Which wood species is best for interior doors: oak, ash, walnut, teak or birch?

Oak is the safest all-rounder for stability and grain; ash gives a pale, linear Scandinavian look; walnut is a premium feature species with strong tonal variation; teak is the traditional choice for humid and coastal regions; birch and engineered birch veneers give a light, uniform, cost-stable surface. In practice, construction and sealing matter more than species — an unbalanced panel in any species will still bow.