DIY Hot Tub Build Guide: Step

Sep 24, 2023

Materials, tools, and clear and complete instructions on building your own off-grid wood fired hot tub for around $2,000

Author

Elias Carlson

Photographer

Elias Carlson

Of all the DIY projects one can conceive, a sauna or homemade hot tub might be the most satisfying. An off-grid cedar hot tub makes a primo addition to any dwelling, creating a place to soak at the end of a long day and recoup in any season. There are loads of hot tub plans and hot tub designs out there, from in-ground soaking tubs to concrete hot tubs to wooden hot tubs to solar-powered hot tubs (there are even inflatable hot tubs, but let's not talk about those). The variety makes starting off on a project daunting, unless you have someone to turn to for advice.

Recently, Field Mag contributor and DIYer Elias Carlson shared insight on how he and his wife Theresa built a two-person stock tank hot tub (affectionally called a "hillbilly hot tub") for $2300 in the backyard of their Idaho abode. They used a mix of materials including a Japanese wood stove as the main water heater and paired the outdoor hot tub with a stock tank pool for cold plunges. For a project rundown, including budget breakdown, inspiration, and lessons learned, check out our interview with the builder here.

For a full-on step-by-step tutorial on how the project actually came together, from start to finish, read on below. Our hope is that between these two articles—and some rudimentary woodworking experience—you should have every bit of information you may need to build a hot tub of your own, too.

[Complete your wellness dreams with an outdoor sauna and cold plunge while you’re at it]

Foundation

Six concrete footings with 4x4 recesses in center, two 4x4 pressure treated posts, pea gravel (optional, handy for leveling foundation blocks)

Deck

2x6 and 2x4 premium grade douglas fir framing timber (2x6 for deck joists & boards, 2x4 for tub frame), plus 2x6 and 2x4 joist hangers plus galvanized nails or joist screws, galvanized corner brackets for attaching to 4x4 posts

Siding

Salvaged 3/4" cedar siding

Stain

One gallon Thompson's Timber Oil (clear)

Screws & Nails

Unknown—we used leftovers

Insulation

4x8 sheet 2" rigid foam insulation, 2'x25' roll of reflective bubble wrap insulation

Chop saw

Table saw

Jig saw or hole saw (for cutting holes in the tub)

Circular saw (can be used instead of chop/table saw if you know what you’re doing and have a steady hand)

Drill

Tape Measure

Level

Framing Square (super handy, highly recommended)

Compressor & brad nailer (for attaching siding)

Chofu Stove, $1200 + tax & shipping

2'x6' galvanized stock tank, $300

All deck materials, $700-$800

Total time: Approximately 60 hours (30 hours x 2 people)

Note the builder was able to salvage much of the wood siding, which helped save on costs

We considered several spaces in our yard for the hot tub project. We have a tent platform near the creek that runs through our backyard and considered building there for the ambiance of the whole experience. But there are tall cedar trees all around the deck, and with bone-dry Idaho summers and the ever-present forest fire danger they bring, we didn't want hot air, sparks, or ash floating up into the branches (Ed Note: electric cedar hot tubs or solar hot tubs are a great alternative for soakers in wildfire prone areas). Also, we really love setting up the tent for friends & family, and often sleep there during the hottest parts of summer to take advantage of the cool air near the creek, which doubles as nature's original and best sound machine. We also considered adding it onto our existing back deck, but nothing about the fit felt quite right with the existing landscaping. Eventually we realized we’d already found the best spot where we set up the cold tank at the edge of the yard, a hose length from the house.

A 2'x6' tub holds 175 gallons of water, which equals about 1,450 pounds of weight. Add in two human beings and you’ve got another 300 pounds or so. Add in the weight of the deck wood, and you’re looking at roughly one ton, give or take. Here in Idaho, we also get anywhere from two to four feet of snow every winter and we wanted it to be able to handle that weight as well.

Knowing this, we went with 2x6 framing, with joists set at 16" on center with six 4x4 pressure treated posts set into concrete footers; three on the back edge of the deck, which would be supporting most of the weight, one in the center under the tub, and two at the front set flush to the concrete blocks to absorb weight even more efficiently.

