In many areas it is acceptable to simply install building wrap and pieces of drip-cap flashing over the tops of windows and doors. However, manufacturers and building codes often call for more elaborate flashings to protect the sheathing, studs, and interior walls. Whether you are installing new windows and doors or siding to existing units, plan and carefully install flashings that meet local codes and provide maximum protection against moisture.
The general principle is: As any moisture (which may collect due to condensation or small gaps in the siding) flows downward, it should not have an opportunity to infiltrate behind the house wrap where it can do damage. So the upper piece of wrap or flashing should go over 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 may install drip-cap flashing over the window itself or over the top trim piece that you install. For a wood unit, drip-cap flashing may be installed over the trim piece (often called brick molding) that comes with the window or door. Consult manufacturer's instructions for the recommended technique.
In addition to metal flashing, self-adhesive flashings, similar to WSU sheets used for roofing edges, are often applied around window or door openings. If you are installing a new unit, the self-adhesive flashing will cover the inside of the framing as well as the sheathing. If the window already exists, the self-adhesive flashing will likely cover the sheathing and any window flanges only. In addition, self-adhesive flashing is sometimes installed at wall corners where it can be attached over the building wrap. Consult manufacturer's instructions, your local building department, or a professional siding installer who works in your area.
When you install a window in new construction, the building wrap overlaps the self-adhesive flashing. In a remodeling job, a simpler arrangement is often used: Self-adhesive flashing strips are applied along the bottom, then the sides, then the top. Then small pieces of flashing are applied over the gaps at the corners.
In this arrangement, felt or building paper is first stapled to the sheathing, with the top pieces overlapping the lower pieces. A bead of caulk is applied to the felt, and the window is set in the caulk. Next comes another layer of building wrap, installed like the first layer. At the top, metal drip-cap flashing is tucked under the siding and on top of the second layer of wrap and the trim is installed just under the drip cap.
When sealing a patio door, follow manufacturer's instructions. In this arrangement, pieces of building paper and self-adhesive flashing are cut to fit and installed in the correct order so water has no chance to seep behind and damage the sheathing.