Aluminum Window Flashing Sequence: Why the Sill Comes First

## The flashing sequence that keeps water out

Water does not usually enter because the frame was the wrong color, the glass was upgraded poorly, or the sealant bead looked thin. It enters because the assembly gave it a better path than the one it was supposed to follow. That path is created or destroyed by the flashing sequence.

A proper [aluminum window installation](https://meichenwindows.com.au/aluminium-window-installation/) treats the window as part of the wall’s drainage system. The visible frame is only the front edge of the system. The real waterproofing happens in the layers hidden behind it, where each piece of tape, membrane, and sealant must overlap the right way so water is forced outward instead of inward.

## Why the sill comes first

The sill is not just the bottom edge of the opening. It is the collection point for everything gravity, wind, and capillary action drive into the joint. Rain hits the head, runs down the jambs, and ends up at the lowest point. If the lowest point is not already protected, the entire opening becomes a basin.

That is why the sill layer goes in before the frame is fixed and before any upper tape is applied. Once the frame is anchored, the sill cannot be flashed correctly without leaving hidden gaps. If the lower edge is installed after the fact, water can slip behind it because the upper layers have already blocked the path that should have directed runoff outward.

A sill flashing also needs side returns or end dams. Without them, it is just a strip of membrane with open ends. Under wind pressure, water can travel sideways to the corner, run behind the membrane, and enter the rough opening or the wall cavity. That failure often shows up months later as staining at the internal bottom corners, swollen timber, or corroded fasteners under the track.

## The only order that works

The sequence is simple, but every step matters.

1. **Prepare the opening so the surfaces will bond.**
   Dust, mortar crumbs, old sealant, and moisture all weaken adhesion. A membrane stuck to a dirty sill edge is decorative, not functional.

2. **Install the sill flashing or sill pan first.**
   It must direct any water outward and down, not inward. The ends should turn up enough to prevent side leakage. The bottom edge should project out far enough that runoff cannot bridge back into the wall.

3. **Set the window and fix it in place without crushing the frame.**
   The frame has to sit in the right plane before the side and head layers go on. If it is forced out of position, the flashing may still look neat while the drainage path is already compromised.

4. **Lap the jamb flashing over the sill ends.**
   This is where many leaks start. If the jamb tape tucks under the sill layer, water can run straight behind the bottom edge. The overlap has to shed water outward, like roof shingles.

5. **Apply the head flashing last, over the jambs.**
   The top layer should never dump water into a lower seam. Head flashing exists to catch anything that sneaks past the exterior face and send it back out again.

6. **Use sealant as a backup, not the primary barrier.**
   Sealant closes the exposed perimeter joint. It is not meant to replace the membrane layers. If the sequence is wrong, a prettier bead only hides the mistake until the first serious storm.

That order is what turns a window opening from a hole in the wall into a controlled drainage path. Reverse it and the system becomes a trap.

## What goes wrong when the order is reversed

The most common failure is the one that looks tidy from the outside. A fresh bead of silicone around the frame can hide a bad flashing sequence for a while, especially on a dry day. Under water load, though, the weakness shows up immediately.

- **Head flashing tucked behind the jambs** creates a shelf that holds water at the corners.
- **Jamb tape installed first** can block the path that should have carried the sill runoff outward.
- **Sealant bridging over weep paths** stops drainage and forces water to collect inside the frame.
- **A shallow or missing sill upturn** allows water to migrate into the rough opening by capillary action.
- **A frame fixed before the sill is protected** leaves no reliable way to recover the correct lap sequence.

One of the clearest examples is a slider installed in a brick veneer opening after the external reveal has already been cleaned up and sealed. The frame looks square, the perimeter bead is smooth, and the operation is perfect on day one. Then a strong southerly drives rain against the wall for an afternoon. Water reaches the sill, finds no proper path out, and tracks behind the inner finish. The leak is not caused by the window style. It is caused by a missing drainage sequence.

## Retrofit installs are less forgiving

New construction gives installers room to build the weather barrier correctly from the start. Retrofit work does not. In an existing opening, the temptation is to rely on sealant alone because the surrounding wall is already finished and nobody wants to disturb it. That shortcut is where many recurring leaks begin.

The problem is not that retrofit windows cannot be watertight. The problem is that the visible joint is doing too much work when the hidden layers were never rebuilt. If the old membrane, reveal, or sill condition is compromised, the new window inherits the failure.

In retrofit work, the sequence has to be even more disciplined:

- clean the substrate down to sound material
- rebuild the sill drainage line first
- maintain clear drainage paths
- keep the exterior sealant joint narrow and uniform
- avoid burying weep points under caulk or trim

That discipline is one reason experienced crews keep [window installation experts](https://meichenwindows.com.au/aluminium-window-installation/) involved when the opening is irregular, the wall is old, or the original waterproofing is unknown. The skill is not in laying more sealant. It is in recreating the path water is supposed to follow.

## How to tell the sequence is right before trim hides it

A good install tells the truth before the architrave or render patch covers anything up. The clues are visible if you know what to look for.

- The sill layer is the first membrane you can identify as part of the weatherproofing stack.
- Every layer above it overlaps outward, never inward.
- No tape ends directly at a corner where water can turn and run behind it.
- Drainage slots and weep paths remain open.
- The perimeter sealant line looks clean, but it is clearly secondary to the membrane work behind it.
- There is no sign that the installer used sealant to bridge a gap that should have been flashed.

A hose test usually exposes bad sequence quickly. If water enters at the lower corners after a few minutes, the problem is rarely the face of the frame. It is almost always a hidden lap error, a blocked drain, or a sill that was treated like a cosmetic edge instead of the base of the system.

## The real takeaway

A watertight window is not created by a better bead of sealant. It is created by a better sequence of layers. Sill first, jambs over sill, head over jambs, sealant as backup, drainage kept open. That is the logic that keeps water moving out of the wall instead of into it.

When that logic is respected, the window can survive the kind of weather that exposes rushed work: wind-driven rain, thermal movement, and pressure at the corners. When it is ignored, even an expensive frame becomes a leak waiting for the next storm.

A strong-looking window is not the same thing as a correctly installed one. The difference lives in the order of the layers nobody sees.