For years now, recessed windows — which mimic the appearance of traditional Southwestern adobe architecture — have been popular in Western states like California, Colorado, Arizona, Nevada, and Utah. Now the trend is moving eastward. Unfortunately, a recessed window — which is typically set anywhere from 2 to 12 inches into the framing — is very difficult to flash properly. I know because my company has been investigating and repairing leaky recessed window openings in Southern California ever since they started showing up on new construction projects 10 years ago.
To understand why recessed windows are so difficult to flash, I find it helpful to look at flush-mounted units. With a traditional installation, water hits the siding (or gets behind it) and travels straight down until it reaches the top of the window, where it hits a watertight head flashing. It then travels horizontally for a short distance, turns downward, and ultimately exits the building either by dripping off the head flashing or continuing down the siding or drainage plane.
Compare that with a recessed wall condition. When the water hits an inset window, it must make six or eight 90-degree turns before it can exit. Surface tension directs the water inward toward the building envelope, while the horizontal surface below the window obstructs the water’s flow. If the flashing around the window isn’t continuous, properly lapped, and free of damage, water will follow a path of least resistance — generally into the wall cavity directly below the window.
By 2001 my company had responded to so many leaks around recessed windows we decided we needed a standard approach to flashing these areas. What follows is our method for flashing inset windows. We use widely available flashing membranes as well as preformed corners that I developed.
Originally we used the “origami method” for the corners, which meant painstakingly folding peel-and-stick into three-sided corners. But our preformed corners — which I’ve dubbed the Corner Flash system — make the job go a lot quicker. Although I devised them for my own jobs, I received frequent inquiries and requests from contractors who saw them in the field, and now I sell them (800/310-7673, tlslabs.com).
Framing the Opening
Before we install and flash the window, we start with a properly prepared opening. A good first step is to confirm the rough-opening dimensions, because if you end up having to make changes, the flashing steps will be a complete waste of time.
We also always check that there’s a minimum of 3 inches between the window opening and the framed recess (see illustration, right). Anything less will prevent proper layering of materials and may result in the nail fin cutting into the flashing.
In addition, we make sure that solid backing (either uninterrupted framing or sheathing) forms the recess. This may sound obvious to builders east of the Rockies, where fully sheathed homes are the norm, but in the West, where homes are not routinely fully sheathed, this step needs to be specified.
Finally, we put a little slope to the horizontal backing immediately under the window. One inch of slope for 12 inches of run (8 degrees) is the minimum — more is better. I like 2 inches in 12 inches.