The idea had been in the back of my head since we poured the basas. The shala fascia boards were going to be exposed to weather on three sides, and paint felt wrong for a building that's supposed to feel like it grew out of the hill rather than landed on it.
Shou sugi ban — or yakisugi — is the Japanese technique of charring wood to seal it against moisture, insects, and rot. It's been used on rural architecture in Japan for centuries. In Costa Rica, you don't find much written about it. We were figuring it out from scratch.
What we used
The lumber is local cedar (Cedrela odorata), bought at a ferretería in Ciudad Colón in 1×6 planks. It's not the same species as the Japanese cedar used traditionally, but it's close-grained, aromatic, and takes heat well.
For the torch we used a standard plomero propano torch — the wide-head kind you'd use for sweating copper fittings. You can do it with a bonfire and a lot more patience, but the torch gives you control.
Tools:
- Wide-head propane torch
- Wire brush (brass, not steel — gentler on the grain)
- Linseed oil for finishing
- Two sawhorses and patience
The process
The goal is an even char across the surface, not a fire. You're looking for the wood to bubble slightly and turn from brown to black, but you pull the flame back before it cracks or catches.
Pass 1 — low and slow, about 15cm from the surface. Get the wood hot. You'll see the grain start to open as the softer early-wood burns faster than the dense late-wood rings.
Pass 2 — closer, quicker strokes. This is where the char deepens to the color you want. For the shala fascia we went moderately dark — not full ebony, more a deep charcoal with visible grain texture.
Wire brush — once the plank cools, brush vigorously along the grain. This removes the loose char and reveals the texture. The contrast between the black char and the now-exposed lighter rings is what makes the wood beautiful.
Linseed oil — one coat, wiped off after 15 minutes. Protects the surface and deepens the color without glossing it.
The smell while burning cedar is remarkable — resinous, warm, almost medicinal. Not unpleasant at all. The workers thought we'd lost our minds. By the afternoon they were asking to try it.
What went wrong the first time
The first plank caught. We held the flame too long on a knot — the resin pocket ignited and we had a small fire rather than a controlled char. The plank was ruined for fascia but made excellent kindling.
The fix: keep moving. Never hold the torch stationary. If you see a dark spot developing faster than the rest of the board, you're too close or too slow.
Results
We've done about 40 linear meters of fascia now. The color is consistent, the grain is textured and beautiful, and the wood feels harder than it did before treatment — the char layer is genuinely protective.
Total material cost above the lumber: roughly ₡6,000 in propane. The linseed oil was ₡4,800 for a liter, which covered everything with some left over.
If you're building in Costa Rica and want a natural finish that doesn't need repainting every two years, this is worth the afternoon.
Next up: the deck boards. We're debating whether to go full char or leave them natural for grip. The full char is beautiful but we'll see how it holds up underfoot.