This tutorial will be about what we used and how we went about making our porthole trailer Sauna. I will try to make it as short and to the point as I can. We have filmed the building and made it to a North & Beyond episode. Watch our sauna building video to the right/below or here on YouTube.
Firstly – what we used was based a lot on what was available to use up here in the Arctic (Tromsø Northern Norway). You may find other materials easier and cheaper to get hold of where you are.

The Trailer

We already had the 2t. aluminum trailer so we used this as a starting point. The trailer is about 450 cm long and 180 cm wide. It has the capacity of 2.000 kg to carry and is a two-axle trailer. It had the green tarpaulin cover and basic frame already. The snow had destroyed it so we completely got rid of the aluminum frame. I welded up a new frame from regular steel to fit under the cover.


Building the sauna wall frames

I started off making the wall frames with the studs divided evenly throughout the length. Making sure that there was one stud to take the join of the plywood sheeting on the outside. This steel frame made the Sauna extremely solid and guaranteed us that we can also drive long distance with it. All the frames were welded using a gas-less MIG welder. Nothing fancy, it was the cheapest one we could buy! And it worked like a dream.

I build the two sides and then the one end at the front first. These were all attached to the trailer by sitting directly on the walls of the trailer. They had an extension from the bottom rail on each end of the sides that continued down into four square 25mm holes into the original trailer walls. These four extensions were then screwed in through the aluminum walls.

The front end wall was then put in position, again, sitting on top of the existing trailer wall and clamped to the side walls ready for welding. Before we welded it all together we put the tarpaulin cover on to make sure it fit and made any corrections and adjustments needed. Once that was done I tacked only the frame into position. Taking it allows any changes to be made if it is needed. And we had to change it several times, since the tarpaulin cover was not fitting on our first try.

Steel Frame Materials

25x25mm SHS (square hollow section) for all the walls and the door.
20x20mm SHS for the “rafters” on the roof.

Steel frame for a trailer sauna
Steel frame or our trailer sauna

Making the curved roof

We now had the three steel walls up and it was time to set about the curved roof….. Its labour intensive!

I measured the width of the inside and the outside of the frame exactly. And we had worked out that in the middle the curve would be the highest at 200mm higher than the walls. I then transferred the measurements onto some graph paper to the largest scale that would fit and that was easy. Eg 1 square = 100 mm. I drew two drawings first from the side view and this gave me the curve of the roof from front to back. In doing so gave me the heights of the other rafters as I moved lower and to the ends. The second drawing was from the front view and it gave me the curve of each rafter from left to right.

I now had the shapes I wanted, So I transferred the scaled sizes into life size on a large full size piece of chipboard. I had the wall frames drawn on the board at full width, with a straight horizontal line joining them. From this line I could measure up in the middle and get mark the highest point of each rafter. Then having divided each side I could measure specific points evenly on each side and just join the dots to create the curve I needed. After doing so I could measure the exact length of each rafter (to the OUTSIDE of the walls) and cut it to length.

Having done so I made a series of cuts (about 12 to a length) nearly all the way through from the top side. This allowed me to bend the SHS easily to fit the curved shape. I screwed some screws into the chipboard along the line and bent the tube around it with some occasional screws on the top side to hold it tight in position while I welded it. Firstly I just tacked every cut in position then followed back through on the other side (still using the screws to hold it in position) and then completely welded everything, both sides. I continued this for each rafter. Also once it had all been welded to shape I was able to mark and cut a birds-mouth in the ends to lock the rafter into position on the top of the wall frames. Again we checked to make sure the tarpaulin would fit then all the rafters were welded into position. I welded a frame at the front to take the plywood on the outside and also to help support the chimney.


Making a curved Sauna roof
Our curved Sauna roof.
Sauna roof
Cladding the curved Sauna roof.

Inside Sauna cladding


On each rafter we glued and screwed a timber batten that was wider than the 20mm SHS. This was so we could easily screw the T&G boards on…

We used 12mm tongue and groove for the ceiling and we installed this first before the walls. Because of the curved roof we couldn’t use anything thicker than this 12mm. I heavily glued all the rafter battens then with the help of clamps and ratchet straps we clamped and strapped all the ceiling boards into place. One at a time and screwed them from the back through the timber battens we had already installed. This way there are no screws showing on the inside.

