The detail that generates the most RFIs and the most callbacks
Of every rough-opening detail you draw, the window flashing sequence is the one most likely to get built wrong in the field and the one most likely to leak when it is. The reason is simple: the correct order of operations is counterintuitive. The instinct on site is to seal everything tight, to caulk the perimeter, to make the opening watertight from the inside out. That instinct fails water. A window assembly is not waterproof — it manages water. Some quantity of bulk water will get behind the cladding and the window fin, and the flashing's only job is to catch it, hold it away from the framing, and route it back out to the face of the wall. Get the lap order backwards and you build a funnel that pours water into the sill plate and the rim joist, where it does its damage silently for years before anyone sees a stain. This page lays out the sequence that drains to the exterior, integrates with the weather-resistant barrier (WRB) correctly, and avoids the two cardinal sins that cause the majority of window-related water intrusion.
- The principle behind every step: water that gets in must be able to get back out, lower and to the exterior.
- Two non-negotiables: the sill pan must drain, and every upper layer must lap OVER the layer below it (shingle-lapping).
- This is a coordination detail as much as a waterproofing one — it touches the window supplier, the WRB installer, and the cladding sub.
Step 1 — Form the sill pan first, with end dams and a back dam
Before the window goes anywhere near the opening, the sill is flashed. The sill pan is the single most important element of the assembly because it is the last line of defense — it is the part that catches everything the upper layers shed and everything the window itself leaks. A correctly formed pan has three features. It slopes or drains toward the exterior so collected water runs out, not in. It has end dams — upturned legs at the left and right ends of the sill — so water that reaches a corner cannot run off the side into the jamb framing. And it has a back dam, an upturned leg at the interior edge, so water can never migrate past the pan toward the inside of the wall. The pan can be a pre-formed product, a fully-adhered membrane dressed into the corners, or a liquid-applied flashing — the material matters less than the geometry. Critically, the sill pan laps OVER the WRB or building paper below the opening, so any water it discharges lands on the face of the drainage plane, not behind it.
- End dams stop corner water from escaping sideways into the jambs.
- Back dam stops water from reaching the interior framing and finish.
- Slope or a drainage gap sends collected water out to the wall face.
- The pan's front leg laps over the WRB below — never tucked behind it.
Step 2 — Set the window in sealant on jambs and head ONLY, never the sill
This is the step that gets reversed in the field more than any other, so it earns its own callout. After the pan is formed, run a continuous bead of sealant on the BACK of the window fin at the two jambs and the head — and leave the sill fin completely free of sealant. The window is then set into the opening, pressed into those three beads, and fastened per the manufacturer's nailing schedule. The reason the sill is left open is the whole point of the assembly: the gap under the sill fin is the drainage path. Any water that gets behind the window — through the frame corners, the glazing, or a failed perimeter seal — collects in the pan and needs an exit. If you bead the sill, you trap that water against the pan with nowhere to go, and you have converted your drainage detail into a bathtub. Sealing the sill is the first cardinal sin of window flashing, and it is invisible once the trim goes on.
- Sealant bead: jambs and head, on the back of the fin, continuous.
- Sill fin: no sealant — it stays open as the drainage exit.
- Set the window into the beads and fasten to the manufacturer's schedule.
- Cardinal sin #1: caulking the sill fin and trapping water in the pan.
Step 3 — Flash the jambs, then the head last, shingle-lapped under the WRB
With the window set, flash the two jambs by applying self-adhered flashing tape over the jamb fins, running it from below the sill pan's end dam up past the head. Then — and only after both jambs are done — apply the head flashing. The head is always last and always on top, lapping OVER the top edges of both jamb flashings. The final, easy-to-miss move is at the WRB above the head: the head flashing must tuck UNDER the WRB head flap (the WRB above the window is lapped over the head flashing), so water running down the face of the drainage plane sheds onto the outside of the head flashing and is carried over the window, not behind it. Reverse this lap — running the WRB behind the head flashing, or taping the WRB on top of the head flashing in a way that directs water inward — and you have committed the second cardinal sin: a reverse-lapped head that channels every drop straight into the rough opening. The order, bottom to top, is the entire discipline: pan, jambs, head, WRB head flap over the top.
