How to Install Door and Window Flashing to Prevent Leaks

Flashing refers to thin sheets of material installed around a home's exterior to prevent water from seeping inside. Helping to direct water down and away from your home, this waterproofing technique is especially important to use on roofs and around windows and doors. In many areas, installing building wrap and pieces of drip-cap flashing over the tops of windows and doors is acceptable. However, some manufacturers and building codes call for more elaborate door and window flashing to protect the sheathing, studs, and interior walls.

Whether installing new windows and doors or adding siding to existing units, check your local codes first, then follow these instructions for installing door and window flashings that provide maximum protection against moisture.

Types of Flashing

Door and window flashing helps prevent any moisture (which can collect due to condensation or small gaps in the siding) that flows downward from infiltrating behind the house wrap, where it can do damage. Therefore, the upper piece of wrap or flashing should always overlap any lower pieces.

The type and location of the flashings will vary depending on the type of windows and doors. If you have vinyl- or metal-flanged windows, for instance, you can install drip-cap flashing over the window itself or over a trim piece that you install. For a wood window unit, drip-cap flashing can be installed over the trim piece (often called brick molding) that comes with the window or door. Consult the manufacturer's instructions for the recommended technique for your windows.

In addition to metal door and window flashing, self-adhesive flashings, similar to waterproof shingle underlayment (WSU) sheets used for roofing edges, are often applied around window or door openings. If installing a new unit, the self-adhesive flashing ($91, The Home Depot) will cover the inside of the framing and the sheathing. However, if the window already exists, the self-adhesive flashing will likely protect only the sheathing and any window flanges. In addition, self-adhesive flashing is sometimes installed at wall corners where it can be attached over the building wrap.

Consult the manufacturer's instructions, your local building department, or a professional siding installer who works in your area to ensure you're installing the flashing correctly.

How to Install Window Flashing

When you install a window in new construction, the building wrap overlaps the self-adhesive flashing. In a remodeling job, apply self-adhesive flashing strips in this order: along the bottom, then the sides, then the top. Then add small pieces of flashing over the gaps at the corners.

How to Install Flashing with Felt or Building Paper

In this arrangement, first staple felt or building paper to the sheathing, with the top pieces overlapping the lower pieces. Next, apply a bead of caulk to the felt and set the window in the caulk. Next comes another layer of building wrap, installed like the first layer. At the top, tuck metal drip-cap flashing under the siding and on top of the second layer of wrap. Install the trim just under the drip cap.

How to Install Flashing Around a Patio Door

When sealing a patio door, check the manufacturer's instructions and follow the directions carefully. To install the flashing, cut pieces of building paper and self-adhesive flashing to fit and install them in the correct order (bottom, sides, top) around the opening. This helps ensure water has no chance to seep behind and damage the sheathing.