Chimney Flashing Repair in Sterling Heights, MI

The bottom layer does the real waterproofing. A one-piece apron (the base flashing) wraps the downhill face of the chimney and laps over the shingles below it. Up each side, step flashing goes in one L-shaped piece per shingle course, each piece lapped over the one below so water steps down and out. The International Residential Code calls for base flashing of corrosion-resistant metal at least 0.019 inch thick (No. 26 galvanized) or heavy mineral-surfaced roll roofing, and for sidewall pieces at least 4 inches up and 4 inches out. Common failures: a continuous L-strip where steps belong, nails through the vertical leg into the brick, and rusted pieces reused from an older roof. Fixing the sides means lifting the shingles along the chimney and weaving in new steps, course by course.

Step flashing stops at the brick face, so something has to cover its top edge. That's the counterflashing (also called cap flashing), and the code wants it in corrosion-resistant metal at least 0.019 inch thick. Done properly, the roofer grinds a slot (a reglet) into a mortar joint, bends the top of the metal into it, wedges it, and seals the joint. On the sides it's cut in a stepped pattern that follows the brick courses. The shortcut version is a strip face-nailed to the brick with a bead of caulk along the top. Caulk ages much faster than the metal it holds. Once it lets go, water runs behind the strip and shows up as a stain on the ceiling beside the fireplace. On a leaking chimney, loose or caulk-only counterflashing is the first thing worth checking.

Water and snow sliding down the roof hit the uphill face of a chimney and stop. On a narrow stack the back flashing can handle that. On a wide one it can't, which is why the residential code requires a cricket (saddle) on the ridge side of any chimney more than 30 inches wide, unless the chimney sits at the ridge. The cricket is a small peaked frame that splits the flow around both sides, covered in sheet metal or the roof material and flashed and counterflashed like the chimney itself. The code even sizes its height from roof pitch: half the chimney's width on a 12:12 roof, a quarter on a 6:12 and an eighth on a 3:12. A wide chimney with no cricket and a rusted pocket of back flashing behind it should have one added during the repair.

Standing seam and other metal panels can't take step flashing the way shingles do, because the panels run in one piece from ridge to eave. Instead, the panel ends are turned up against the chimney behind a closure, a cricket or diverter is built on the uphill side, and a counterflashing cut into the mortar laps over all of it. The flashing metal should match the panel metal, or at least be compatible with it, so the two don't corrode each other. The code applies the same logic to fasteners: galvanized for steel roofs, copper alloys or stainless for copper. Most panel makers publish their own chimney detail, and that's the one to follow. If you're replacing a shingle roof with metal, the chimney flashing is a new design, not the old pieces reused. More on panels is on the standing seam metal roof page.

Some chimney leaks skip the flashing entirely and come in from the top. The code requires a masonry chimney to have a concrete, metal or stone cap (the crown), sloped to shed water, with a drip edge and a caulked bond break around the flue liner. A cracked or flat crown lets water into the brick below it, and an open flue with no rain cap lets water straight down the liner. Soft brick and missing mortar soak water up like a sponge and pass it into the house below the flashing line. A good inspection checks the crown, the cap and the brick before replacing perfectly good flashing. HomeGuide puts crown repair at $150 to $700 for small cracks and $600 to $1,500 or more for larger damage, and a new chimney cap at $200 to $850.