6 min read · Guide
A window that never leaked and then starts after new siding or a window replacement is telling you something specific: the water management at the opening was disturbed and not rebuilt correctly. The window unit itself is rarely the culprit. Water gets into a wall at openings more than anywhere else, and the difference between a dry opening and a leaking one is a sequence of flashing details most homeowners never see. This guide explains how a correctly flashed opening sheds water, and maps the leak patterns that appear after exterior work to the detail that failed.
The opening is a flashing assembly, not just a window
A window is set into a hole in a water-managed wall, and the job of keeping that hole dry belongs to the flashing around it, integrated with the wall's water-resistive barrier. Water that runs down the wall has to be directed over the head of the window, past the jambs, and out at the sill — never behind them. When that layering is correct the window can weep and the wall still stays dry; when it is reversed or missing, even a perfect window leaks. The order of operations is everything, and it is exactly what gets disturbed when siding comes off or a window is swapped.
The sill pan — the detail most often missing
The single most important, and most often omitted, detail is a sill pan: a flashed, back-dammed tray under the window that catches any water reaching the sill and drains it back out to the face of the wall. A window installed without a sill pan has nothing under it but the rough framing, so water that gets past the unit soaks the sill plate and the wall below. The signature is staining or soft framing directly beneath a window, or interior damage at the sill after driving rain. A correct sill pan — sloped to drain, dammed at the back and ends — is inexpensive during install and nearly impossible to add later without opening the wall.
Head flashing and reverse laps
Above the window, a head flashing directs water out over the top of the unit and onto the face of the cladding. If the barrier above the window is lapped under the head flashing instead of over it — a reverse lap — water is funneled behind the window instead of over it. This is a classic re-side error: the old flashing is removed, the new WRB is run continuously and neat, and the head detail is lapped the wrong way. The tell is a leak at the top corners of a window that only shows in wind-driven rain. The fix is rebuilding the head flashing with correct shingle-style laps, per the CA Residential Code opening-flashing requirements.
Why replacement windows leak when the old ones didn't
An insert (pocket) replacement window slides into the existing frame and never touches the wall's flashing, so it usually inherits whatever water management was already there. A full-frame replacement removes the old unit down to the rough opening — which means the flashing has to be rebuilt, and that is where leaks are introduced. If a window that was dry for twenty years leaks the first winter after replacement, the flashing was disturbed and not correctly reintegrated with the WRB. The window brand is almost never the answer; the opening detail is. This is why we integrate window flashing with the wall barrier as one system rather than treating the window as a drop-in part.
Sealant is not flashing
The most common bad repair is running a bead of caulk around a leaking window. Sealant at the perimeter of a window is a secondary, sacrificial measure — it is not what keeps the opening dry, and it fails on a predictable cycle. A window that depends on its exterior caulk to stay dry will leak again the moment that caulk ages, because the actual water management behind it was never built. Worse, sealing the bottom of a window traps any water that does get in, turning a small leak into rot. Flashing sheds water by geometry and layering; caulk only closes a gap. The two are not interchangeable, and a contractor who answers a leak with more caulk has not diagnosed it.
What to do about a leaking window
Start by noting when and where it leaks: bottom corners in wind-driven rain points to head or jamb flashing; staining directly below the sill points to a missing or failed sill pan; a leak only during sprinkler operation points to irrigation, not the window. Because the flashing lives behind the cladding and trim, confirming the cause usually means removing trim and some siding at the opening — which is also the only way to fix it correctly. This is not a caulk-and-hope repair. If the leak followed recent exterior work, it is reasonable to raise it with the installing contractor; verify anyone you bring in through the CSLB first.
Key takeaways
- Windows leak at the flashing, not the unit — the opening is a flashing assembly
- A missing sill pan is the most common and most damaging omission
- Reverse-lapped head flashing funnels water behind the window instead of over it
- Full-frame replacements introduce leaks because the flashing is rebuilt; insert windows rarely do
- Sealant is a sacrificial secondary measure, never a substitute for flashing
- Confirming and fixing a leak means opening the trim and siding at the opening
FAQ
Quick Answers
Wind-driven rain pushes water upward and sideways, testing the head and jamb flashing that ordinary rain never reaches. A leak that appears only in storms usually points to a head-flashing reverse lap or a jamb detail, not the window unit itself.
Rarely, if the fix is real. The flashing that keeps a window dry lives behind the trim and cladding, so correcting it means opening the wall at the opening. A repair that stays on the surface — more caulk — treats the symptom and buys a season at most.
Almost never. A window that was dry before replacement and leaks after it points to disturbed flashing, not a defective unit. The opening detail — sill pan, head flashing, WRB integration — is where the leak was introduced.
A sill pan is a flashed, back-dammed tray under the window that catches and drains any water reaching the sill. It is the cheapest insurance an opening has and nearly impossible to add later without opening the wall, so yes — a correctly built opening has one.
Sources
Authoritative references
- Building Science Corporation — window installation & water management
- CA Residential Code — flashing at exterior wall openings (R703)
- CSLB — verify a California contractor's license
External links to government, code, and manufacturer sources. Sierra Siding is not affiliated with these organizations; references are provided for verification.