We’ve noticed water stains at the bottom corners of several windows and some rot under the carpet under one of these windows in the house we rent. I checked the outside siding and it fits tight to the wood window frames so I’m wondering what possible ways water is getting into the walls and floor?

The most likely and most common reasons for leaks around windows that appear inside the wall cavities are ice damming at the roof eaves and missing head flashings over window and doors. Since this leakage seems to be happening during seasons other than winter, the reason for the leakage is probably the latter. Unfortunately in the past, builders, window installers and siding installers often omitted head flashings over these openings. Some trades people relied on caulking or stucco coats to form seals between the upper horizontal window and door “brick moulds” and the siding. Because this brick mold forms a flat horizontal ledge, water that runs down the siding lays on this surface creating an opportunity to wick into the wall cavity. Sometimes lady luck smiles and the window or door doesn’t leak. Otherwise, expect problems.

However, there are times when head flashings are not necessary. Here are some head flashing guidelines for times when head flashing installation is important to ensure windows don’t leak at these vulnerable locations. Measure the horizontal distance from your exterior wall to the far leading edge of the eave of your house (just under the soffit roof overhang). Next, measure the vertical distance from the bottom of this eave soffit to the top of the window or door trim (brick mold). A head flashing is required if this vertical distance is greater than one-quarter of the horizontal distance of the overhang at the eave soffit. Now, lets consider the gable ends of the building. There is often little or no eave protection for these window and door openings so these are the most vulnerable penetrations through the building envelope. You can see how likely leaks develop over these openings without any head flashings.

If you are retrofitting head flashings, they should be installed at least 2 inches (50mm) up and under the sheathing paper and existing siding then returned over the existing brick mold. Just to complicate matters, there are newer window and door designs that occasionally incorporate a head flashing detail into their design. With these windows and doors, the installer is required by Code to embed the exterior flange of the window or door into a bed of caulking that seals the brick mold joint to the exterior wall sheathing. The general rule of thumb in all construction is to “drain the horizontal plane” away from the building. This is a good rule to follow when dealing with all horizontal construction surfaces as well as the ground and slopes around the building.