Materials / Step and Counter Flashing
Step and Counter Flashing
accessoryStep and counter flashing form the watertight assembly used where an asphalt shingle roof plane meets a vertical sidewall, chimney, or dormer. Step flashing is a series of individual L-shaped metal pieces, one per shingle course, interwoven with the shingles as the roof is laid; counter flashing is the wall-side lap - siding, masonry counterflashing, or a cap flashing strip - that overlaps the vertical leg of the step flashing to complete the seal. IRC R905.2.8.3 (Sidewall Flashing) requires this base or step flashing be not less than 4 inches in height and 4 inches in width, and R905.2.8.1 requires corrosion-resistant metal of at least 0.019-inch nominal thickness, under the general flashing mandate of R903.2.
- per-course interweaving directs water continuously off the wall and onto the roof surface
- code-minimum 4-inch legs under IRC R905.2.8.3 fit most siding and masonry conditions
- corrosion-resistant metal at a minimum 0.019-inch thickness resists degradation for the life of the roof
- leaving the vertical leg unfastened to the wall lets roof and wall move independently without tearing the seal
- must be removed and reinstalled with every roof replacement rather than reused
- improperly lapped counter flashing at siding or masonry is a leading cause of sidewall leaks
- installed piece by piece with each shingle course, adding labor time versus continuous flashing
- masonry counterflashing needs a reglet or mortar joint, pulling in a separate trade
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- up.codes verified 2026-07-28