New-Install Water Intrusion: Pan Flashing vs Sealant vs Sill Commissioning Decision Tree

Why this matters

When a freshly installed window leaks on its first storm, the cause is almost always one of three install-stage defects: a missing or compromised sill pan, a sealant joint that was over-relied on or applied wrong, or a sill that was not commissioned (not sloped, weeps blocked, back dam missing). These look similar from the inside, a wet sill, but the fix for each is different and the wrong one re-leaks. Commissioning the install correctly, and diagnosing it correctly when it fails, is the difference between a one-visit fix and a recurring callback that poisons a referral.

The governing principle from ASTM E2112 is that the sill pan, not sealant, is the primary line of defense. Sealant is a secondary, sacrificial joint. Any diagnosis that ends at "re-caulk it" without confirming the pan is doing its job is incomplete.

Symptom presentation

• Water at the interior sill within hours of rain, especially driven rain: a drainage-plane failure (pan/sill), not just a joint.
• Water entering at one jamb or corner: end-dam or jamb-flashing defect.
• Water only in prolonged or wind-driven rain at the head: head-flashing or sealant-at-head issue.
• Weep holes dry while the sill cavity holds water: blocked weeps or no drainage slope, water backing up over the interior edge.

Quick checks

• Sill pan presence. Pull the stool or inspect from inside/outside to confirm a sill pan exists, that it has end dams turned up at both jambs, and that it has a back dam at the interior edge to stop water from running indoors.
• Slope and weeps. Check that the sill pan and exterior sill slope to drain outward and that the weep holes are open and unobstructed. Pour a small amount of water into the pan and watch it drain out the weeps, not pool or run inward.
• Sealant joints. Inspect head and jamb sealant for continuity, adhesion, and correct geometry (hourglass joints fail; proper joints have backer rod and the right width-to-depth ratio). Confirm the sill is NOT sealed continuously in a way that traps water behind the pan.
• Lap direction. Verify jamb and head flashing shingle-lap over the pan and integrate with the WRB so water sheds outward at every horizontal.

Isolation tree

1. No sill pan, or pan without end dams / back dam.
   • Primary defense is missing. Water that gets behind the cladding has nothing to catch it and runs to the interior. This is the most serious and most common new-install leak. Cause: pan flashing. Sealant repairs will not save an absent pan.
2. Pan present, but water enters at a jamb/corner.
   • End dam not turned up, or jamb flashing not lapping over the pan upstand. Water rounds the pan end into the wall. Cause: pan end-dam / jamb-lap detail.
3. Pan present and dammed, but sill holds water and weeps stay dry.
   • Commissioning failure: no outward slope, weeps blocked by foam/sealant/debris, or the sill was bedded flat. Water collects in the pan and overtops the back dam indoors. Cause: sill commissioning (slope + weeps).
4. Pan and sill fine, leak only at head in driven rain.
   • Head flashing or head sealant. Confirm head flashing laps under the WRB and over the window head fin, and that head sealant is continuous. Cause: head detail / sealant.
5. Everything detailed correctly, isolated water test at a sealant joint reproduces leak.
   • Sealant adhesion/geometry failure at a specific joint. Cause: sealant. This is the only branch where re-sealing alone is the fix, and only because the primary drainage plane is intact.

Confirming diagnosis

• Pan flashing confirmed when inspection shows no pan, no end dams, or no back dam, and a directed water test per ASTM E1105 reproduces interior wetting that a properly detailed pan would have drained.
• Sill commissioning confirmed when the pan exists but holds water, the weeps are blocked or the pan lacks outward slope, and clearing/sloping to drain stops the intrusion.
• Sealant confirmed only when the full drainage plane (pan, end dams, back dam, laps, slope, weeps) checks out and a water test localizes entry to a discrete failed joint.

Remediation

• Pan flashing: Remove the unit as needed and install a code-compliant sloped sill pan with turned-up end dams and an interior back dam per ASTM E2112; reintegrate jamb and head flashing in shingle-lap with the WRB. This is the correct, durable fix; do not substitute sealant for a pan.
• Sill commissioning: Re-establish outward slope (sloped sill or shim the pan to drain), clear and open the weep holes, and remove any foam/sealant that bridged the weep path. Verify water drains out the weeps.
• Sealant: Cut out the failed joint, install proper backer rod, and tool a correctly proportioned joint with a compatible sealant. Treat sealant as the secondary defense it is, never the primary.
• Re-commission after any repair: pour-test the pan, confirm weep drainage, and run a directed water test before closing up.

The discipline that prevents most new-install leaks is commissioning at the time of install, not diagnosing them after. Before the trim goes on, pour a cup of water into the installed sill pan and watch where it goes: it should run to the weeps and exit outboard within seconds, with nothing pooling at the interior edge or running past the back dam. If it does not drain, the slope or the weeps are wrong and you fix it then, with the unit still accessible, instead of on a callback after a finished interior. Stage the water test the same way you would diagnose a failure: start low at the sill and work up, one zone at a time, so a positive result localizes the entry rather than soaking the whole assembly. A commissioned, pour-tested, water-tested sill is the cheapest insurance against the most expensive callback in this trade, the one where you have to pull a finished window to find an absent pan.

References

• ASTM E2112, Standard Practice for Installation of Exterior Windows, Doors, and Skylights (sill pan, end dams, back dam, weeps, sealant as secondary defense).
• ASTM E1105, Standard Test Method for Field Determination of Water Penetration of Installed Exterior Windows, Skylights, Doors, and Curtain Walls.
• AAMA InstallationMasters / FMA-AAMA InstallationMasters program (pan flashing and commissioning procedures).
• International Residential Code (IRC) R609 (exterior window and glass door installation and flashing requirements).