Sidewall and Headwall Flashing on a Metal Roof
Where a roof meets a wall, the flashing has to go behind the cladding, not be caulked to it — and the missing kick-out at the bottom of a sidewall rots more walls than anything else.
Roof-to-wall junctions cause more concealed damage than any other detail on a house, because when they fail the water does not appear on the ceiling. It goes into the wall cavity and stays there — wetting sheathing, rotting studs and sill plates, and doing it quietly for years before anything shows inside. The detailing is not complicated, but it has to be right, and it has to be done in the correct sequence with the wall.
Sidewall and Headwall Are Different Problems
A sidewall is a wall running parallel to the slope — the roof rises alongside it, as at a dormer cheek or where a lower roof runs up against the side of a two-storey section. Water travels down the roof beside the wall and the flashing has to keep it out of the wall for the whole run. A headwall is a wall at the top of a slope, perpendicular to it — where a porch or addition roof terminates against the main house wall. There, water is arriving at the wall and has to be turned back down onto the roof.
Headwall Detailing
At a headwall, the panel ends have to be turned up. Each panel gets an upstand at its top edge, an inch or two high, formed the same way as a ridge termination. That leg is the physical stop that prevents wind-driven water from running up the pan and over the panel end into the assembly. Foam or formed metal closures fill the profile gaps where a ribbed panel meets the flat flashing.
Over the top of that, a headwall flashing — essentially a Z-shaped or L-shaped piece — has a horizontal leg that laps down over the panel upstand with its own hemmed drip, and a vertical leg that runs up the wall. The vertical leg goes behind the wall's water-resistive barrier and behind the cladding, so anything running down the wall sheds onto the flashing and out onto the roof. That is the whole job: get the wall's drainage plane to hand off cleanly to the roof's.
Sidewall Detailing
At a sidewall on a metal roof, the panel nearest the wall gets an upturned leg on its wall side, just as it would at a rake. A wall flashing covers that leg with a vertical leg running up behind the water-resistive barrier and the cladding. Because the panel needs to move with temperature, the flashing must not be screwed through the panel into the wall framing — the connection is by hem and cleat so the panel can slide behind it.
The vertical leg height matters more in this climate than in a mild one. Snow drifts against a sidewall and sits there for months; meltwater under that drift is effectively standing water against the wall. A leg height that would be fine in a rainy but snow-free climate is not adequate here, and running the self-adhered membrane well up the wall behind the flashing is standard practice on any Ontario job worth the money.
Two-Piece Flashing and Counter-Flashing
Wherever the wall is masonry, or the cladding is not going to be disturbed, the correct approach is two pieces: a base flashing fixed to the roof side, and a separate counter-flashing coming down over it from the wall side. The counter-flashing is let into a mortar joint or tucked under the cladding, and it overlaps the base flashing without being fastened to it.
- ✓The overlap lets the roof and the wall move independently — buildings settle, panels expand, masonry shifts — without tearing the flashing.
- ✓It allows the roof to be replaced in future without disturbing the wall, and the wall to be reclad without disturbing the roof.
- ✓It removes any reliance on a sealant joint between two rigid materials that move in different directions.
- ✓It is the only reasonable approach where the roof meets brick, stone or stucco.
Kick-out flashing. Where a sidewall runs down and ends at the eave — the classic case being a roof plane meeting a wall that continues past the eavestrough — a kick-out flashing at the bottom of the run diverts the water out of the wall junction and into the trough. Without it, every drop that ran down that sidewall goes straight into the wall cavity at the bottom. This single missing piece of metal is responsible for an extraordinary amount of hidden wall rot, and it is invisible from the ground once siding is on.
Sequencing With the Wall
The recurring mistake on wall junctions is treating them as roofing details applied after the wall is finished. The order that works is: wall sheathing, then the roof-side flashing installed with its vertical leg up the wall, then the water-resistive barrier lapped down over that leg, then the cladding. Water running down inside the cladding meets the barrier, the barrier drains onto the flashing leg, and the flashing drains onto the roof. Reverse any of those laps and the water goes into the wall instead.
When the wall is already finished, the honest answer is that some of the cladding has to come off in a band along the roof line to install the flashing correctly. Surface-mounting a flashing to finished siding and running a bead of sealant along its top edge is not a flashing detail, it is a maintenance item with a short life, and it will fail during a freeze-thaw cycle without warning.
What Failure Looks Like
Soft or discoloured siding at the bottom of a sidewall run. Peeling paint or efflorescence on a wall below a roof junction. A window below and inboard of that junction with staining at its corners. Rot in the rim joist or sill plate below. Interior finish damage that appears in a wall rather than a ceiling. None of these read as roof problems to a homeowner, which is exactly why they run for years before anyone connects them to the flashing.
Frequently Asked Questions
Why is the wall below my roof line rotting when the roof does not leak?
Almost always a wall junction. Either the flashing is surface-mounted to the cladding rather than lapped behind the water-resistive barrier, or there is no kick-out flashing where the sidewall ends at the eave. Both dump water into the wall cavity where it never reaches the ceiling and never announces itself.
Can the top of a wall flashing just be caulked?
Sealant on top of a flashing leg is a temporary measure. Two rigid materials moving in different directions will open that joint, and in a freeze-thaw climate they will do it quickly. The permanent detail is either a counter-flashing let into the wall or the flashing leg lapped behind the water-resistive barrier and cladding.
Does the siding have to come off to flash a roof-to-wall junction properly?
Usually a band of it along the roof line, yes. That is what allows the flashing leg to go behind the drainage plane instead of on top of it. It adds work, and it is the difference between a detail that lasts as long as the roof and one that has to be maintained.
How high should the vertical leg go up the wall?
Higher than in a mild climate, because snow drifts and sits against sidewalls all winter. Combine a generous leg with self-adhered membrane run up the wall behind it. The specific dimension depends on the exposure and the assembly, and it is worth asking the installer what they are planning rather than assuming a minimum is adequate.
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