Window and Door Flashing Details in a Metal Siding Wall
Openings are where walls leak. Sill pans, jamb flashing, head flashing with end dams and a drip — the sequence and the reasoning behind each layer in a metal-clad wall.
If a wall is going to leak, it will leak at an opening. Openings interrupt the drainage plane, interrupt the cavity, introduce four changes of direction, and sit exactly where wind pressure concentrates. Everything else in a metal wall assembly can be done reasonably well and the wall will still fail if the openings are wrong. Everything below is about sequence and about giving water a way out.
The Governing Principle: Layer Like a Roof
Water runs downhill. Every layer in an opening must lap over the layer below it so water crossing a joint always travels outward and downward. Get one lap backwards and you have built a funnel that delivers water into the wall — and it will be invisible once the cladding is on.
This is why the sequence matters as much as the materials. There is a correct order, and it goes from the bottom up.
Step One: The Sill Pan
The sill pan goes in first, before the window. It is the last line of defence for everything that gets past the window itself, and a window will eventually pass some water — through a failed corner seal, around a worn gasket, or under sustained wind-driven rain.
A sill pan is not a strip of tape across the rough sill. A proper pan has three features that make it work.
- ✓A back dam — a raised lip at the interior edge. Without it, water in the pan runs inward across the rough sill onto the subfloor. The back dam forces water to go outward instead.
- ✓End dams — turned-up ends at both sides. Without them, water in the pan runs sideways out of the pan and into the wall at the jambs.
- ✓Slope to the exterior, or a positively drained profile, so water actually leaves rather than sitting in the pan.
The pan's front edge laps over the weather-resistive barrier below the opening — outward and downward, like every other lap. The pan can be formed metal, a moulded plastic pan, or built up from flexible flashing membrane, provided the dams are real and the corners are sealed.
Do not seal along the bottom of the window between the frame and the sill pan. That bead is what stops the pan from draining. Seal the jambs and the head; leave the sill open so anything that gets into the pan can get out. This one detail is reversed on job sites constantly.
Step Two: Set the Window, Then Flash the Jambs
The window goes in over the sill pan. Shim and fasten per the manufacturer's instruction, then flash the jambs with flexible flashing that laps over the window's mounting flange and extends onto the barrier beyond it. Critically, the jamb flashing laps over the sill pan flange — it goes on after, not before.
Step Three: Head Flashing
The head is where a metal wall differs most from other claddings, because a metal wall above the opening delivers a lot of water to this line — metal sheds fast and absorbs nothing.
- ✓Use a formed metal head flashing with a positive outward slope, not just a strip of membrane. Membrane at the head is for integration with the barrier; the metal is what carries and throws the water.
- ✓The flashing needs a drip — a hemmed edge that turns down and slightly out. Surface tension will otherwise carry water back along the underside of the flashing and straight onto the window head.
- ✓End dams at both ends, so water travelling along the flashing cannot run off the end into the wall.
- ✓The flashing extends past the opening at each side and laps over the jamb flashing beneath it.
- ✓The weather-resistive barrier above laps over the head flashing's upturned leg. The upturn must be behind the barrier, always.
- ✓Leave the ends of the head flashing unsealed. Sealing them closed traps any water that reaches the flashing behind the trim.
How the Cladding Trim Fits Around All of This
Once the opening is flashed and integrated with the barrier, the metal siding's own trim goes on — receiver channels at the jambs, a head trim over the flashing, a sill trim or a return at the bottom. That trim is doing three jobs: closing the panel ends, giving the opening a finished edge, and shedding the bulk of the water. It is not the waterproofing. The flashing behind it is.
This distinction matters when people ask about caulking. Sealant at the trim face is a first line that reduces how much water gets behind the trim, and it is a maintenance item — it will need attention at some point over the life of the wall. The wall must be built so that when that sealant fails, water lands on flashing and drains out. Any detail where a bead of caulking is the only thing between rain and the sheathing is a detail that will eventually leak.
Openings in a Rainscreen Cavity
A furred wall adds a complication: the cavity has to be closed around the opening so wind and insects do not get in, without blocking drainage from the cavity above.
The way this is resolved is with a sloped head flashing that spans the full cavity depth and carries water out over the face of the cladding below, combined with a designed drainage and ventilation path at the sides. Where the cavity is interrupted by a wide opening, some assemblies leave a deliberate gap at the head, screened, so air can still pass. A cavity that is blocked solid above a wide window becomes a sealed pocket with its own moisture and no way to dry — which is the opposite of why the cavity was built.
Ontario-Specific Failure Modes
Two things happen here that do not happen in milder climates, and both concentrate at openings.
The first is freeze-thaw on trapped water. A detail that holds a small amount of water is tolerable in a mild climate. In Southern Ontario that water freezes, expands, works the joint open a little, and thaws — dozens of times a winter. A sealed head joint that traps a few millilitres of water is a joint that is being mechanically worked apart all season.
The second is snow and ice loading at sills. Snow accumulates on a projecting sill, melts against the sun-warmed glass, refreezes at the outer edge, and can back water up against the sill joint. Sills need real outward slope and a drip, and sill trim that drains rather than collects.
Doors and Deck Ledgers
Doors are windows with a threshold that sees foot traffic and standing snow, so the pan under a door threshold matters even more and takes more abuse. Where a deck ledger or a roof meets a wall near an opening, the flashing of those elements has to integrate with the opening flashing rather than run past it — and a kickout at the bottom of a roof-to-wall intersection is essential wherever a roof edge dies into a wall above a metal-clad surface. Without it, the entire roof's runoff is directed behind the cladding at one point.
Frequently Asked Questions
Does every window need a sill pan?
It is sound practice on every opening in an Ontario wall. The pan costs very little at the framing stage and is the only thing standing between an eventual window leak and the framing and floor below. Retrofitting one later means removing the window.
Should the window be caulked all the way around?
Jambs and head, yes. The sill should be left open so the pan can drain. Sealing the bottom of a window converts the sill pan from a drain into a bathtub, which is worse than having no pan at all.
Is membrane flashing enough at the head, or do I need metal?
Membrane integrates the head into the barrier, but it cannot form a slope, a drip or end dams — and those are what actually get water away from the opening. On a metal-clad wall with a lot of runoff arriving at the head, formed metal head flashing over the membrane is the durable answer.
What is the most common flashing mistake at openings?
A reversed lap — barrier tucked behind head flashing instead of over it, or jamb flashing installed before the sill pan so the pan flange sits on top. Both create a direct path into the wall, both are hidden by the finished cladding, and both are avoidable simply by working bottom to top.
Can existing windows be reflashed when new metal siding goes on?
Partly. With the cladding off you can inspect and improve the head and jamb flashing and correct reversed laps. A proper sill pan generally requires pulling the window, so that decision is usually made when the window is being replaced anyway. It is worth assessing at the start of a re-cladding job rather than after the barrier is on.
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