The first course of brick should always have at least two-thirds (2/3) the thickness of the brick wythe (unit) bearing directly on its support. This keeps the weight of the brick masonry over the support. This limitation applies to foundations and shelf angles and is consistent with the corbelling requirements of the International Residential Code and International Building Code. Projecting more than one-third (1/3) the thickness of the brick wythe can lead to wall instability, cracking of the brick masonry and failure by collapse. Occasionally, the differing construction tolerances of various materials and errors in construction make achieving adequate bearing challenging.