Rain Water Enters at the Window Corner: Head Flashing vs. Sill Dam vs. Jamb Seal Decision Tree

Why this matters

Corner water entry is the most commonly misdiagnosed water problem in window replacement work. Most technicians caulk the exterior corner and call it done. Three months later the callback comes back because the underlying flashing plane was never addressed. The entry point - which corner and which height - tells you which flashing layer failed before you touch the wall. Getting the hose test right means fixing the right layer on the first visit.

Step 1: Record exactly where the water appears inside

Water that enters at a window corner travels before it becomes visible. The interior wet spot is usually lower than the entry point. Before any testing, note:

• Top corner (at or near the head of the window, upper left or upper right): suggests head flashing or the lap at the head-to-jamb intersection.
• Bottom corner (at or near the sill): suggests sill pan failure, sill slope reversal, or the jamb-to-sill intersection gap.
• Middle of the jamb height, appearing at the corner: suggests the jamb-flashing-to-sill-pan integration is incomplete and water is running down the jamb and finding the gap where it meets the horizontal pan.

Write down the location before running any water tests. Testing with a hose in the wrong sequence wastes time and can make the source harder to isolate.

Step 2: Hose test - head flashing plane only

Conduct each plane test separately. Use a garden hose at low pressure - no spray nozzle - and have a second person inside watching for entry.

Head flashing test:

1. Direct water only at the wall above the window, 12 inches above the rough opening, running water horizontally across the wall for 2 minutes.
2. Do not water the window itself yet.
3. If water appears inside at the top corners during this test, the head flashing is the source. Head flashing may be missing, reversed in lap direction, or not integrated with the window's nail fin or installation flange.

If no entry appears, move to the next test. Do not combine the tests.

Step 3: Hose test - jamb flashing plane only

1. Direct water at the side of the window frame only, running water down the left jamb for 2 minutes. Stop. Watch for entry.
2. Repeat on the right jamb.
3. Water appearing at the bottom of the jamb during the jamb-only test, particularly in the corner where the jamb meets the sill, points to a break in the jamb flashing or tape, or a gap at the jamb-to-sill-pan joint.

The jamb-to-sill joint is a 90-degree inside corner where two different flashing planes meet. If the sill pan was installed first and the jamb flashing tape was not turned down into the pan (or the pan end dams were not sealed to the jamb), water running down the jamb hits that joint and enters.

Step 4: Hose test - sill and sill pan only

1. Direct water at the exterior sill area only, below the window frame, for 2 minutes.
2. Water entering at the bottom corners during this test points to the sill pan.

Sill pan failures take three forms:

• No end dams: a flat pan with no turned-up lips at each end allows water to run off the side of the pan and behind the jamb.
• Reversed slope: the pan slopes toward the interior rather than toward the exterior weep holes or the drainage plane.
• Pan not connected to the drainage plane: the sill pan drains onto the sheathing rather than out through the siding at the bottom edge, allowing water to pool and wick into the rough opening.

Repair path by diagnosis

Head flashing failure (water enters at top corners during head-only test): Remove exterior trim or casing at the window head. Verify that the head flashing laps over the top of the window's installation flange or nail fin from above, not under it. Install or replace head flashing so water sheeting down the wall goes over the top of the window, not behind it. If through-wall flashing is present, verify the lap direction at each layer. Reinstall trim with appropriate sealant at the top edge only (never seal the bottom of trim - that traps water).

Jamb-to-sill intersection failure (water enters at lower corner during jamb test, not during sill test): The repair is at the 90-degree inside corner where the vertical jamb flashing meets the horizontal sill pan. Apply a flexible, paintable flashing membrane that fully wraps the corner and bonds to both planes. Seal all lap edges with compatible sealant. The fix does not require window removal in most cases.

Sill pan failure (water enters during sill-only test): Minor sill pan repairs (adding end dams or correcting the drainage path) can often be done from the exterior without removing the window. Full slope reversal usually requires removing the window, rebuilding the sill pan, and reinstalling.

References

• Building Enclosure Moisture Management, flashing plane integration guidance
• ASTM E1105 standard test method for water penetration of installed windows by uniform or cyclic static air pressure difference
• Industry flashing guidelines for window and door rough openings, sill pan and head flashing sequencing