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May  2004

TECHnical-ities

It’s Not as It Seems
Window Leaks That Aren’t Necessarily Window Failures 
by Brad Oberg

A classic window surrounded by dramatic brick veneer details can be very attractive, but there is nothing more frustrating than trying to solve a window leak that is really associated with inadequate water management details surrounding the window, and not the window unit itself. It is important for window manufacturers to understand how water leaks at the side jamb, head and sill areas can look like a window failure but, in reality, are caused by not following proper flashing details surrounding the window unit.

Building Science of Brick
Because brick and mortar are porous structures, they absorb water and yield quickly to water in a rainstorm. Although front surface runoff is usually apparent during stormy weather, most of the water that drains from the brick usually drains down the back of the brick. This water needs to be accounted for when installing a window into a brick veneer façade.

Head Flashing
When bricks terminate over a window, water has the potential to be deposited on the head of the window. If mortar partially fills the cavity and the brick ties slope toward the house instead of away, water can jump the gap and run down the drainage plane and hit behind the lip of a flanged window, causing leaks that look like a window failure. Through-wall flashing and/or house wrap flashed over the window head and outside the jamb flange is required for water control. Bow and bay windows are difficult since the brick veneer must be supported on the slope of the roofline. Typically the metal roof of a bow is flashed and counter flashed to the brick, but water can penetrate behind the brick and past the pan flashing under the brick. Through-wall flashing integrated with the basic building drainage plane will pick up this water and prevent a leak in the window area.

Jamb Flashing
The details of the jambs should be set up so the brick terminates behind the face of the window frame—in such a way that a small backer rod can be inserted between the window and the brick. This rod acts as a bond break for the elastic sealant placed between the brick and the window frame and also creates a capillary break along the flange. 

In many cases I have seen, a problem area exists where the window is installed directly against the framing and a thin flexible sheathing material is lapped over the window head and flange. This strategy works adequately at the top of the jambs but becomes reverse flashed at the bottom of the jambs, where the lower sheathing material has now created a trap for water. This problem will look like a window failure but must be solved at the flashing stage.

Sill Flashing
A third area of concern is the brick sill. Sill flashing extending from under the window must overlap the drainage plane to move any water that does penetrate the window system out and away. A brick ledge or sill must slope at least one inch in six for positive drainage. The mortar in the head joints must be full depth, fully compacted, and rodded (a steel tool is used to compact and smooth the mortar increasing resistance to water penetration) to resist water penetration. The brick ledge or sill must end under the window unit sill, or a secondary flashing piece must be extended over the brick to continue water movement away from the window.

Other Areas
Windows that are aligned over each other are also sources of confusion. An upper window with any of the faults mentioned will deliver water to the lower window. This will send the search party to the wrong place (the lower window), when in fact the flashing of the upper window is the culprit. Flashing details on both windows are critical.

So the lessons are, if the window is in brick, check the brick detailing first, then the window. If the window is flashed properly into the drainage plane of the house, even a brick veneer failure will not show up as a leak. Careful execution, using more than one way to overcome a problem, and redundancy are the keys.

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