How roof type and chimney width size a flashing job

Flashing is a roof detail as much as a chimney detail

Flashing seals the joint where the roof meets the masonry. One layer lies under the roof covering and turns up against the chimney. A second layer, the counter flashing, is let into the brick and laps down over the first. Neither layer works unless the roofing around it is lapped in the right order.

So the roof covering decides much of the work. Replacing flashing properly means lifting some of the roofing around the chimney and laying it back, and each covering comes up differently. The code reflects this: FBC Residential gives shingle roofs and tile roofs their own flashing rules, and both point to the roofing manufacturer's instructions.

The metal is set by code as well. Table R903.2.1 gives minimum thicknesses for copper, aluminum, stainless steel, galvanized steel and other flashing metals. The metal also has to suit what it touches, since some metals corrode against others or against fresh mortar, and that choice is written on the estimate.

Asphalt shingle roofs

Shingles are the simplest covering to flash. FBC Residential R905.2.8.3 allows a sidewall to be flashed by the step-flashing method or the continuous L flashing method, while R905.2.8.4 leaves the chimney details to the shingle manufacturer. Shingles next to the chimney are lifted, the old metal comes out, new metal goes in course by course, and the shingles go back or are replaced.

The scope grows when shingles are brittle and break as they are lifted, since those have to be replaced with a match. It also grows when the shingle roof as a whole is worn out. In that case the flashing and the roof should be planned together, and the roof replacement belongs to a roofer.

Tile roofs, concrete or clay

Tile is heavier work at the chimney. Tiles around the stack are lifted and set aside, and some crack at their fasteners however carefully they are handled, so matching replacement tiles belong in the plan from the start.

For tile, FBC Residential R905.3.8 wants both layers, flashing and counterflashing, at every junction with a vertical surface, built to the roofing chapter, the tile manufacturer's installation instructions and the recommendations of the FRSA/TRI high-wind tile manual. Those details differ from shingle steps, and the work follows them. Crossing a tile roof also calls for walk boards and careful footing, so the tile away from the chimney survives the visit.

Metal roofs

Metal panels do not lap under flashing the way shingles do. The flashing at a chimney on a metal roof is bent to follow the panel profile and tied into the panel system's own trim, following the roofing maker's details. The uphill face needs particular care, because water running down long panels builds up against the stack there.

The scope grows with a panel profile that needs custom-bent pieces, with fasteners that have to be replaced, and with any panel that has to be lifted or cut. Panel work beyond the flashing detail belongs to the roofer.

Low-slope and flat roofs

On a roof with little pitch, water sits longer, so the chimney joint is sealed with a membrane base flashing that turns up the masonry, and metal counter flashing laps over its top edge. The membrane has to match and bond to the roof's own membrane, which ties the job to that roofing system.

Low-slope roofs give the roofer the largest share of the work. The written scope separates chimney work from the roofing contractor's work in plain terms.

Width, and the cricket on the uphill side

A narrow chimney sheds water around its sides. A wide one dams it. FBC Residential R1003.20 calls for a cricket where the chimney measures over 30 inches parallel to the ridge without reaching it, and R903.2.2 sets the same 30-inch trigger from the roofing side. A cricket is a small peaked roof built behind the chimney to split the water around it.

A wide chimney without a cricket is a flashing job that keeps coming back. Building one involves framing and roofing, so the estimate names who builds it and who flashes it. Width also adds metal on every face, and a chimney that sits in a valley or beside a wall adds its own detail.

Reseal, reset or reflash

Not every flashing complaint needs new metal. Counter flashing whose sealant has dried out can be resealed. Counter flashing that has worked loose from its joint can be reset into a freshly cut reglet. Step or base flashing that has rusted through, or was lapped wrong from the start, needs a full reflash. The cost to reflash a chimney sits at the top of that range, and chimney flashing replacement cost on tile or metal runs above the same job on shingles.

A reset keeps the step flashing and renews only the counter layer's seating and seal, so it depends on the steps underneath being sound. If lifting the counter flashing shows rusted or missing steps, the job moves up to a reflash, and that change is explained to you before it happens.

Face-sealed counter flashing is the shortcut to watch for. The metal is glued to the brick face with a bead of sealant instead of being let into a mortar joint, and it fails as soon as that sealant ages. It is high on our list of checks at any leaking chimney.

Flashing jobs in order of size

Holding height and pitch equal, flashing jobs line up this way:

• Resealing sound counter flashing on one face of the chimney.
• Resetting loose counter flashing into a cut reglet.
• Reflashing a narrow chimney in a shingle roof, all four sides.
• The same full job on a tile or metal roof, with tiles or panels lifted and reset.
• Reflashing on a low-slope roof, tied into the membrane.
• A wide chimney with a cricket to build, flash and roof.

Height, pitch and the state of the roof

A steep pitch or a second-story stack moves any of those jobs further up the list, because staging and safety gear take time before any metal is cut. A roof covering near the end of its life can turn a flashing job into a joint plan with a roofer.

The estimate lists the roof covering, the faces to be flashed, whether the counter flashing is let into a reglet, any cricket, and which trade does each part. That list is the fairest way to compare two quotes.