Due to the traditional through and through resaw pattern optimized for wood longevity and dimensional stability, as well as the nature of how wood dries, there is often cupping of approximately 1/16″ over the board widths. Therefore, we spec two face nails every 16″ OC to keep a flat plane. Note samples cup more than final product due to short lengths and low moisture content. Also keep in mind this is non-standard millwork and form follows function with shou sugi ban. Slight cupping is all you have to give up to achieve maximum wood longevity and dimensional stability in all other dimensions over decades of exposure without maintenance.