- Order is strict: jambs first, head last, each upper layer over the one below.
- Head flashing laps over both jamb flashings at the upper corners.
- WRB above the window laps OVER the head flashing (head tucks under the WRB flap).
- Cardinal sin #2: a reverse-lapped head that drains water into the opening.
How this maps to your drawing set and your phases
The flashing sequence lives in your details, not your plans, and where it lands in the set depends on the phase. In Design Development you are typically establishing the wall assembly and the drainage-plane strategy — which WRB, which cladding, and the basic principle that the window integrates into a drained, back-ventilated wall. By Construction Documents this becomes a numbered, step-by-step jamb/sill/head detail at a real scale (commonly 3 inch or larger), keyed to the window schedule and the wall types, with the lap directions drawn explicitly rather than implied. The most useful version is an exploded or sequential detail — a small series showing the pan, then the window, then the jambs, then the head — because that is what removes ambiguity for the installer and pre-empts the RFI. Many firms also carry this same sequence into the submittal and mock-up review, where the field mock-up of a typical window is the real test of whether the drawn sequence is buildable. A clean, annotated, step-numbered detail is also the fastest way to settle a field dispute, because it shows order, not just final position.
| Step | Layer | Action | Lap / drainage rule |
|---|---|---|---|
| 1 | Sill pan | Form pan with end dams + back dam before window goes in | Front leg laps OVER the WRB below; slopes/drains to exterior |
| 2 | Window | Bead sealant on jamb + head fins only, set and fasten | Sill fin left OPEN — it is the drainage exit |
| 3 | Jamb flashing | Tape over both jamb fins, sill-to-above-head | Laps OVER the jamb fins; runs above the head line |
| 4 | Head flashing | Apply last, over the top of both jambs | Laps OVER jamb flashing; tucks UNDER the WRB head flap |
| 5 | WRB head flap | Lay WRB above the window down over the head flashing | WRB laps OVER head flashing — sheds water to exterior face |
Frequently asked
Why can't I seal the window sill if I want it watertight?
Because the assembly is designed to drain, not to be sealed. Some water will always get behind the window fin through the frame corners or glazing. The sill pan catches it, and the open (unsealed) gap under the sill fin is the only way that water can drain back out to the exterior. Bead the sill and you trap water in the pan against the framing — you turn a drainage detail into a reservoir. Jambs and head get sealant; the sill stays open.
What do end dams and a back dam actually do?
They define the pan as a three-sided tray instead of a flat sheet. End dams are upturned legs at the left and right of the sill that stop water from running off the ends into the jamb framing. The back dam is the upturned leg at the interior edge that stops water from migrating past the pan toward the inside of the wall. Without them, a 'pan' just spreads water onto the framing it was supposed to protect.
What is shingle-lapping and why does the head go on last?
Shingle-lapping means every upper layer overlaps the top edge of the layer below it, exactly like roof shingles, so water always sheds down and over a joint rather than into it. That is why the order is sill pan, then jambs, then head last: the head laps over the jambs, and the WRB above laps over the head. Install the head before the jambs, or run the WRB behind the head flashing, and water is directed into the opening instead of over it.
Does the window flashing sequence change for stucco or rainscreen cladding versus lap siding?
The flashing sequence at the rough opening — pan, jambs, head, WRB lap — is the same regardless of cladding, because it is about the drainage plane, not the finish. What changes is what happens outboard of it: the size of the drainage gap, the use of a rainscreen mat or furring, and how the cladding terminates at the head and sill. Always confirm the window manufacturer's installation instructions and the WRB manufacturer's details, since their warranties depend on the specified sequence.
Should the sill pan be sloped, or is a flat pan with a drainage gap enough?
Either approach can work as long as collected water has a clear path to the exterior face of the wall. A sloped or back-pitched pan actively drains water out; a flat pan relies on the open sill-fin gap and the pan's front leg lapping over the WRB to discharge water. What you must avoid is a dead-flat pan with no exit, which lets water pond against the back dam. Draw the drainage path explicitly so the installer cannot accidentally seal it shut.