No shou sugi ban isn't 100% waterproof it is water resistant. Although some say that charring it does weather proof the wood I would lean more towards thinking of it as weather resistant. After charring the wood you should seal it with either natural oils or polyurethane which will also help with weatherproofing it.
Best Wood Preservation Techniques Mother Earth News. 28 Oct 2010 a new coat of waterproofing on wood surfaces that are exposed to the elements. Tree preservation and timber management on a small woodlot. . After charring the outside in a pit fire we used a large propane torch to. Get-Prices
Wood charring also known as Shou sugi bun refers to the method of timber colouring and preservation which is conducted through applying flames to the surface of the timber planks boards. The flame lightly chars the exterior of the timber planks creating a visually appealing natural look which can be tailored to look slightly grey charred to deep black in appearance.
2018. 6. 27. Setting wood on fire may not seem like the most logical way to make it stronger but the ancient technique known as ’shou sugi ban’ has proven its effectiveness time and time again. Charring any species of wood (usually with a blowtorch these days) wraps it in a layer of carbon that protects it from moisture mold insects and even fire.
2020. 4. 11. The value in charring wood surfaces goes beyond visual effect. In Japanese building charring wood surfaces is known as shou-sugi-ban; this technique is valued because it wraps up wood in a layer of carbon that's highly resistant to mould insects water and even fire. It also creates a powerful visual effect.