Repair & Maintenance · FAQ

Counter-flashing or step flashing — what’s the difference?

You need both. Step flashing is the L-shaped pieces woven into the shingle courses, providing the primary water seal between roof and chimney. Counter-flashing is a separate piece that mortars into the chimney’s brick joint and overlaps the step flashing, providing the secondary seal.

The step flashing typically lives 20 to 25 years in Illinois weather; the counter-flashing 30 to 40 years because it’s mostly out of direct UV. Trill replaces both during a chimney flashing repair if the home is more than 20 years from the last replacement; replaces just the step flashing if the counter-flashing still has good mortar engagement; and replaces just the counter-flashing if the step flashing was recently done by another contractor. Knowing which to replace is the experienced-eye part of the repair.