Chimney Flashing on a Metal Roof: Crickets and Counter-Flashing
A chimney is a wall in the middle of your roof that water has to get around. The cricket behind it and the counter-flashing let into the masonry are what make that work.
A masonry chimney is the most demanding penetration on a residential roof. It is large, it is rigid, it is made of a material that moves and settles independently of the house, it interrupts the flow of water down the roof, and it collects snow and ice behind it all winter. It is also, on a lot of older Southern Ontario houses, the place a roof was already leaking before the metal went on.
Why a Cricket Is Not Optional
The uphill side of a chimney is the problem. Water running down the roof arrives at a wall and has to go somewhere. On a narrow chimney it splits and runs around each side without much trouble. On a wide one, the back pan behind it becomes a shelf where water slows down, debris collects, and — in this climate — snow piles and ice forms and stays for weeks.
A cricket, or saddle, is a small framed and sheathed structure behind the chimney with two sloped planes meeting at a ridge, which pushes water and snow around either side. The rule of thumb the trade uses is that any chimney wider than roughly two to two and a half feet across the slope gets one, and in a snow climate the argument for building one on a narrower chimney is strong. Confirm the actual requirement with your local building department.
How a Cricket Is Built
For anything of size, a cricket is framed — a small ridge from the back face of the chimney to the roof deck, with valley rafters or a simple sheathed saddle either side — then covered with sheathing, wrapped with self-adhered membrane, and flashed in metal. The two planes it creates form small valleys where they meet the main roof, and those valleys are built like any other valley: a formed pan, membrane beneath, panels hemmed and stopped short.
For a narrow chimney, a one-piece formed or soldered metal saddle can do the job without framing. What does not work is a flat back pan with a bit of upstand, which is what a lot of older roofs have. It holds water and ice by design.
The Base Flashing
The base flashing is the part attached to the roof. On a metal roof it takes a different form than the shingle-roof step flashing most people picture, because there are no courses to interleave with.
- ✓The downhill face gets an apron: a formed piece with a vertical leg against the masonry and a horizontal leg that laps over the panels below with a hemmed drip edge.
- ✓The sides get formed pans with an upturned leg against the masonry and a hem where they meet the roof panels, so the panel and the flashing interlock rather than relying on a fastened lap.
- ✓The uphill side gets the cricket, flashed into the main roof on both sides.
- ✓Self-adhered membrane runs up the masonry face and out onto the deck well past the flashing on every side, before any metal goes on.
- ✓The panels themselves are hemmed where they meet the flashing, and the flashing must not pin a panel that needs to move.
Counter-Flashing Into the Masonry
The second half of the assembly is the counter-flashing, and it is the half most often done wrong. The correct method is a reglet: a groove cut into a mortar joint, generally around three-quarters of an inch deep, into which a folded lip of the counter-flashing is inserted. Lead wedges hold it in place, and the joint is then sealed with a compatible sealant. The counter-flashing hangs down over the base flashing, overlapping it without being fastened to it.
The counter-flashing must never be fastened or soldered to the base flashing. A masonry chimney and a wood-framed roof move independently — different materials, different rates, different responses to temperature and moisture, and chimneys settle. The overlapping two-piece detail is what allows that relative movement without tearing anything. Join the two and something will crack.
Why Surface-Mounted Flashing Fails
Surface-mounted counter-flashing — a piece of metal fixed flat to the brick face with masonry fasteners and a bead of sealant across the top — is quick and it is everywhere. It is also a maintenance item from the day it is installed. The sealant is doing all the work at the top edge, it is fully exposed to sun and to every freeze-thaw cycle, and it is bridging a joint between a rigid metal edge and a porous, moving masonry surface. It cracks, water gets behind the flashing, and it runs down the inside face of the metal and into the roof. On a roof intended to last for decades, a reglet is the detail that matches.
Chimney Condition
Re-roofing is the point at which the chimney above the roof line becomes accessible, and it is worth looking at while the scaffolding is there. Deteriorated mortar joints, spalling brick from freeze-thaw, a cracked or missing crown, and a missing rain cap are all common on older chimneys and all of them let water into the masonry from above — which no flashing detail can address. Flashing a chimney that is itself absorbing water is fixing the wrong problem, and the leak will continue.
Metal Chimneys and Flues
Factory-built metal chimneys and appliance flues are a different assembly. They use a flashing base sized for the pipe and a storm collar above it, and — importantly — they have clearance-to-combustible requirements set by the appliance and chimney manufacturer that govern how close framing and roofing material may come. Those clearances are not a roofing judgment call; follow the manufacturer's instructions and the requirements of the authority having jurisdiction.
Frequently Asked Questions
Do I really need a cricket behind my chimney?
If the chimney is wide across the slope — roughly two feet or more — yes, and in a snow climate the threshold is arguably lower. The alternative is a flat back pan that holds snow, ice and debris against the masonry for months. Confirm the requirement with your local building department.
Can the existing chimney flashing be reused with a new metal roof?
The counter-flashing sometimes can be, if it is let into a reglet and the masonry is sound. The base flashing cannot — it is part of the roof assembly, it is formed for the old roofing material, and it comes off with the old roof.
Why does my chimney only leak in winter?
Two usual causes. Snow and ice build up behind the chimney, hold meltwater against the flashing, and force it past details designed to shed running water. Or the masonry itself is absorbing water through failed mortar or a cracked crown, freezing, and driving moisture inward. Both are winter-specific and neither is fixed by caulking the flashing.
Is aluminum flashing acceptable against masonry?
Bare aluminum in prolonged contact with fresh or wet mortar is a poor combination — the alkalinity attacks it. Compatible materials are chosen for masonry contact, and it is one of the reasons a specific material is specified rather than whatever is on the truck.
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