Chimney Leak and Woven Valley After a Slate-to-Shingle Reroof
A 1890s Victorian in Salem has a 12:12 main roof that was reroofed last fall by another contractor, converting from slate to architectural asphalt shingles. The masonry chimney is 44 inches wide, sits on the main slope, and is coated with stucco; the old counterflashing was reused and bedded in roofing cement. The homeowner reports water staining on the second-floor ceiling in a band running from the chimney downslope, worst during rain-on-snow events and after wind-driven rain from the northeast. The valley between the main roof and a cross gable was woven with the shingles over one layer of felt, and the same ceiling stain tracks along the valley line. The prior contractor's proposed fix is to trowel mastic over the chimney base and glue an EPDM patch into the valley. You are asked to diagnose and price a correct repair.
1. Diagnose the chimney leak and specify the flashing assembly that should have been installed. Explain why the mastic and patch approach fails.
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Diagnosis. A 44-inch-wide masonry chimney on a slope needs a cricket, because chimneys wider than about 30 inches create a dam that backs water and snowmelt upslope. This chimney has none, so water ponds against the masonry and finds the reused, cement-bedded counterflashing. Cement and mastic crack and lose bond as the chimney and roof move differently, and the northeast wind drive pushes water directly at that joint.
Correct assembly. Build a framed and flashed cricket on the upslope side, with a valley that directs water around the chimney. Install new base flashing and step flashing in corrosion-resistant metal, stepped with each shingle course and lapped so upper pieces shed over lower ones, with each shingle course lapping over the step flashing leg. Cut a reglet or mortar joint and let the counterflashing at least 1 inch into the masonry, then seal the top edge with a compatible polyurethane sealant, not surface mastic. Use lead or copper where the flashing meets masonry on this building. Install ice-dam membrane along the chimney sides and upslope before flashing. Repair the stucco and crown so water does not enter the masonry itself.
Why the patch fails. Mastic is an adhesive, not a flashing; it cannot accommodate thermal movement, degrades under ultraviolet exposure, and traps water behind itself. An EPDM patch at the chimney base does nothing about the missing cricket and the reused counterflashing, so the water path remains. Both proposed fixes convert a repairable flashing defect into hidden rot.
2. Evaluate the woven valley and describe the correct valley detail and material for this roof.
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Evaluation. Woven valleys are permitted for asphalt shingles by some manufacturers, but they are a poor choice on a 12:12 roof in a snow country with ice damming and a history of interior staining. A woven valley has no positive channel, the shingle edges cross the flow line, and every nail penetration near the centerline sits where water concentrates. One layer of felt underneath is minimal protection if ice backs up.
Correct detail. Strip the valley back several courses on both sides. Install ice-dam membrane along the valley centerline over the deck, then a corrosion-resistant metal valley flashing at least 24 inches wide, centered on the valley line, with a stiffened center rib or crimp to control flow and hems that do not puncture the membrane. Set the shingle courses so they stop short of the centerline and are trimmed parallel to it, with the upslope-side shingles lapping over the flashing and the downslope side trimmed clean. Fasten shingles away from the centerline so no fasteners penetrate the metal in the water path. Where the valley meets the wall or chimney, run the flashing uphill into the wall flashing rather than terminating it in the field of the roof.
Judgment. Because the valley and chimney share a stain line, repair both in one scope. Doing the valley alone will leave the owner believing the leak persists.
3. How do you verify the repair is complete before you close it out and collect final payment?
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Verification plan. First, document the existing conditions before repair: photographs of the flashing, the cricket area, and the valley after tear-off, keyed to the stain locations inside. Second, inspect in stages. Photograph the ice-dam membrane, the cricket framing, the step flashing at each course, the counterflashing reglet depth, and the metal valley before any shingle covers it. Third, water test. A controlled hose test at the chimney base and valley, run at low volume for 20 to 30 minutes from the upslope side, with a second person inside watching the stained area, is the practical field check. Do it before final payment and after the flashing is complete. Fourth, schedule a building inspection if a permit was required for the repair, and get a written statement from the owner that the interior staining has not recurred after the next significant rain.
Closing out. Provide a written scope describing the cricket, counterflashing, and valley work, with photos and material specifications, plus the manufacturer's flashing and sealant compatibility note. Do not offer a no-leak guarantee on the chimney without addressing the stucco and crown, since masonry saturation can mimic a flashing leak. If the ceiling stain returns after the repair, the next step is an interior inspection of the sheathing and chimney framing, not more sealant.