Window flashing is material installed around windows designed to prevent water from entering between gaps in adjoining building surfaces. Wind-driven rainwater can enter even small spaces left between the frame, or casing, of a window and the siding, or trim, of the home. A lack of properly installed window flashing can lead to water infiltration to inner walls that can cause wood rot and structural damage as well as insect infestation.

Proper installation of window flashing requires that a total barrier be formed between the window and the siding. The nails along the top of the window frame are removed and the frame is pulled out slightly from the siding in order to leave space for the window flashing to be slid in. Dirty areas should be cleaned and dried and any old caulking should be removed with a putty knife. After the flashing is in place under the siding, it is nailed on using the original nail holes. A bead of sealant is often added along the window edge as an extra measure of protection against unwanted moisture.

All window flashing needs to be weather-resistant, durable, and compatible with the surrounding building materials used in the dwelling. New window flashing and/or sealant can be added to existing flashing, but compatibility of all materials must be thought out to avoid an undesirable chemical reaction. Vinyls are particularly likely to react negatively with other materials such as sealant, so always follow product warning labels.

Window flashing is available in 3 basic types: sheet metal, vinyl, and tape. Sheet metal varieties are widely used and include aluminum, copper, zinc, and galvanized and stainless steel among others. Vinyl window flashing is also called PVC flashing and is quite inexpensive. Tape flashing, also called self-adhering membrane, is the newest form of window flashing.

All 3 types of window flashing have advantages and disadvantages. Metal window flashing is durable and fairly easy to install, yet some metals can be difficult to work with or have sharp edges. Both stainless steel and galvanized steel window flashing can be especially difficult to work with. Vinyl flashing is easy to work with and can be cut to fit difficult areas, but it is not strong and may snap in cold temperatures. Self-adhering membrane is easy enough for even beginners to work with, but may degrade in high temperatures or in ultra violet light.

Most homes built today have window flashing installed. Window flashing is especially important in coastal areas or other locations prone to storms and heavy rain.