Why is the direction of wood grain important when building with wood? Wood is a natural substance that is much stronger when the grain is continuous. Wood is a natural polymer. That is, it consists of parallel strands of cellulose fibers held together by a lignin binder. Think about lining up thousands of straws all lined up and packed together. One straw is weak but altogether, they can be quite strong.
When you split wood with the grain, you’re breaking lignin bonds (easy); when you break across the grain, you’re snapping cellulose fibers which is much harder.
To take full advantage of a wood’s strength, pay attention to the grain direction. Always orient the grain so the fibers support the load. Whenever possible, cut the parts so the grain is continuous, running the length of the board.