PSA for anyone considering shou sugi ban siding: the species underneath matters more than the char

Just replaced a client's shou sugi ban siding that was only 6 years old. The char looked great, deep, even, properly sealed. But underneath it was southern yellow pine and once moisture found its way in at the butt joints, the pine rotted from behind the char layer. From the front it still looked fine. From behind, it was crumbling. Lesson: shou sugi ban is a SURFACE treatment, not a preservation method. The char adds UV and insect resistance but does not fundamentally change the wood's durability class. Pine is still Class 5 (non-durable, <5 years ground contact) underneath that char. If you want shou sugi ban that lasts 25+ years, start with a Class 1 or 2 base species: - Cypress heartwood (Class 2) - Thermory/Abodo thermally modified (Class 1) - Accoya (Class 1) - Sapele or ipe (Class 1, overkill but very durable) Best combo I've found: Thermory spruce (already Class 1 from thermal modification) with shou sugi ban char finish. The thermal mod ensures zero rot risk and the char gives you the aesthetic. I source the Thermory from a specialty hardwood supplier, they stock it and can do the charring/texturizing at their facility so it shows up ready to install. Don't let anyone char pine or hemlock for your exterior and call it "shou sugi ban." That's ak lesson waiting to happen.