When using dimensional lumber as floor joists, in what position should the crown be placed?

• A. Upward
• B. Downward
• C. Flat
• D. Sideways

Answer: (A)

The main idea is how the natural warp in lumber interacts with load. Lumber often has a crown, a convex bow along its length. When you use dimensional lumber as floor joists, placing that crown upward makes the joist start higher in the middle. As people walk on the floor, the joist deflect downward and tend to flatten. With the crown up, this downward deflection helps bring the floor surface toward level, producing a flat, stable floor. If the crown faced downward, the warp would tend to push the floor surface away from level, making a more noticeable unevenness. Flat or sideways would not take advantage of the wood’s natural tendency to flatten under load, leading to a less even floor.