We also used 2x6 Douglas fir framing timber for the deck surface, knowing they’d give us additional rigidity and strength over your standard 1" or 5/4" pine or cedar deck boards. Our deck is probably overbuilt. But we want it to last, and I figure too strong is much better than not strong enough.

Take your time here. Getting everything square and level early will make the whole process easier, and you’ll have way less of a chance of measurement creep in the later stages.

Our first step was to frame a simple 8'x8' square using the 2x6 lumber. Then we positioned it where we wanted it to be in the yard, made sure it was perfectly square, and marked the corners.

Next we moved the concrete cinder blocks roughly into place and marked the edges, took them back out, and started digging out the lawn where they’d set in. A bag of pea gravel proved very helpful in leveling the blocks.

Our stock tank hot tub is on a gradual slope, so on the front edge we dug out our concrete blocks so they were nearly flush with the lawn. We knew we wanted the front edge of the deck to rest directly on these blocks, so we made sure the tops were level with each other by setting a 2x6 across them and putting a level in the middle, then adjusting the depth of the higher block until there was a match. The back edge of the deck was 10-12 inches lower, so we only dug enough here to get the blocks level, knowing we could cut the 4x4 posts to size to achieve a perfect level regardless of elevation.

Once all four corners were set and level, we put the frame on top of the cinder blocks, and used 4x4 posts to anchor them to the footings. Because the front edge had already been leveled, leveling the back edge was simple. All we had to do was set a 4x4 post in the footing, raise the corner of the deck till level, mark the post at that spot and cut it flush. We attached the deck frame to the 4x4 posts using galvanized corner brackets for maximum strength.

This part is by far the easiest if you’ve done a good job leveling and squaring your foundation and frame. First, measure off your 16" on-center joist marks. Then tack them into the frame with screws or nails (I recommend screws). Add your joist hangers to each end of the 2x6 joists for additional strength and secure them with galvanized joist nails or screws.

Next, lay out your deck boards and pick the best ones for the front part of the deck. Move any boards with chips, cracks, or noticeable warping to the back of the deck, which will be covered by the tub.

Lumber sorted, we secured the front board to the deck first, then used a ⅜" spacer between the boards as we set them in one-by-one, to ensure an equal distance between each one.

Quick Tip: As you set the deck boards in, screw down only the outermost ends of each board first. Boards will flex a bit, so if there is a slight warp in the board and your ⅜" gap isn't perfectly uniform, this allows you to pry the board apart (or closer together) before you tighten it down for good.

You should now have a basic 8’x8’ deck that is solid and square. We treated ours with clear Thompson's Timber Oil to keep the natural look of the doug fir boards.

You can stop here if you want. All you need to do now is raise your tub four inches above the level of your stove (to allow for the correct height for the stove's thermosiphon to work properly), hook it up—find those steps below—and you’ll have your own hot tub, homemade DIY-style!

However, we highly recommend framing in the tub and adding siding so you have a seating area, wood storage, and a super-legit professional look.

Place your stove and tub roughly where you want them to go on the deck and calculate for the width of the framing on either side of the tub, accounting for the length of the inlet/outlet ports from the tub to the stove.

If you want your siding to be flush with the edge of your deck you’ll need to account for that too. For example, our siding was ¾", so I cut 1 ½" off the 8’ framing boards so each end would be flush to the deck once the siding was added.

The Chofu stove generates very little heat on its exterior. Almost all of the heat from the fire goes either into the tub water or out the chimney. Because of this, you need very little room on either side of the stove for fire-safety reasons. We have about two inches of air space on either side of ours alongside the frame, but we did leave most of the stove open to the air to allow for chimney space.

With the basic location determined it's time to frame in the tub. With metal stock tanks, Chofu recommends raising the base of the tub four inches above the base of the stove. The stove pulls cold water through the bottom port, which is then heated by the fire, and because hot water rises, it's pushed out through the top port and into your tub. To achieve this, you need a 3-4" rise for every foot of pipe. To give myself more flexibility in where I set the hot water port, I settled on a 6" rise, so I used 2x6 boards set on edge to frame around the base of the tub.

I then dropped in a layer of 2x6 scrap wood, spaced 6-8" apart, and layered two pieces of 2" rigid foam insulation on top of those. This brought the base of the tub roughly level with the top of the 2x6 frame and created a nice container for the foam, which is critical for retaining heat in the tub overnight.

