Douglas Fir Lumber is ideal for a variety of residential projects. Flat grain lumber is milled with the annual growth rings running parallel to the face, creating a unique and prominent grain pattern on each piece. Since knots start in the center of a tree and grow outwards, any knots found in flat grain lumber generally will be round or oval-shaped. Flat grain lumber is a perfect option for most projects. It is cost-effective, maintains similar durability properties, and has more character with its visible wavy grain and round knots.