DO THIS, NOT THAT
When to avoid mitering.
Most joinery is designed to accommodate the movement of wood—expansion and shrinkage—due to changes in temperature and moisture content. For example, doors are designed with unfastened panels to mitigate the seasonal movement of the stiles and rails and the panels themselves; held tight, joints would open or panels crack. Other joinery is not as easy to conceal. Consider the miter joints on door casings. Seasonal wood shrinking across the grain always causes the joint to open on the inside corner; high humidity causes the joint to open on the outer edge. A flexible filler and paint typically are used to mask the problem, and it’s no wonder corner blocks became popular, as they make a miter unnecessary. Suitable joinery becomes even more important outdoors, as in porch floors.