What's the Difference Between a Check and a Crack in a Wood Framing Member?
A check is a split in wood that runs parallel to the grain, across annual ring growth. It is also described as the separation of wood tissues along the grain of sawn lumber. So, if you're looking at a rafter, a check would run along its length. A crack runs across not parallel to the grain. So, if you're looking at that same rafter, the crack is vertical or at an angle.
What you need to know: A check is naturally occurring and is caused by shrinking during drying or seasoning. Naturally occurring meaning that the structural member's place in the structure did not cause the check. In an older house, the check has been there a long time. You can tell because the inside of the split is as aged as the rest of the wood. If the check is in a framing member that isn't otherwise defective or deformed you should carefully examine the evidence before you recommend that it be repaired.
On the other hand, a crack is a defect caused by something that went wrong. A crack and its cause needs to be corrected.