Step Flashing Along Walls and Dormers: Where Midwest Roofs Leak Most Often

When a Midwest roof leaks, the wet spot on the ceiling rarely sits under an open field of shingles. Far more often, the trouble traces back to a spot where the roof runs up against something vertical: a sidewall, a chimney, or the cheeks of a dormer. These transitions are the hardest-working joints on the whole roof, and the piece of metal that protects them is step flashing. Get it right and the joint stays dry for the life of the roof. Get it wrong and water finds its way behind the siding within a few hard seasons.

Step flashing is a series of small L-shaped metal pieces, each one bent roughly 90 degrees so that one leg lies on the roof deck and the other leg runs up the wall. The pieces are not one long strip. They are individual squares, typically around five by seven inches, and they are woven into the roof one at a time. Each piece tucks under the shingle course above it and laps over the shingle course below it, so the entire run of the wall is shingled with metal underneath the visible roofing. Water that runs down the wall hits a piece of flashing, gets directed onto the top of a shingle, and sheds down the roof instead of slipping behind it.

The reason this matters so much in Illinois and the broader Midwest is the sheer variety of weather a wall joint has to absorb. Wind-driven rain pushes water sideways and upward against the wall. Freeze-thaw cycles work moisture into any gap and then expand it. Snow piles against the wall line and melts slowly, sitting on the flashing for days. A single continuous strip of bent metal, which is what cut-rate crews sometimes substitute to save labor, cannot handle that movement. It looks tidy on day one, but it has no way to shed water at each shingle course, and the caulk holding it eventually cracks and lets water run straight down behind it.

Dormers are where step flashing gets tested hardest because a dormer creates several of these wall transitions at once. The sidewalls of the dormer need a full run of step flashing. The front wall needs apron flashing where it meets the lower roof. And the points where the dormer roof ties into the main roof create valleys and corners that have to be flashed in the correct order. The most common failure we find on Midwest dormers is at the bottom corner, where the sidewall flashing meets the front apron. If those two pieces are not lapped correctly, that corner becomes a funnel that channels water directly into the wall cavity, and the resulting stain often shows up on a ceiling several feet away.

A detail that separates a lasting installation from a future leak is the relationship between the step flashing and the siding. Step flashing belongs behind the siding or behind the wall cladding, never face-nailed on top of it and smeared with sealant. When the metal sits behind the siding, water running down the wall is forced out over the top of the flashing and onto the roof. When someone surface-mounts the flashing and relies on caulk to bridge the gap, that caulk becomes the only thing standing between the house and a leak. Caulk is a maintenance item with a short life in our climate, so a joint that depends on it is a joint on borrowed time.

There is also the matter of what happens during a reroof. Flashing is not automatically replaced when shingles are. Some crews reuse old step flashing to save money and time, bending the existing pieces back into place under new shingles. Old galvanized flashing that has already rusted at the fold, or aluminum that has worked loose from years of expansion, should come out and be replaced with new metal as part of the job. Reusing tired flashing under a brand-new roof is one of the quieter shortcuts in the trade, and it is the reason some homeowners see a wall leak appear within a year or two of paying for a full replacement.

For a homeowner trying to judge a roofing proposal, wall and dormer flashing is worth asking about by name. A straightforward question, such as whether the step flashing will be new and woven into each course, tells you a great deal about how the crew works. A contractor whose answer is specific, who talks about lapping the corners and tucking metal behind the siding, is describing the method that actually keeps a Midwest roof dry. An answer that leans on sealant and surface-mounted strips is describing a repair that will need to be repeated.

None of this is exotic work. Proper step flashing is a long-established roofing practice, and a crew that has done it correctly for years can install it without drama. What it requires is the discipline to do it the slow, correct way at every wall and every dormer, instead of reaching for a tube of caulk when the angles get awkward. Those awkward angles, the dormer corners and the sidewall transitions, are exactly where Midwest roofs leak most often, and they are exactly where careful flashing earns its keep.