Chimney Flashing Systems: Step Flashing, Counter Flashing, and Reglet Cuts

The Complete Four-Part Flashing Assembly

A code-correct chimney flashing system includes: an apron (base) flashing at the front face where the downslope roof meets the chimney; step flashing woven into each shingle course up both sidewalls; counter flashing embedded in the masonry overlapping the step flashing; and a cricket or saddle behind the chimney when it is wider than 30 inches (required by NCRC R905.2.8.3 for chimneys wider than 30 inches on the upslope side).

The cricket diverts water around the back of the chimney instead of allowing it to pond against the masonry and search for a seam.

Step Flashing Woven Shingle by Shingle

Each step flashing piece is a small L-shaped sheet tucked under one shingle course and lapped over the one below, creating a shingled waterfall up the chimney's sides. Pieces must be secured only by the overlying shingle—nails through exposed flashing defeat the lamination.

On premium architectural jobs, AGCS uses pre-bent aluminum or copper step flashing sized to the shingle exposure.

Counter Flashing and the Reglet Cut

Counter flashing is the sheet metal embedded in the chimney's masonry that overlaps and sheds onto the step flashing below. The correct installation is a reglet cut—a saw kerf ground into the brick mortar joint about 1 to 1.5 inches deep—into which the counter flashing's top edge is inserted and sealed with polyurethane or silicone sealant (AGCS standard: MasterSeal NP1 or SikaFlex).

Surface-stapled counter flashing caulked to the brick face, a common shortcut, peels away within a few freeze-thaw cycles.

Mortar Crowns and Sump Pans

Water often enters chimneys from the top, not the sides. A cracked mortar crown lets water saturate the flue-to-masonry joint. AGCS evaluates crowns, flue caps, and—on older masonry chimneys—the condition of the firebox sump pan during every roof replacement estimate.