This article originally appeared on SIGA’s blog:

What is window flashing?

What is it? Why does it matter? And where can you learn more?

Like many things in our collective memory, windows aren’t as simple as they used to be, which means window flashing, too, has reached a whole new level of complicated.

Case in point – there are 4 different flashing standards prescribed by the American Architectural Manufacturer’s Association (AAMA). That’s not factoring in the dozens of window flashing manufacturers with unique interpretations of those methods.

And that’s just for residential flanges windows. Combined with the hundreds of window producers and overlaid by some actively-advancing energy codes: the number of possible window flashing variations for each project is boggling.

So how do you as an installer Window flashing comprises the bulk-water diversion steps of the window installation process.  Simply put – it is the connection between window and wall.  make good decisions in the field? Remember a couple fundamental building envelope truths:

  • Water runs downhill
  • Windows will leak*


 What is flashing?


Any durable material that is installed to divert bulk water away from sensitive areas is a flashing.

Flashing’s are used in many different exterior trades. As a result, you’ll primarily see them in roofing and siding– and they generally operate within 2 distinct layers of the building assembly:




2. SECONDARY DRAINAGE PLANE ( weather-restive barrier )



Depending on which layer of the wall they’re in, flashing can take on a number of forms to repel water:

  • Sheet metal (stainless steel, galvanized steel, copper)
  • Tapes or self-adhered membranes
  • Flexible non-adhered sheets
  • Gunnable liquids


What is window flashing?


Window flashing comprises the bulk-water diversion steps of the window installation process.  Simply put – it is the connection between window and wall.

In terms of the definition stated above, window flashing is the bridge between the secondary drainage plane (weather resistive barrier) to the primary water shedding surface (the window).


Why are windows special?


Windows all start as a really big hole in the wall (often called a fenestration).

Even though we fill the hole with arguably the most sophisticated building component there is (a window), the hole itself presents an inherent vulnerability.

Windows also command special attention because they are 3-dimensional. It’s much simpler to manage the flow of water (straight down) in the area of the [flat] wall plane.

However, reach the window, and bulk water is on a downward path and is diverted out/in/around/along any number of directions.

Thankfully the path water takes is predictable.  With a little building physics know-how, the right products, and the correct installation, you can mitigate water penetration risk.

Today we design buildings with a backstop: a secondary drainage plane and associated flashing work to reduce the risk of water penetration.


A quick run-down on the two primary steps of window flashing


The process of flashing a window is broken into two main parts:

  1. Pre-flashing of the rough opening (hole in the wall) before the window goes in,
  2. Counter-flashing the installed window

Like fish scales, each part must nest underneath the previous one.  Layering from bottom-to-top is called water-shedding or weather-lapping fashion.


What is pre-flashing the rough opening?


“Pre-flashing” is an extension of what is historically known as a sill-pan: a custom, soldered piece of brake-formed metal installed at the bottom of the rough opening.

Metal pans are still around, but the more common practice is to choose a self-adhered material to flash the sill – Wigluv is a high-quality tape option here. Better-than code-minimum installations won’t stop at the sill but rather seal the entire perimeter of the rough opening.

Pre-flashing in this way helps to integrate the secondary drainage plane (weather-restive barrier) with the opening. And when (not if) the window itself leaks, your horizontal wood or metal sill is protected.


TIP: How to help window flashing perform better


Providing a drainage path at the sill when setting the window can help the window flashing to perform better.

  • All windows can be installed on setting blocks, which provides and air space at the sill.  (Make sure they are structurally sound and spaced properly.)
  • Flanged windows also benefit from a few 1/16” horseshoe shims behind the sill flange, for the same reason.


What is counter flashing the window?


After the window is installed, another layer of self-adhered material is installed to the sides (jambs) and top (head) of the window.

The main goal of this flashing is to