# Small Wood Sauna Cabin Construction How a small backyard sauna cabin is built layer by layer, from a square floor and framed walls to foil insulation, cedar lining, a sloped metal roof and the heater\. By Thibault Terray Topic: Engineering [Open the interactive Jig](https://ask.jig.com/j/p5Mgx4fML4iU) ## Transcript ### 1. A tiny room for heat This little wooden cabin is a backyard sauna: a hot room where people sit, sweat and relax\. It is about 1\.5 metres wide, 1\.2 metres deep and 2\.3 metres tall, roughly the size of a garden shed\. Let us build it layer by layer and see why each layer is there\. Drag the scene to look around\. Scroll or pinch to move closer\. ### 2. A floor off the ground Everything starts with a strong, flat floor\. An outer frame of pressure\-treated 2x4 boards, wood soaked with a preservative so it resists rot and insects, is screwed together at the corners with long outdoor deck screws\. Measure both diagonals: when they match, the frame is square, which means every corner is a right angle\. Then cross pieces called joists are screwed in to stiffen it, and a sheet of plywood is screwed down on top\. Common practice, not from the plans: fix the plywood with a screw about every 20 centimetres into each joist\. Treated wood is used only down here, never inside where people sit\. Look for the small silver screw heads: they go in right after the boards they hold\. ### 3. Frame the walls Each wall begins as a skeleton of studs: upright 2x4 boards about 40 centimetres apart\. Long deck screws go through the top and bottom boards, called plates, into the end of every stud\. The front wall leaves a gap for the door, with a strong header beam screwed in above it\. The front wall is taller than the back, so the roof will slope and rain will run off behind\. Common practice: frame each wall flat on the ground with two screws into each stud end, and check its diagonals match\. Later the finished walls stand on the floor, are clamped square and joined at the corners with long structural screws\. Look for the silver screw heads at plates and corners\. ### 4. A tough outer skin Grooved plywood siding, called T1\-11, is nailed to the outside of the frame with galvanised siding nails, which are coated so they do not rust\. Check the wall is square first, then nail into every stud, so the sheet locks the frame square for good\. The siding stiffens the box and keeps out wind and rain\. Pale cedar boards are screwed over the corners and around the doorway\. Common practice: a nail about every 15 centimetres around the sheet edges and every 30 centimetres in the middle\. The siding is stained dark charcoal, so the cedar trim stands out\. Look for the rows of nail heads along each stud and the pairs of screws in the trim\. ### 5. Keep the heat in Foam boards are cut to fit snugly between the studs, like a warm coat, so they stay in place without glue or fixings\. Next, shiny foil is rolled over the foam, shiny side facing the room, and stapled to the studs\. Every seam is sealed with foil tape, a metal tape that copes with heat; plastic or duct tape is not used\. The foil bounces heat back into the room and keeps damp air out of the wooden frame\. Common practice: overlap each foil strip at least 10 centimetres over the last one\. One wall is cut away here so you can see the layers: siding, frame and foam, then foil\. Look for the tiny staples running down each stud\. ### 6. Line it with cedar First, thin wooden strips are fixed over the foil along every stud\. They hold the cedar about 6 millimetres away from the foil, so air can dry the wood after every sauna\. Then thin cedar boards are nailed across the strips, one row at a time\. Cedar is chosen because it stays comfortable to touch, smells good and copes with heat and moisture\. The boards are shiplap: their notched edges overlap, so there are no straight cracks for heat to escape\. Common practice: start at the floor with a level first board and nail each board into every strip\. Look for the screws in each strip and a small nail head on every board\. ### 7. Put on the roof Rafters, the sloping roof beams, are screwed in from the tall front wall down to the lower back wall, and cedar boards line the ceiling underneath\. Plywood is screwed on top, then a sticky waterproof sheet called ice\-and\-water shield is stapled over it\. Ribbed metal panels go last, sealed with butyl tape, a sticky rubbery tape, and fixed with roofing screws that have rubber washers\. The slope sheds rain, and the overhang keeps it off the walls\. Suggestion: follow the panel maker’s screw spacing and tighten until the rubber washer is snug, not squashed\. The hottest air collects under the ceiling, so it gets the same insulation and cedar as the walls\. Look for the steel rafter ties and the roofing screws with black washers\. ### 8. Benches, heater and door Two slatted benches step up the back wall\. Their frames are screwed to extra blocks built into the wall, and the slats are screwed down with small gaps, so air flows and the wood dries\. Common practice: sink screw heads below the wood, because hot metal can burn skin\. In the corner, an electric heater with stones on top sits inside a wooden rail so nobody touches it, and a cedar door with four glass panes is screwed on with hinges\. An electric heater needs its own circuit, fitted by a qualified electrician, and the clearances in the heater maker’s instructions\. Look for the screw heads on the bench slats, the guard rail and the door hinges\. ### 9. Make some steam The heater warms the stones and the air, usually to about 70 to 100 degrees Celsius\. Pour water on the hot stones and it bursts into a cloud of steam that Finns call löyly\. Hot air rises, so the top bench is the warmest seat and the lower bench is cooler\. A sauna is never sealed tight: vents let fresh air in and out, sized from the heater’s manual\.