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Extension > Environment > Housing Technology > Moisture management > Water managing doors, windows, and skylights

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Water managing doors, windows, and skylights

By Richard Stone, Extension Educator in Housing Technology

Unlike complex roof flashing details which are entirely on the exterior, doors, windows and skylights are installed across the building envelope from the interior to exterior finished surfaces of a home. Doors, windows, and skylights, in addition to being architectural features of the home, are usually operational, providing access and ventilation. Protection of the structure and its occupants from the elements are also basic functions of doors, windows, and skylights. This article provides an overview of installation details that are used to fully integrate doors and windows with the drainage plane and provide the structure with maximum protection against water intrusion and exposure to Minnesota weather.


Photo by Richard Stone, University of Minnesota

Because the correct flashing of roof openings seems to be more visible and more intuitive than the details of side wall openings, skylights provide a good example of integration into the drainage plane. Flashing components and detailed instructions for the installer are usually packaged with skylights. Millwork manufacturers are beginning to make flashing packages available for side wall installations as well. Door and window flashing installation kits and guidelines are also provided by a number of weather resistant barrier (WRB) and flashing product suppliers and manufacturers. While all these sources of guidance for door and window installation are useful, the information may not be getting out to everyone that needs it. Many out of date or questionably effective practices are still frequently seen in new construction.


Photo by Richard Stone, University of Minnesota

The concept is simple. Every component of the drainage plane from the footings to the roof must be applied "shingle fashion" to direct water down and out. By thinking of the outside surface of doors and windows as "shingles" in the system that protects the walls from water intrusion, the details of a well integrated drainage plane are easily understood and applied.


Photo by Richard Stone, University of Minnesota

To prepare for a door or window installation, a sill flashing is installed on the subfloor or rough opening to direct any water that may leak through a door or window assembly back to the outside surface of the weather resistant barrier as shown by the window corner with correctly completed sill and side flashings or directly out to the exterior finished surface.

When installing doors and windows, the top and sides of the flange should be sealed to the surface behind them with a bead of good quality caulking. However, the outside of sill flanges should NOT be caulked or sealed. Caulking or taping the bottom flange as shown in the examples can trap water in the sill instead of allowing it to drain back to the WRB or an outside finished surface.

The example showing the incorrectly taped sill flange also shows the use of WRB tape on the side flanges where flashing tape should have been used. Side flanges should be sealed to the weather resistive barrier using flashing tape that has been designed for that purpose and will stay flexible to allow for normal movement and seal nail penetrations.


Photo by Richard Stone, University of Minnesota

Where the water runs down the weather resistant barrier and on to the window, flashing tape is installed over the top flange (or metal head flashing) and side flashings. The weather resistant barrier is then lapped over the flashing tape and held in place with WRB tape until the siding is installed, as shown in the example of correctly installed side and head flashings.


Photo by Richard Stone, University of Minnesota

Too frequently, the flashing tape is applied over the flange and on top of the weather resistive barrier. When the head of a door or window is reverse flashed, the adhesive component of the flashing tape must stop any water running down the wall. Weather conditions or improper installation can lead to a failure of the adhesive which may allow water to enter the window rough opening and window assembly.


Photo by Richard Stone, University of Minnesota

If the housewrap or building paper (WRB) is lapped over the flashing tape and fastened in place as shown in the correctly completed corner detail, gravity will direct the water out even if the fastening tape and flashing adhesives degrade over time. If weather conditions are a concern, use mechanical fasteners to hold flashing tapes in the correct position until the next step can be completed. In the case showing adhesive failure there are at least three obvious opportunities for improvement visible. The flashings are installed behind the head and side flanges of the window (reverse flashed) and staples could have been used to hold the flashing tape in place until the weather resistant barrier was completed by another crew.

Gravity has been working reliably for much longer than the most effective adhesives on the market today. Using dependable products that are applied in a "shingle fashion" will give the best results when managing water on building exteriors. When choosing and installing building products, always follow the manufacturer's recommendations. To assure proper performance of every building assembly, read the technical information for all materials used on the job and, when needed, provide the appropriate specialized training to the trade professionals who will be responsible for completing each task. Finally, it is important to remember that there are many excellent products to choose from but the outcome largely depends on individual craftsmanship and a good understanding of building science and the building process by everyone who touches the job.

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Reviewed 2010

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