I then framed around the tub with 2x4 lumber, leaving the top edge of the frame ¾" short of the lip of the tub, so the top layer of siding would end up flush to the tub. For the seating area at the back of the tub I used 2x4 joist hangers tied into an 8’ length of 2x4 to make sure it had enough structural strength to tolerate the weight of an adult or two.

We used salvaged cedar siding for our hot tub lid and exterior of the build. The easiest way to do this is to pull the tub out of the frame, assemble the appropriate amount of siding to cover the top, and flip the tub upside down on top of it. Then you can use the tub edge as a template to trace a line you can follow with the jigsaw.

However, I wanted the lid to overlap the siding by about 1" on all sides. The tub is 24" wide, so I added 2" to that measurement (26"), then divided that in half (13"). Then I tied a string to a pencil, and marked out a 13" length. I positioned the pencil 1" past the length of the tub, centered the 13" piece of string on the 24" width, and used the string and pencil held taught as a rudimentary compass to trace a 26" circle onto the wood. Then I repeated the process on the other end.

I now had a lid tracing 1" wider than the tub on all sides. I trimmed the straight edges of the lid to size with a table saw and used the jigsaw to carefully cut the circular curve.

Next, I cut a length of insulated bubble wrap and laid it out on the wood, then attached five horizontal pieces of 1" cedar scrap wood to hold the lid together and keep the insulation in place. I further secured the insulation with a construction stapler anywhere the supports weren't pinning it down. Then I added two handles, making sure the screws went through the siding and into the horizontal supports so it’d be super solid.

This is the scariest part of the build! But it's essential for having a heated hot tub. One minute you have a perfectly good stock tank, and the next you’ve got a $300 piece of metal with giant holes in it. Read your Chofu instructions closely here. Measure twice, cut once, and you’ll be ok.

With your stock tank and stove in place, carefully measure the location of your inlet and outlet ports as well as the drain location. Mark these holes and use the appropriate sized hole saw or jigsaw to cut the holes. I found the hole saws to be really difficult to use, so I used the plastic port hardware to trace each hole onto the tub using a black Sharpie. Then I drilled a smaller hole with a carbide drill bit to allow enough room for the metal blade of a jigsaw to fit through. Then I slowly cut the hole with the jigsaw, and filed the rough edges smooth with a round file.

Drop the tub back in and mark the drain hole on the rigid insulation, then pull the tub and cut away the necessary foam to accommodate the brass pipe, and cut a notch in the wood frame for the spigot to come out.

At this point you can wrap the tub in reflective bubble wrap insulation, trim it to size, and cut holes for the ports. I used a bit of packing tape to secure the bubble wrap, and then used two lengths of parachute rope to secure it further. Stock tanks typically have indented ribs in the sides, and I lined up the parachute rope with these, so when pulled tight the ropes sink into the grooves and lock the insulation in place.

This is pretty straightforward now that you’ve got all the holes cut. The Chofu kit comes with a drill bit and self-tapping screws, which you’ll need to assemble the stove pipe. Hook up all the hardware, fill the tub to your predetermined fill level, and you fire that bad boy up. You now have a functional hot tub! If you’re impatient like us, you can use it while you finish up the build. The stove is also a great place to burn lumber scraps from the build.

We treated each piece of siding in the Japanese Shou Sugi Ban method. Traditionally cedar is used, but pine and fir are also common. Charring the wood creates a naturally weather-resistant carbon layer, made further weather-resistant by treating it with tung oil, boiled linseed oil, or Danish oil. We went with Thompson's Timber Oil as it was fairly cost effective for one gallon and easier to get at our small town ACE Hardware store. You do not have to treat charred cedar boards with oil, as cedar is naturally rot-resistant. However, the oil treatment really amps up the weather protection and has the added bonus of sealing in the char so you don't get black marks rubbing off on your skin or clothes.

We borrowed a big propane burner attachment from a neighbor and carefully charred each piece of wood on the main face. We used a smaller propane burner to char the edges and any knots in the wood (which are much denser, and char more slowly than the rest of the wood). We left the backs of the boards un-charred, but treated them all with timber oil. Some of the long boards warped a bit due to the high heat of the charring process, and I found that wetting the back side of the board before charring really helped with this. Shorter boards seemed to be less prone to this effect than longer boards.

