Window flashing is the layer of waterproofing that directs water away from an opening before it reaches the framing. Installed in the right order, flashing makes a window shed water for decades. Installed in the wrong order, it collects water behind the siding and sends it to the sill, the sheathing, and the interior. The same shingle-lap logic that governs roof flashing installation applies at every window: upper layers overlap lower layers so water runs off instead of in.
Water intrusion at windows accounts for a large share of leak complaints in houses, and most of those leaks trace back to installation details rather than the window itself. The sequence, the materials, and the overlaps determine whether the assembly works, which is why flashing is specified and inspected before the siding goes on.
The Water Management Strategy
A window opening has three vulnerable zones: the sill, the jambs, and the head. Water that hits the glass or the frame runs down and meets the sill first, so the sill pan is the first line of defense. Water that gets past the exterior trim meets the jamb flashing, and water that runs down the wall above the window meets the head flashing. Each layer sheds water onto the layer below it.
The complete approach to watertight window installation works from the sill up: a pan at the bottom, flashing tape up the jambs, and a head detail that keeps running water out of the top of the opening.
Shingle Lapping and the Sequence
Flashing follows one rule: each piece overlaps the piece below it, the way shingles overlap on a roof. Start at the bottom of the opening, work up the sides, and finish at the top. That order guarantees that water always flows over a flashing edge rather than behind it.
Why the Sill Is First
The sill is the first surface water meets, so it gets the first layer of flashing. A sill pan that slopes to the exterior and seals at the corners sends incidental water back outside before it ever touches the wall cavity.
Most window leaks do not start at the glass. They start where the flashing stops short, where a corner was cut, or where two materials meet without a seal. The building paper and house wrap shed bulk water, but the flashing at the opening is what directs that water around the frame and back out to the cladding.
Flashing a Flanged Window in Order
Most modern windows come with factory flanges, and the flashing sequence for a flanged window is well established: pan first, then the window, then jamb and head flashing.
- Prepare the rough opening: check that the sill is level and slopes slightly outward, then install the sill pan.
- Set the window in the opening, shim it plumb and square, and nail the flanges, leaving the bottom flange for last.
- Apply flashing tape over the jamb flanges, starting at the bottom of each jamb and working up, with the tape lapping onto the pan.
- Install the head flashing so it covers the top flange and extends past the jambs.
- Seal the top corners so water cannot enter at the joint between the jamb and head flashing.
The flashing detail for flanged window jambs at Fine Homebuilding shows the failure that causes most field leaks: tape that stops short of the sill pan, which lets water run down behind the jamb flange.
Nail placement matters as much as tape placement. The side and top flanges are nailed first, with the bottom flange left loose until the sill pan and the first course of siding can lap over it. Shims support the frame at the sill and the jambs, and every shim gap gets packed so the frame does not bow when the nails seat.
Tools and Materials for the Job
- Peel-and-stick flashing tape, 4 to 9 inches wide.
- A sill pan: site-built membrane, metal pan, or prefabricated pan.
- A utility knife and a roller or squeegee for tape adhesion.
- Window shims, screws or nails, and exterior sealant for the final bead.
Tape Adhesion Depends on the Surface
Peel-and-stick tape bonds to clean, dry sheathing. Dust, frost, and wet wood cut the bond dramatically, so the rough opening gets a wipe-down and a dry weather window before the tape goes on.
Sill Pans: Site-Built and Prefabricated
The sill pan catches whatever water reaches the bottom of the opening and routes it outside. It has to slope outward, hold sealant at the corners, and extend past the jambs so water cannot turn the corner.
Both site-built and prefabricated sill pan solutions have their place, and the choice comes down to the crew, the schedule, and how the sill is framed.
Three Ways to Build a Sill Pan
- Peel-and-stick membrane formed into a pan with folded corners, the fastest site-built option.
- A metal pan bent from aluminum or copper, which adds a rigid, sloped surface.
