Chimney Flashing on a 5/12 Steel Shingle Roof
A contractor is installing 24-gauge steel shingles on a 5/12 roof. A masonry chimney is 28 in. wide measured perpendicular to the slope, 8 ft tall above the roofline, and its uphill face sits 3 ft from the ridge. The contractor proposes to run the shingles tight to the chimney, install surface-mounted 4-in. step flashing over the shingles, and seal the top edge with polyurethane caulk. No cricket is planned because the chimney is less than 30 in. wide. The roof has a 6-in. eave overhang and new edge metal. The homeowner has had a leak at the chimney during wind-driven rain. The contractor says caulk will solve it and that a cricket is required only for chimneys wider than 36 in. The crew has cut the step flashing in 8-in. lengths and nailed it through the top face.
1. Evaluate the contractor’s flashing plan and cricket claim. Identify what is wrong and what is recommended for this chimney.
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The plan is unacceptable. Surface-mounted step flashing laid over the shingles and sealed at the top is a lap in the wrong direction: water running down the roof can get behind the flashing. The step flashing must be integrated with the underlayment and each shingle course, with the upper piece lapped by the shingle above, and it must turn up into the masonry. A counterflashing cut into a reglet in the chimney, with a sealant that is not the primary barrier, is needed. The chimney is 28 in. wide, so the common 30-in. threshold does not require a cricket; the contractor’s statement that the threshold is 36 in. is wrong. However, a 28-in. chimney on a 5/12 roof, 8 ft tall and only 3 ft from the ridge, is a significant obstruction; a cricket or saddle is strongly recommended because it diverts water and snow around the uphill face. The pan flashing at the base, corner pieces, and step flashing must be continuous. Nailing through the top face of the flashing is also wrong: fasteners should be concealed or placed where the lap above covers them. Caulk alone will not solve wind-driven rain.
2. Describe the correct installation sequence for the chimney flashing and the metal shingles around it.
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Install the chimney flashing in this order: complete the underlayment and ice/water protection up to the chimney; install a base pan flashing that turns up the sides and front and extends under the shingles; install step flashing on the sides, one piece per shingle course, lapped by the shingle above and turned up into the masonry; install a cricket or saddle on the uphill side with its own flashing; install counterflashing in a cut reglet in the masonry, with the lower edge lapping over the step flashing; seal the reglet, not the step-flashing laps. Keep the metal isolated from fresh mortar or dissimilar metals to avoid corrosion. Do not run shingles tight to the masonry; leave a gap for movement and drainage, and use a backer or sealant joint at the vertical contact. At the front, the pan flashing must lap over the shingles below. Fasten only where the lap will cover the fastener, and do not puncture the horizontal flange. Check that the cricket outlet drains to the main roof, not into the chimney.
3. What inspection or water-test steps would confirm the detail is watertight before closeout?
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Test and inspect before closeout. Spray the chimney from a hose, starting low and working up, while an observer watches inside for leaks; do not blast water into the reglet. Check the counterflashing is continuous, the step flashing is not face-nailed, and each shingle course covers the top of its flashing. Look for gaps at the corners, at the cricket, and where the pan flashing meets the step flashing. Verify that the chimney has a proper cap and that the flashing height is at least a few inches above the roof surface. Photograph the detail from several angles. If the leak persists, remove the shingles adjacent to the chimney and inspect the underlayment-do not add more caulk. Confirm the repair with the manufacturer’s detail and the local code official; a 28-in. chimney may not require a cricket by the letter of the code, but the installer should document why one was or was not used.