New Flashing Leaks At First Storm: Decision Tree

Why this matters

Flashing is where most roof leaks live, and freshly installed flashing that leaks on the first storm is almost always a detailing or sequencing error, not a material failure. The metal is new, the sealant is fresh, so the leak is telling you the water is getting behind the flashing rather than through it. Diagnosing this correctly means thinking like water: it follows gravity, then capillary action, then wind pressure, and it exploits any lap that runs the wrong direction. A flashing callback solved by lapping order and let-in details stays fixed; the same callback solved by a bead of caulk comes back next season.

Symptom presentation

The leak shows up at or near a roof-to-vertical transition: a sidewall, a headwall, a chimney, a skylight curb, a parapet, or a dormer cheek. Interior staining typically appears below or slightly downslope of the flashed feature, often at the top of an interior wall or ceiling corner. A telltale of a flashing miss versus a field leak is that the water volume correlates with rain direction and intensity rather than total duration: wind-driven rain against the vertical surface pushes water behind a poorly lapped flashing leg.

Quick checks

• Identify which vertical feature the leak sits under. Stand inside during rain if possible and watch the highest wet point.
• Note wind direction during the leaking storm. A sidewall that only leaks when wind drives rain at it points to an open step-flashing lap or a missing kick-out.
• Look for a kick-out diverter at the bottom of every sidewall that terminates at a roof edge or gutter. Its absence is one of the single most common new-flashing leaks.
• Check whether counter-flashing is surface-mounted and caulked or actually let into a reglet or behind the cladding. Surface-caulked counter-flashing on masonry is a first-storm suspect.

Isolation tree

Branch 1: Sidewall (rake-side wall running up the slope). Pull the lowest shingle course at the wall. Each step-flashing piece must be one per shingle course, bent over the wall and onto the roof, with each piece overlapping the next downslope piece by the headlap of the shingle. Failure modes: continuous L-flashing substituted for steps (water rides under it), steps face-nailed through the vertical leg into the roof (creates a hole), or no kick-out at the eave so the wall sheds water behind the gutter and into the wall cavity. The kick-out absence is confirmed by water in the wall below the eave, not on the ceiling.

Branch 2: Headwall (wall running across the top of the slope). The apron/head flashing must lap up the wall, out over the shingles, and tuck under the wall cladding or counter-flashing. New-install failure: the head flashing nailed to the roof shingles on its downslope leg and caulked at the top instead of being run under the cladding. Water gets behind the top edge.

Branch 3: Chimney. Confirm four pieces are present and correctly ordered: base/apron on the downslope face, step flashing up both sides, head flashing or a cricket on the upslope face, and counter-flashing let into the mortar joints over all of them. The two dominant new-chimney leaks are a missing cricket on a chimney wider than roughly 30 inches across the slope, and counter-flashing surface-sealed to the brick rather than cut into a reglet.

Branch 4: Skylight curb. Verify the head, sill, and step (jamb) flashing kit was installed in order with the head flashing tucked under upslope shingles and the sill apron over downslope shingles. A curb leak that is actually condensation (clear water, no rain correlation) is a different problem and belongs on a separate diagnostic.

Branch 5: Parapet or coping on a low-slope roof. Check that the membrane base flashing runs up the wall to the required height and is terminated under a counter-flashing or coping, and that coping joints are covered by splice plates. A leak directly below a coping joint with no splice plate is the source.

A field reminder on sequencing: most first-storm flashing leaks are not a missing piece of metal but a piece installed in the wrong order. Flashing only works when each upslope component overlaps the one below it and the cladding overlaps the top. A crew that nailed the head flashing on first and then ran siding over it in the wrong sequence produces a detail that looks finished from the ground and leaks from the top edge. When you open a detail, verify the order of operations, not just the presence of the parts.

Confirming diagnosis

Water-test from the bottom up, one flashing leg at a time, with a helper watching inside. For a sidewall, wet the eave detail and kick-out zone first, then walk uphill course by course. For a chimney, test the downslope apron, then sides, then the upslope cricket last, since that is the highest-pressure point. Hold each zone several minutes. When the interior wets, isolate by drying and re-testing the single suspected lap. Document the failing lap with a photo before remediation.

Remediation

Replace the wrong detail with the right one. Install true step flashing where continuous was used. Add a kick-out diverter at every sidewall-to-eave termination. Re-run head flashing under the cladding. Cut reglets and install let-in counter-flashing at masonry. Build and flash a cricket where one was omitted. Pull face-nails from vertical legs and refasten through the roof leg only, under the next course. After repair, water-test the corrected detail and update the closeout flashing photos.

References

• 2021 International Residential Code R903.2 (flashing) and R905.2.8 (flashing for asphalt shingles), including valley, wall, and penetration flashing requirements.
• NRCA Roofing Manual: Steep-slope Roof Systems, base, step, head, and counter-flashing detailing; chimney cricket sizing guidance.
• SMACNA Architectural Sheet Metal Manual, flashing and counter-flashing fabrication and reglet detailing.
• ASTM D5957, Standard Guide for Flood Testing Horizontal Waterproofing Installations (controlled water-test methodology).