Once the wood was cool to the touch, we treated it with the timber oil and let it sit for several hours before attaching it to the tub frame with a brad nailer. Once attached, give the wood 24-48 hours before using the tub so the oil will have time to fully cure.

You can save yourself a lot of time and effort by simply staining the siding, but I’ve wanted to try the Shou Sugi Ban method for years, and wanted something that could stand the test of time. Plus it looks fucking cool, if you ask me.

One quick tip: No matter how good a job you’ve done leveling, squaring, and measuring your frame, framing lumber is inherently imprecise due to minor warps and twists. As such, you’re always going to find an edge or a corner that is off by ¼" somewhere. Because of this I recommend measuring EACH siding board to fit so you can account for variances in the lumber and framing versus pre-cutting everything to a standard length and hoping it will fit perfectly—it won't.

You’ve now got a badass wood fired hot tub, built to stand up to harsh weather for years of soaking pleasure. Kick back, relax, and let the good times roll.

You won't regret it. The benefits of cold water immersion are well documented. You’ll learn to love it. You’ll become obsessed with it. And if you get it before you start your DIY hot tub build, it makes for an excellent way to keep cool on a 90-degree Saturday.

Using water straight from the tap means there aren't any chemicals in it (as opposed to your standard jacuzzi or swimming pool). Which means you can use the tub every freaking day and your skin will never get dry and crackly like it does from chlorinated water. The downside of course is that untreated water gets funky after a while. We drain our tub every other day and refill (fortunately we are on a well so our water bill is $0, and we do not live in a drought-prone region). But this process has a silver lining too! Since the water is untreated we can re-route it. I run a length of hose from the drain spigot to two small apple trees we have near the tub so we get multiple uses from our water. If your tub is near a garden you could absolutely hook up a drip system to take full advantage of this feature.

Alternatively, if you’re close enough to a power source, you can purchase a circulation pump/filter for the tub to keep the water clean and avoid changing out the water altogether.

Hot water and cold water stratify, so you’ll need to stir your tub a few times while it's heating up or you’ll have a layer of 110-degree water on top and 60-degree on the bottom. After you’ve stirred it well, and the overall temp is nearing 90 degrees, avoid the temptation to pile on the wood. The hot water heats exponentially, so that last 12 degrees to a perfect 102 goes really quick and you’ll shoot past it in a hurry. This is also where the drain spigot comes in handy. Most nights we end up at 106 despite our best efforts and have to drain an inch off the tub and pipe in some cold water to get the balance right.

The better your wood, the less time it’ll take to heat your tub. If you’re lucky enough to have access to good hardwoods like maple or oak you’ve got it good. Here in North Idaho the best firewood we have access to is tamarack (larch), Douglas fir, and some birch. Lodgepole pine is pretty decent and I’ll burn it if I’ve got it. Grand fir and bull pine are barely worth the trouble to cut unless that's all you’ve got. I find it takes four or five pieces of good Doug fir cord wood split into 2" pieces to heat our tub from cold. It takes half the time or less when reheating the next day because our insulation keeps the water between 85-90 degrees in the summer (TBD in winter). If I start a small fire after we get out of the tub at night, the water is often still close to 100 degrees the next morning.

The Chofu is remarkably efficient and rips a hot fire in short order. Once it's really going, secondary combustion kicks in—that's when the heat of the fire is strong enough to ignite and burn off the smoke generated by the wood—and it burns really clean, generating almost no smoke. We’ve never seen sparks coming out the top, but we do get the occasional bit of ash floating down. Because of this, we may purchase the optional chimney cap to prevent anything hot floating into the trees.

The chimney, on the other hand, will burn the shit out of you. This was part of the reason we left the back corner of the frame open. This feature creates an intuitive visual and spatial barrier for anyone seated on the back of the frame, discouraging folks sitting too close to the chimney.

And there you have it. If you've read this far, you're all the wiser for it. Maybe even ready to get building your own DIY hot tub. Just remember, measure twice cut once. And many hands make like work!

Now check out how a self-taught woodworker built a DIY cabin in 55 days.

Published 05-05-2023