We did the same thing with the battens on the walls then used 20mm T&G boards to clad the walls. Screwing and Gluing as we went one at a time…


It was now time to make the rear wall where the door and windows are. This was made to slide inside and sit from ceiling to floor. We screwed it to the floor, the ceiling and the walls. Because we were lacking in height I needed to make the door before the wall was installed and install the whole lot together as there is not enough room to lift the door off the hinges and they needed to be welded onto the wall and the door. While making the door I also welded the frame (an octagon) to hold the porthole window in the middle of the top half of the door.

Sauna cladding
Installing the inside Sauna cladding

Framing the Sauna floor

After this I cut long wedges of timber to glue and screw to the floor at 450mm intervals. This allowed the water to flow to the middle of the floor then down and out the drain we would install later. I left room for the stone plate to go onto the floor to take the sauna oven. We needed to install the oven base and the oven at the same time so I welded two threaded rods to the bottom of the oven and drilled two holes through the floor on either side of the draw bar.

Installing the Sauna oven

The stone plate was placed on a layer of heavy Bitumen to absorb some shock then the oven sat on top with the threaded rods all the way through the bottom of the trailer. We then screwed that up tight with a length of 25mm SHS under the draw bar connecting the two threaded rods… Oven Super secure. After the oven was installed we could then proceed with the flooring working from one side to the other and secret screwing through the tongue half of the joints into the timber wedge battens on the floor.

The Sauna oven

We bought the Harvia M2 with a extra 40 liter water boiler, sauna stones and smoke pipe for about 500 Euro in Finland. We got a very good offer so you might end up with a slightly more expensive setup. The Harvia M2 is for a size of the sauna room is 6–13 m3. Using the Sauna now for some time, the oven is absolute the right size for this kind and size of a trailer Sauna.

Sauna oven M2
Installed Sauna oven in the trailer.

Sauna Windows


We are fortunate enough to have a glass cutter right next door to our office. He laminated two glass plates together and sold it to us for a great price. 2 windows (100cm x 30 cm) for 500 NOK. I then had to build the timber frame around the windows so that we could secure them in.
I made the timber frame so that it was a tight fit in the metal frame. And with my router out of one long length of timber for each window I routed a groove into one side of the length that was just 1/2mm wider than the window itself. I then cut 45 degree miter joints on each end to suit and put a generous bead of high strength clear silicone in all 4 grooves.
I squeezed them together hard and clamped them while I screwed the corner joints from the back side. Once they had dried I installed them in the back wall. Then I clad over them with the timer T&G boards inside and out. The door was a bit different. I had to clad it first then we cut out the hole for the porthole window then the holes were drilled and the window installed.

Sauna Windows
Installed Sauna windows and porthole.



The D- mouldings I made with a round-over bit on the router, running one pass from each side of a piece of timber then running it through the table saw to make individual D – corner moulds… buying them here was just far too expensive.. I also madde 90 degree external moulds to cover and finish the window edges by cutting an internal square block out of a length of timber again on the table saw, leaving me with an L shape to finish the windows with.


Sauna benches


The seating framework was made from large size timber that we cut to the desired widths on the table saw and then used the router with a round-over bit to finish the edges. They were then screwed into position all in place as everything had to be measured individually due to the sloping floor.


Sauna Electrics


Led lights run around a groove under the foot rest that are attached to a dimmer switch as well as a normal cut off switch.. as are the external led lights on the porch. These are connected to a 55AH AGM battery in an insulated box built in under the seat also with USB charging points and a built in charger which can be connected to mains power from outside without uninstalling anything. We also installed a tiny 40mm computer fan that runs off a temperature sensor to sucking outside air IF the battery box gets too warm inside. There are also vent holes in the floor and the wall to allow air circulation.


Air and Water


There is a 120mm air vent behind the oven on the floor to suck air in and another 100mm vent in the top of the door to let the old air out ( we would like to install another in the door)… this prevents the sauna from becoming stale and stuffy inside. We installed a drain in the floor under the door to drain the water out the back.


Metal flashings


We had custom fitting flashings made for the back of the sauna to finish off all the timber work and around the door frame. This is made from 0.7mm galvanised steel and just screwed into place, it can be cut with tin snips.


Henning and I hope that this brief post about how we built the sauna can be of some use to you all and at least give you some ideas if you want to try building your own, If you have any questions please don’t hesitate to contact us and we’d be happy to help out in any way we can.

If you would like to have a sauna built by us we can discuss what your requirements are and your budget and try to find something that will work for you.


If you would like to rent this sauna you can visit us at Rent-a-Sauna

Sauna light and benches
Installing the sauna lights under the benches
Sauna metal flashings
Installed metal flashing on our trailer Sauna.