may not end up using much.
For a small garage shop, plywood is probably the simplest material to use to build a box. This box, though, would need to hold hundreds of pounds of hot water, and unlike most furo-style tubs that are drained, the goal was to keep water in this indefinitely.
Can you really build a hot tub from plywood?
Plywood, of course, is made from gluing many thinner layers of wood together, and that glue delaminates when exposed to moisture over time. Plywood also is made with formaldehyde, not something I would want to soak in. This includes marine plywood, which is made to last longer in wet conditions, but is not designed to live below the water line. I decided to by regular plywood and coat it with a low-toxic marine-grade epoxy resin.
To join the box I used a combination of Dado groves, epoxy resin, and stainless steel screws. Once assembled I added another coat of resin to the corners.
At the time I was focused on sealing off the plywood from water and making sure the corners would not leak. I had not done any calculations on the force of the water pressure!
The dimensions were dictated by trying to optimize the use of standard 3/4″ 4’x8′ plywood sheets. I used two sheets of plywood for this. The sides and bottom had a length of 48″, the width 24″, with a height of 28″. From what I could figure this was actually significantly bigger than many ofuros. I was not trying to build a tub to stretch out in, but something compact that would fit off my back patio where I could soak in neck-deep water.
For a drain I cut a hole in the floor just big enough to stick a wine cork in. I had no plan for draining the water off away from the tub, something I would learn about later after flooding my patio.
While I let the epoxy resin cure in the tub for a few days I built a flat paver pad for the tub where I wanted it to live. It was critical to make sure the tub sat evenly so that the weight of the water did not cause the wood to deform and settle, potentially causing cracks and leaks.
How Do You Heat a DIY Hot Tub?
This is a fun one. Rather, this might be the most difficult part with a wide variety of options. Heating 100+ gallons of water to hot tub temps takes a lot of energy. Heating a tub from a wood fire came to mind as the most elemental, primal source, but in reality this is very difficult and presents a significant fire danger. It would take several hours to heat and would require constant fire maintenance. This might be fun in the right setting, but I’m more or less in an urban single-family home neighborhood, so that’s not very safe or realistic.
I had recently installed a whole-house tankless water heater that runs off our natural gas line. It can heat water well above hot tub temps on demand, without limit. I just had to hack into my hot water somewhere and run out a line and fill my new tub like a large bath tub. The drawback was that I didn’t have a way to maintain heat, but this would work for my first soak.
Writing this post over a year later with much more experience, I’m actually amazed that this tub worked not only then, but has held water in it without leaking this entire time. After my first soak I noticed the walls bulging out with the force of the water, so I quickly added some simple modifications to keep it from pulling apart. I added a tight reinforcing brace on the bottom using 2×4’s just screwed together. For the top I cut a 2×6 about 6 inches wider than the rim and then cut grooves out so the board would sit on the top lip, holding it together in the grooves. This also made a decent bath tray.
The Plywood Hot Tub Experience
Most importantly, I could not believe how excellent the soaking experience was. It was like a hot tub, only better. There was no chlorine or chemicals to inhale and worry about burning in, and I actually loved the peaceful experience without the jets or motor. It was very much like being in a very clean, private hot spring at home. I also did not mind the relatively short length. I did not feel the need to stretch out. Being in the hot water, neck deep, was really all I wanted for the complete therapeutic experience.
With this proof of concept working, I immediately began planning an upgrade to a cedar tub. If the 3/4″ plywood tub worked so well then surely a cedar tub of greater thickness would be even better….