Wilmington chimney leak: it was the counterflashing, not the roof
Two roofers had caulked it. It still leaked. The counterflashing had never been let into the mortar joint, and no amount of caulk was going to change that.
The history
A 1990s house between Wilmington and I-55. Water stain on the ceiling next to the chimney every spring. A roofer caulked the flashing. A year later another roofer replaced a few shingles and caulked it again. Both times it held for a season and failed. The homeowner called us because someone suggested it might be the crown. The crown was fine.
How flashing is supposed to work
Chimney flashing is two layers. Step flashing is L-shaped metal woven into each course of shingles and turned up the chimney wall. Counterflashing is a second layer set into a groove cut in a mortar joint, called a reglet, and bent down over the step flashing. Water running down the brick goes behind the counterflashing, lands on the step flashing, and runs out onto the roof. The two layers are not sealed to each other; they overlap so the chimney and roof can move independently. On this house the counterflashing had been surface-mounted, laid flat against the brick face and caulked along the top edge. Caulk on brick lasts a season or two in an Illinois freeze-thaw. Then water goes behind it and straight down the chimney.
The rebuild
We removed the old counterflashing, cut a reglet into the mortar joint a full inch deep on all four sides, checked and reset the step flashing, and installed new counterflashing let into the reglet with a lead wedge and a flexible sealant that is protecting the joint rather than carrying the water. The chimney sits on the downhill side of a fairly steep roof and is wider than 30 inches, so we also built a cricket behind it to divert water and snow. No stain this spring. The roof was never the problem.
If this sounds like your house: we handle chimney repair and flashing across Kankakee County, and our Wilmington page covers what we usually find there. Call (815) 449-4264, 7am to 9pm, any day.
Quick questions
How do I know if it is the roof or the chimney leaking?
Stains right next to the chimney, worse after wind-driven rain, usually mean flashing or crown. We check both from the roof.
Why does caulked flashing fail?
Caulk on a brick face does not survive freeze-thaw. Counterflashing has to be set into the mortar joint.
What is a cricket?
A small peaked diverter behind the chimney that keeps water and snow from piling against it. Code requires one on wide chimneys.
Can you do flashing in winter?
Yes if the roof is safe to work on. It does not involve mortar curing.
Published October 6, 2026 · By Jacob Dove, Friendship Chimney Sweep, Bourbonnais IL