Aluminum Window Head Flashing: The Lap Detail That Prevents Costly Leaks

## The detail that decides whether the wall stays dry

A head flashing is easy to misunderstand because it looks like a simple piece of bent metal. The real job starts only when it is tied into the wall wrap correctly. The difference between a dry opening and a hidden rot problem usually comes down to the [head flashing detail](https://meichenwindows.com.au/aluminium-window-head-flashing/) above the window, not the aluminum itself.

When the lap is correct, the wall sheds water by gravity. When it is reversed, the wall stops draining and starts funneling rain into the cavity.

## Why the lap order matters more than the material

Water does not need a big opening to enter a wall. It only needs a continuous path. That is why a reverse lap at the head is so destructive. The water-resistive barrier is supposed to catch water that gets behind the cladding and deliver it onto the flashing surface. The flashing then throws that water outward with a projecting drip edge.

If the sequence is flipped, the WRB sends water behind the flashing instead of over it. The metal still looks correct from the outside, but it has been turned into a shelf that directs runoff into the rough opening.

Correct sequence:

1. Rain runs down the cladding face.
2. Any water that gets behind the cladding lands on the WRB.
3. The WRB laps over the top leg of the flashing.
4. The flashing projects outward and breaks surface tension at the drip edge.
5. End dams keep water from spilling off the sides.

That sequence is not optional. It is the drainage system.

## Why sealant cannot compensate for a bad lap

Sealant fails for three reasons: movement, pressure, and age. Aluminum and wall assemblies move at different rates as temperatures swing. On a hot day, the metal expands; on a cold night, it contracts. The sealant is asked to stretch, compress, and stay bonded while also resisting water exposure and UV.

That is too much to ask from a bead of caulk, especially when the geometry is already wrong.

A sealant bead can stop incidental drafts. It cannot turn a reverse lap into a drainage path. Once water is sitting on the wrong side of the assembly, gravity keeps feeding it inward. Pressure from wind-driven rain makes the problem worse. The bead may look intact for months, then split or lose adhesion in one corner and let water start moving in the wall where nobody can see it.

The most expensive repairs on window openings do not begin with a dramatic failure. They start with a detail that looks almost right.

## Why the head is the most unforgiving place on the window

The head of the window gets the full runoff from the wall above it. That makes it different from the sill, which can usually drain, and the jambs, which deal with less water volume. The top edge has to manage everything coming down from above while also staying clear of the opening below.

On an exposed facade, that means the head flashing may be asked to redirect water from several square feet of wall every time it storms. A tiny mistake in lap direction is enough to overwhelm the detail because the water load keeps arriving at the same spot.

That is why the head is where leaks often show up first as staining, swollen trim, or a persistent musty smell inside the room. The failure is usually outside and above the visible damage.

## What a correct detail actually protects against

A properly lapped head flashing does more than keep out rain. It also protects against the failures that show up later:

- water tracking behind cladding by surface tension
- capillary action at the top edge of the window
- wind-driven rain being forced up and under a loose edge
- lateral runoff moving off the ends of the flashing
- moisture soaking the sheathing and framing long before interior signs appear

That last point is the one that matters most. A bad head detail is not just a leak. It is a concealed wetting cycle. The wall gets damp, dries slowly, then gets damp again. Timber starts to soften. Fasteners begin to corrode. Insulation loses performance. Mold can follow.

By the time the interior paint blisters, the problem has usually been active for a long time.

## How the cost multiplies once the lap is wrong

The repair cost is not driven by the flashing itself. The cost comes from access.

If the issue is caught early, the fix may only require opening a short section of cladding above the window, correcting the lap, and reinstalling the trim. That is a manageable repair.

If the problem is ignored, the scope expands fast:

- cladding removal
- WRB replacement or patching
- framing repair
- insulation replacement
- mold cleaning or remediation
- repainting and finish restoration
- possible interior plaster repair

What began as a detail problem can turn into a multi-trade job. On finished homes, especially upper-story openings or walls with difficult access, that shift can move the bill into the thousands quickly.

## What to verify before the wall gets closed up

The safest inspections are the simple ones. Before cladding goes back on, the head detail should answer a few questions without debate:

- Does the WRB lap over the top leg of the flashing?
- Does the flashing project far enough to throw water clear of the wall face?
- Are end dams formed at both sides?
- Are fasteners kept out of the water path?
- Is the flashing sitting behind the cladding, not exposed as a ledge that can collect water?
- Is there any place where water could be directed inward by a reversed overlap?

If any answer is unclear, the detail is not ready.

A separate strip of metal can look right and still fail if the overlap is wrong. A modest amount of extra sealant cannot fix a misdirected drainage plane. The only reliable cure is correct layering.

## The rule that keeps the opening dry

The simplest way to think about the head detail is this: every layer above the window must shed water onto the layer below it, and never the other way around. That is the entire system in one sentence.

The window frame, the flashing, and the WRB are not separate defenses. They are one drainage sequence. When that sequence is reversed at the head, the wall stops protecting itself.

When it is built in the right order, the opening stays quiet, the framing stays dry, and the expensive repair never gets a chance to start.