Q: Most chimneys I see have a counterflashing that is set into a reglet in the masonry. I don't know that I have seen anybody use through-wall flashing at chimneys. Why is through-wall flashing required in some walls but not chimneys?
A: In most cases, chimneys are fairly small and the masonry extends down through the roof to the firebox and eventually to the foundation. It is true that through-wall flashing is rarely used in these chimneys. The problem with adding through-wall flashing is that you disrupt the bond at the base of the chimney. Since the portion of the chimney that rises above the roof functions as a cantilever, the bond cannot be disrupted or else the chimney may blow over under high winds.
If the chimney is small and relatively straight, problems with water penetration and leakage are rare. Water that does penetrate is generally absorbed by the masonry below. Metal counter-flashing sections are typically inserted into saw cut reglets to seal the edge between the masonry and the roof base flashings.
However, in larger chimneys, through-wall flashings are often provided. The interior masonry is designed to handle the load structurally and the veneer is not counted on when calculating the resistance of the chimney to wind loads. The through-wall flashing can extend from the face of the back-up through the wall underneath the veneer above the roof flashings. In these larger chimneys, reduction of bond area either from a saw-cut reglet in the masonry walls or from a through-wall flashing should be included in any calculations to determine the overturning capacity of the chimney.