- A prefabricated pan molded or extruded to fit the opening width, installed before the window.
The Slope Test
Check the pan with a level: it should shed water toward the exterior. A pan that sits level or dips inward holds water against the window frame, and standing water finds the screw holes.
Back dams and end dams turn a flat pan into a tray. A back dam at the interior edge stops water that creeps past the sill, and end dams at each jamb keep water from running sideways into the wall. Pans without dams rely on sealant alone, which is a weaker setup.
Whatever the pan type, the ends extend 4 to 6 inches up the jambs, and the jamb flashing laps over the pan, never under it.
Jamb and Head Details
Jamb flashing runs up the sides of the opening, covers the window flange, and laps onto the pan at the bottom. The tape runs continuously from the pan to the head, and any seam in the jamb run is lapped shingle-style with the upper piece over the lower.
For openings where the trim needs a custom fit, the step-by-step method for custom aluminum window flashing gives crews a repeatable way to fabricate head and sill pieces on site that match the exact opening width.
Head Flashing Options
- Peel-and-stick tape over the top flange, cut to extend past both jambs.
- Z-flashing or drip edge metal installed over the top trim.
- Liquid-applied flashing at the corners and around fasteners.
The Drip Edge Rule
The head flashing must extend past the side jambs and end in a drip edge or downward bend, so water running down the wall falls free of the window instead of wrapping around the head.
Corners fail more often than straight runs. The joint where jamb tape meets the pan, and the joint where head flashing meets jamb tape, both need a full-width overlap with the upper piece outside the lower. Skipping that overlap shortcuts the whole water management system.
Flashing Materials Compared
Material choice affects cost, durability, and how forgiving the installation is. Peel-and-stick tape dominates residential work because it conforms to the sheathing and seals around fasteners. Metal flashing lasts longest but requires bending and careful fastening. Liquid-applied membranes bridge gaps and seams but need dry, clean conditions to cure.
The same principles extend beyond windows: the proper flashing techniques for cantilevered deck joists apply the identical shingle-lap and drip-edge logic to keep water out of structural connections.
Material Comparison
| Material | Cost | Durability | Best use |
|---|---|---|---|
| Peel-and-stick tape | Low | 20+ years | Sill, jambs, and head of flanged windows |
| Aluminum flashing | Medium | 30+ years | Head details, custom pans, trim |
| Copper flashing | High | 50+ years | Historic work, visible details |
| Liquid-applied membrane | Medium | 15+ years | Corners, fasteners, retrofit repairs |
| Prefabricated pan | Medium | 25+ years | New construction, repeat sizes |
Compatibility Checks
Metal flashing in contact with aluminum windows or certain treated lumber can corrode, so check the compatibility of the metals before pairing them. Tape and liquid membranes need a compatible primer on some substrates.
Self-adhered tape on bare plywood or OSB usually bonds without a primer, but dusty sheathing, cold weather, and some treated panels call for a primer first. A quick adhesion test, tape pressed and peeled after a few minutes, tells you whether the bond will hold.
Field Checks and Modern Wall Assemblies
A flashed window deserves a water test before the siding hides it. Spray the opening from outside with a hose, starting low and working up, and check the interior for drips. The test is cheaper than the repair.
The Final Inspection List
- The pan slopes outward and extends past the jambs.
- Jamb tape laps over the pan and runs continuous to the head.
- Head flashing extends past the jambs and ends in a drip edge.
- All tape is rolled, with no bubbles or wrinkles at the corners.
- Flashing is installed before the siding and weather barrier lap over it.
Modern wall assemblies pair flashing pans with drainage planes and rainscreen gaps, and the flashing pan systems used in modern wall assemblies assume the flashing is part of a system, not a single band-aid over the opening.
Window flashing is cheap, fast to install, and nearly invisible once the siding is up. The openings that leak are almost always the ones where the sequence was skipped or the laps were reversed.