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A masonry chimney rising through a roof plane creates one of the more demanding intersections a roofer has to address, and counterflashing is the component that finishes the job started by step flashing and base flashing lower down. Where step flashing handles the sidewalls of the chimney and an apron or base flashing covers the front and back, counterflashing is the separate piece set into the masonry itself that laps down over the top edge of the step and base flashing, shedding water away from the joint and keeping it from working its way behind the assembly.
There are several accepted methods for installing counterflashing into masonry. One common approach involves cutting a horizontal reglet, a shallow groove roughly one inch deep, into a mortar joint of the chimney with a grinder, then inserting the bent top edge of the counterflashing into the groove and packing it with a compatible mortar or sealant. Another method uses surface-mounted counterflashing secured with masonry fasteners and sealant along the top edge, which is faster to install but generally considered less durable because it relies more heavily on sealant that degrades with sun and temperature cycling over time. A third approach, sometimes seen on chimney rebuilds, integrates a through-wall flashing membrane into new masonry courses as the chimney is constructed or rebuilt.
Material choice for chimney counterflashing generally mirrors the step and base flashing beneath it, with galvanized steel, aluminum, and copper all in common use, though copper is favored on some higher end installations for its longevity and the way it visually complements certain masonry finishes. Whatever the material, the counterflashing needs to lap over the top of the step flashing by a sufficient margin, generally a few inches, so that thermal movement in the roof system does not open a gap between the two components during a hard freeze thaw cycle.
Because counterflashing is set into or against masonry rather than into the roof deck, its condition and its replacement scope are frequently evaluated somewhat separately from the shingle field itself. A chimney with cracked mortar joints, spalling brick, or a deteriorated crown may need masonry repair independent of any roof replacement, and the counterflashing reglet may need to be recut and reset if the chimney is repointed. Conversely, a chimney in sound structural condition may only need the counterflashing removed, the underlying step and base flashing replaced, and the counterflashing reinstalled into the existing reglet, which is a narrower scope than full chimney work.
Inspecting this detail in the field usually means looking at the reglet for gaps, checking whether sealant has failed and separated from the masonry, and examining the step flashing legs below for corrosion or hail-related deformation. Photographs from a ladder or drone, along with a physical check of mortar hardness near the reglet, are typical documentation methods used by roofers, adjusters, and appraisers alike. Determining what a reasonable repair or replacement scope looks like at a given chimney comes from what that inspection shows, since chimneys of similar age and design can be in very different physical condition depending on prior maintenance, exposure, and construction quality.
This article is general education about how the appraisal process commonly works. It is not legal advice, and specific procedures can vary by state and policy.
Russ Lis is a working property insurance appraiser and umpire based in Minnesota, serving clients nationwide. Contact Appraisal Resolution.