Counterflashing in brick
St. Louis is a brick town, and its roofs pay for it in joints. A chimney, a dormer cheek, a rear addition tucked against the original brick wall: every one of those is a place where a flexible, layered roof meets a rigid masonry face, and the seam between them is the most engineered detail on the house.
- 1 Ridge caps. First to fatigue in a climate that swings hard in both directions.
- 2 The field. Thaw cycles work the covering harder than the calendar suggests.
- 3 Valleys. Litter holds water against the covering exactly where it should be leaving.
- 4 Flashing. The chimney, and where a lower roof runs into a brick wall.
- 5 The vent boot. A rubber part with a life measured in years, not decades.
- 6 Eaves and gutters. Refreezing and a plugged box gutter back water up under the edge.
Two metals, one joint
The seam is built in two parts. Step flashing weaves into the shingle courses and turns up the brick; counterflashing is let into the masonry itself, set into a cut mortar joint, and folds down over the step flashing like a shingle over a shingle. Water running down the brick face is handed outward at every course. When it is done well, the joint outlives the covering around it.
The failure modes are specific. Counterflashing surface-mounted onto the brick with a bead of sealant instead of let into a joint depends entirely on that sealant, which this climate’s thaw cycles chew through in a few seasons. And old lime-mortar joints, common in the pre-war stock, can be soft enough that the flashing’s anchorage crumbles, a masonry conversation as much as a roofing one.
The re-roof shortcut to watch for
On a replacement, new step flashing is not negotiable. It is woven into the new courses. Counterflashing is where bids quietly diverge: cutting and resetting it in the mortar is careful, slow work, and a thin bid reuses the existing metal, bent up and bent back down, or caulks over the old joint. Both put a brand-new roof’s most vulnerable seam in the hands of its oldest and most disturbed component.
A chimney adds one more layer: the brick above the roofline weathers faster than the sheltered wall below, and a chimney whose joints are eroding will defeat any flashing attached to it. On the region’s older homes it is worth having the chimney’s condition stated in the roofing quote: a good roofer will name what is theirs to fix and what belongs to a mason, in writing.