I'm building a large chimney for a residence. Normally, I don't use through-wall flashing, but simply an exterior stepped counterflashing set into a reglet (a sawcut or raked-out slot about 3/4 inch deep in mortar joints). This chimney, though, is 12 feet wide and extends about 10 feet above the roof surface, so a lot of masonry is exposed to the weather. Should I use through-wall flashing in this case? If so, how should it be detailed?
A through-wall flashing should be used. If not, wind-driven rain reaching the sides of the chimney can penetrate the veneer and outer wythe, causing efflorescence in the interior masonry or water damage. Before installing such a flashing, however, consult a structural engineer to make sure the smallest dimension of the chimney is large enough to withstand strong winds. Chimneys will attempt to bend under wind load at the roof line. For this reason, the highest stresses are located at this point. Through-wall flashing reduces the chimney's strength because it interrupts the bond.
Through-wall flashing should begin in the reglet in the wythe just behind the exterior wythe. Once installed, the flashing should pass through the wall and cover the top edge of the roof flashing (figure, right).The roof flashing should consist of a saddle on the high roof side of the chimney and step flashing along the sides. The step flashing should weave into the shingles (figure, below).
This figure is taken from the Architectural Sheet Metal Manual, published by Sheet Metal and Air Conditioning Contractors National Association Inc. (SMACNA).