Building your 21 Cube

Set up the first two sections

Don't join the end walls yet. Start with the back section that has the power box on it — the unit that runs the heater, lights and speaker is already mounted to it, and its cable comes out at the back bottom, which tells you the orientation. Pair it with the matching front window section.

Pick the right window: the two front windows aren't interchangeable. Check which side of the door each one lines up with at the metal peg connectors. The door also has a smaller gap at the bottom, the step-over, which tells you which way is up.

Stand the two sections facing each other, one stave-length apart, with the obvious curved edge on the ground.

Set the first stave level

The first stave sets the line for everything after it. Put it on near the top, drive one screw into the first section, then measure from the ground to it. Go to the other section, measure the same height, and screw that end in.

Now your first stave is straight and square across, and you can work up and down from there.

Work up and down from that stave

Add staves above and below the first one until you have enough done to hold its shape — around the first bottom corner (11 thin staves) and onto the floor. Seat, tap both ends, screw.

Add the door section and the back center

Once the floor reaches the middle, add the door. It connects to the window section with two square metal pegs: press them in, then slide one direction to lock. There are pegs on both sides of the door, since it connects to both windows.

The back center section goes on at the same time and is simpler — no pegs. Line it up with the support boards already attached and screw it on.

The glass is very fragile. Have one person hold and guide the door section the whole time, keep it padded, and keep the mallet away from it.

Keel stave and floor across the middle

Carry the floor across the door section: 3 wide staves, the keel, then 3 more. The keel stave (bead on both edges) sits in the exact middle of the floor. Past the keel the curves face the other way, so each new stave's cove seats over the bead before it.

The floor is 14 wide staves plus the keel: 7 on each side of it.

Add the third sections

With the floor past the door, add the third front section (the other window, pegs and slide) and the third back section (screws into its support boards). The end walls are now complete.

Finish the floor and both bottom curves

Add the last 4 wide floor staves, then the second bottom corner (11 thin staves). You can prop the assembly up to reach more of the curve. The more of the curve you finish now, the less awkward work there is later.

Brace the far side with temporary staves

Before standing it up, screw one or two staves across the far end (what becomes the roof) so the front and back stay square and firm. Any stave will do — they come back off later.

Stand it upright, on its skids

Roll the assembly up onto its floor. With only about a quarter to a third of the staves on, 2–3 people can lift and shift it into position once it's upright.

Set the four black-painted 4×4s (included) down first, on the pad or wherever the sauna is going to live. They run side to side, evenly spaced from front to back, and the sauna stands up onto them.

Build up both side walls

Stack 14 wide staves up each side with the bead facing up. Add one on each side in turn so both walls rise evenly. Pull the temporary braces off as soon as they're in the way.

Round the top corners

Continue around both top curves with 11 thin staves each, still bead up and cove down.

Roof staves toward the middle

Work across the roof from both sides: 4 wide staves on each side, seated, tapped and screwed as usual. That leaves 7 for the middle — 3 wide on each side plus the crown.

Dry-fit the last 7 staves

Set the remaining middle staves — 3 wide, the crown, 3 wide — in place without any screws, slanted upward so the group sits as a shallow tent above the final line. Slanting them is what lets the last ones in — flat, they'd be too tight to start.

Press the group flat, then screw it down

Press the whole group down together until it lies flat and every joint is tight. That pressure is what forces the fit. You can stand on the group to press it down, or work it down with the mallet.

You'll be up on top of the sauna for this, so have the ladder set and a second person steadying it. Now screw all of them down, including the crown. The shell is finished.

Seal the exterior wood (optional, recommended)

Before any roofing goes on, it's worth sealing the outside. We suggest one gallon of Sauna Shield C2 Guard, a sauna-safe natural sealant.

Brush it on generously over all exposed exterior wood with a wide paint brush — one heavy coat, or two quick coats, following the label. One person, 30–50 minutes. Seal before the straps go on.

Fit and tighten the two metal straps

Two metal straps wrap the sauna and pull the whole shell together. They go on after the sealant and before the barrier and shingles.

Every stave has a screw at each end, so there's a row of screws near the front and a row near the back. Put one strap near each end, lined up to cover that row of screws. Put the tightening mechanism wherever you like, then crank it down hard.

Watch the front walls as you tighten: the strap pulls them together, closes any gaps between staves and brings the door into better alignment.

Water and snow barrier

The peel-and-stick barrier is included; you cut it to size. Measure the front-to-back length and add about 4 inches, so you get a 2-inch overhang at the front and the back.

Cut two pieces, one for each side. Roll each one down the slope, tar side down, letting it hang to where you want the shingles to stop — 65 inches from the ground is a good line. That leaves a gap along the top middle.

Cut a third piece the same length and run it across the top middle, overlapping both side pieces generously. Fold the overhangs tight and staple or nail them . The front and back trim covers them later.

Shingles

These are hexagonal bitumen shingles — strips with hexagon tabs, the type used on garden buildings. Fasten with roofing nails or staples .

Start at the bottom edge and work up. Stagger the hexagons course to course, and lap each course all the way down to the bottom edge of the one below, so only hexagons show — like the photos.

Nail only in the narrow areas between hexagons. A nail anywhere else won't get covered by the next course, and an exposed nail is a leak.

At the top, cut the last course to meet the shingles coming up the other side, then cap the middle with cut tabs. Warm sun helps the strips bend over the curve and seal down.

How much of each course should show (the exposure), per the supplier's sheet

Trim the front and back edges

The trim screws onto the front and back around the roof edges and covers the barrier overhang. It looks best with the inside face flush with the staves, so the trim stands proud on the outside. Final placement is the owner's call.

Benches and backrest

Straightforward: screw in the braces on the walls first, then screw the seats down onto them. Two tiers — a lower bench and an upper bench — plus the backrest.

The accent lighting is already built into the seat back. It plugs into the obvious plug; no wiring to figure out.

Mount the heater

Follow the heater's own manual for mounting and clearances. Load the stones as the manual shows.

9 kW · 240V single phase · 50A breaker. A wiring diagram is included. All hookup work should be done by a licensed electrician on a dedicated circuit, to local code.

Controls, thermometer and timer

The control module (the panel with the blue lighting) runs everything: heat, lights and music.

Put the thermometer on the side wall opposite the heater, at about shoulder height. The timer goes wherever the owner likes it.