Preventing Water Ingress at Metal Window Head Flashings

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Water ingress at window head flashings is usually caused by flashings that lack adequate slope, drip edges, end dams or continuous sealing at the frame junction. Preventing it requires designing the flashing as part of the wall drainage system, not as a cosmetic trim.

Why window heads leak

The head of a window is one of the most exposed points in a facade. Rain runs down the wall, meets the top of the opening and, if the flashing does not shed it outward, the water is driven into the joint between frame and masonry. Over time this shows up as staining below the sill, rust on steel lintels, or damp plaster inside.

Most leaks trace back to a few repeatable causes: a flat or back-sloped flashing, a missing drip edge, an unsealed corner, or a flashing that terminates short of the jamb.

Detailing the flashing profile

A head flashing should project beyond the face of the wall and carry a clear drip edge so that water falls free rather than tracking back under the lip. The outer leg should be deep enough to protect the top of the frame and turn down far enough to cover the wall face.

Specify a continuous outward slope toward the drip edge.
Include end dams at both jambs.
Lap the flashing over the frame or into a sealed reglet.

Slope, laps and end dams

Slope is the single most important factor in shedding water. Confirm the design slope on the drawings and check that the fabricated profile holds it once installed. Laps between sections should run in the direction of flow, so water never runs into a joint.

End dams stop water from migrating sideways along the flashing and entering the jamb. Without them, even a well-sloped head can channel water into the side of the window.

Sealants and junctions

Sealant should support the flashing, not replace it. Use it at the frame-to-flashing junction and at corners, and select a product compatible with both the metal and the frame finish. Avoid bridging sealant across the drip edge, which traps water behind it.

Where dissimilar metals meet, such as stainless steel fixings against galvanized or powder-coated steel, specify isolation details to limit galvanic corrosion.

Maintenance and inspection

Flashings should be inspected at least once a year and after severe storms. Look for lifted laps, cracked sealant, paint breakdown at the drip edge and staining on the wall below the head. Clearing debris from the top of the flashing keeps water moving freely.

Check sealant for gaps and adhesion loss.
Clean the drip edge and the top surface.
Record findings with photos for future comparison.

Repairing an existing leak

When a head leaks, first identify the source before replacing anything. Water often enters at a point well away from where the stain appears. Inspect the flashing profile, laps and end dams, then decide whether localized sealing and a new drip edge are enough or whether the flashing must be remade.

Remaking the flashing to measure, with correct slope and dams, is usually more durable than repeated patch sealing.

Frequently asked questions

What slope should a window head flashing have?

The flashing should fall continuously toward the drip edge so water sheds outward. The exact value is set by the design and checked with the engineer for each project.

Is sealant enough to stop a window head leak?

Sealant helps at junctions and corners but cannot compensate for a flat or back-sloped flashing. The profile, slope and drip edge should do the primary work.

Why do end dams matter?

End dams stop water running sideways along the flashing into the jamb. Without them, water can enter the window side even when the head itself is well detailed.

How often should head flashings be checked?

Inspect them at least once a year and after severe storms, looking for lifted laps, cracked sealant and staining below the head.

Common mistakes to avoid

Designing the head flashing as a trim without slope or drip edge, and relying on sealant to fix a water path that the profile itself creates.

Our recommendation

Detail the head flashing as part of the wall drainage system, specifying slope, drip edge, end dams and isolation of dissimilar metals on the drawings, and have the fabricated profile checked against them before installation.

Summary

Preventing water ingress at window head flashings comes down to correct profile, continuous slope, drip edges, end dams and sensible sealing, followed by regular inspection.

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