wood thickness vs. expansion

The thickness doesn’t really matter. It’s all about the grain… a flatsawn board will move the most, that’s because wood moves the most with the tangential grain (flatsawn). Second most movement comes from quartersawn (radial grain), but it usually is ALOT less noticable than flatsawn (tangential) grain. If you’re that worried, try to get quatersawn, or riftsawn material. But then you have virtical lines for grain rather than cascade grain look. Also, like you said, it depends on the species of wood. You can pick up “understanding wood” by R. Bruce Hoadley as he describes in detail about wood movement according to species. Hope this helps.

-- Childress Woodworks