6 Things to Know About Window Flashing
Understand how this crucial waterproofing process works to protect your home from rot, mildew, and structural damage.
During the construction, a builder will take many precautions to protect a house from water damage, and one of the most important is the installation of window flashing—thin material that prevents water from seeping in around a window. Over time, even a tiny gap around a window that allows water to enter can result in mold growth, wood rot, and structural damage that can end up requiring costly repairs. If you’re concerned or simply curious about the flashing around your windows, keep reading to learn the basics of this key component of keeping your home watertight.
1. Window flashing protects the window—and the house—from water damage.
While a home’s siding serves as its main source of protection against the elements, penetrations through the siding windows require additional waterproofing. Flashing—whether it’s on a window, roof, or chimney—is designed to be installed in a layered sequence, similar to the way shingles are layered on a roof. Each successive layer overlaps the layer below to direct the flow of water down and away.
2. Several types of flashing can be installed on a window.
The three main types of window flashing are drip cap, flashing tape, and molded vinyl—and each type suits a different part of a window. Another construction material, known as a vapor barrier, is also frequently used to flash windows at the time of their installation.
- Drip cap: Available in Z-shaped sheet metal strips, drip cap is designed to fit over the top of a window, much like a small canopy, to shed water outward and keep it from seeping behind the window.
- Flashing tape: This self-stick membrane comes in four-inch and six-inch rolls and is cut to fit along the inside edge of the rough opening (the frame constructed in the wall to hold the window). When firmly in place, flashing tape effectively prevents water from reaching the wood of the rough opening.
- Molded vinyl: Often called a “sill pan,” this type of window flashing is rigid and pre-formed to fit the inside of the rough opening along the bottom edge. It features a slope that encourages water to drain down and out. Sill pans are commonly used in the installation of vinyl-clad windows but not as often on wood windows.
- Vapor barrier: While not technically a type of flashing, a vapor barrier—the house wrap that installs over the entire outside of the wood sheathing before the siding is installed—is sometimes used as flashing during window installation by wrapping it around the inside edges of the rough opening.
3. Correctly flashing a window requires knowing when to install each type.
Window manufacturers will recommend the method of flashing best suited to their specific windows, and in order to maintain the warranty, that process should be carefully followed. Some windows now have built-in channels along the top that eliminate the need to install drip cap over the top. While the manufacturer’s specs should always take precedence, the following sequence is standard for flashing windows.
- First, the vapor barrier (house wrap) is cut and wrapped around the inside of the window. Before windows are installed, a breathable vapor barrier is wrapped around the entire exterior of the house sheathing (un-sided plywood walls), including over the rough openings for the windows. The window installer will cut the vapor barrier over the window (often in an “X” pattern), and then pull the flaps through the window to the inside and staple them in place. Note: Not all window manufacturers intend for the vapor barrier to be wrapped; some want it cut evenly with the edges of the rough opening because the type of flashing tape they recommend adheres better to bare wood than to the vapor barrier.
- Next, self-stick flashing tape is installed along the inside of the rough opening—over the wrapped vapor barrier (unless the manufacturer specifies otherwise)—to form a watertight seal all along the inside edge of the rough opening. The tape is first installed along the bottom (sill), then along the sides (jambs), and finally along the top of the rough opening (head). By installing in this sequence, the edges of each successive membrane strip overlap the strip below, encouraging water to shed downward.
- Then a sill pan (if called for) is placed over the membrane along the bottom edge of the rough opening. The window is then set in the rough opening, leveled, and nailed in place. New windows come with nailing flanges that extend flat along the outside surface of the house sheathing.
- More flashing tape next installs over the side and top nailing flanges to seal the nail holes and to cover the seam between the sheathing and the flange. The key here is that no flashing tape should be installed over the bottom nailing flange—in case any water inadvertently enters the window in the future, it needs a way to escape.
- Finally, if the window requires a drip cap, it should be installed at this point. The drip cap attaches to the sheathing above the window, and then a strip of flashing tape is applied over the nails used to attack the drip cap. This completes the flashing process, and siding can now be installed.
4. It’s best to leave window flashing installation to the pros.
Flashing is an important part of window installation, and setting windows can be tricky, so the entire process is best left to the pros. Depending on the rules in your community, a local building inspector may check to ensure that the flashing (along with all aspects of window installation) meets local building codes. A DIYer with a solid working knowledge of carpentry may opt to install windows and flashing; if so, you may need to pull a permit, so contact your local building authority before getting started.
5. Flashing errors are a common cause of window leaks.
While flashing a window isn’t a difficult process, if the flashing tape isn’t installed in the correct sequence or per manufacturer’s specs, leaks can occur. The most common flashing mistakes include:
- failure to layer flashing materials in a “shingle” fashion.
- sealing the bottom of the window and trapping water inside.
- stretching the flashing tape during installation, which can cause it to contract and pull away, leaving a gap where water can enter.
- failure to securely adhere the flashing tape. Pressing the tape in place with your hands usually isn’t sufficient. A flashing membrane roller (sold on the same aisle near the flashing tape) should be used to get a tight seal.
- not following the manufacturer’s specs. Manufacturers may call for a specific brand of flashing tape, and some may require additional sealant (caulk) during the flashing process. Not following the specs can result in a poorly flashed window that leaks.
6. It isn’t always easy to tell if existing flashing is defective.
Once the window is installed, the only flashing that’s visible is the front edge of the drip cap (if the window requires one). If you spot a thin strip of sheet metal coming loose from above the window—one side hanging out or the whole thing coming out and falling on the ground—it’s a sure sign that the drip cap was either poorly installed or that the wood sheathing holding the nails in place has rotted, allowing the drip cap to fall out.
A leak could also point to a flashing problem. If water leaks into the house from the window. it could indicate that the flashing was incorrectly installed—or it could be that a different part of the window is leaking. Either way, a qualified window installer should inspect the window, because a damaged drip cap or any sort of leak should be inspected and repaired promptly to prevent water damage.