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When it comes to installing windows, there is nothing more important than following the manufacturer’s installation instructions.

The performance of the window can be completely compromised if the installation is not done correctly. Installing a window in new construction is quite different than replacing an existing window. Windows come with and without nail fins, and with different types of drainage systems (actually this is what we will talk about a lot in this article). If you ask 100 installers to tell you the best techniques to install a window you’re likely to get 200 (or more) different suggestions. With our industry partners, WDMA has developed a series of installation guides that cover most typical installations in a variety of building wall types with equally diverse cladding options. Whether the wall is wood framed with vinyl cladding, or a cinderblock wall that will be left exposed, there are three simple concepts that every installation should follow: proper preparation of the opening, understanding water management, and making sure the window is plumb, level and square. For this article I will provide a high-level overview of how these concepts apply to the installation of windows in a conventional new construction, light-framed wood wall.

Delivery, Storage and Handling

In a perfect a perfect world, windows would arrive to the jobsite and the contractor would install them all in the same day, right? Well we all know that in construction there is no such thing as perfect timing. Typically, material arrives well before we are ready for them or have space on site to store them. Proper planning is needed to make sure that windows are stored properly so that damage does not occur to the windows or worse, someone gets injured by accidently. When storing windows remember to do the following

  • Store the windows in a location away from traffic areas and away from areas where workers are working.
  • If the windows are shipped in protective crates or cradles, keep them in the containers until you are ready to install. Heavy equipment may be necessary to move the containers.
  • Always, always, always handle the units carefully and make sure there is enough manpower to lift the units. Again, for larger units be sure to use proper equipment such as a lift or pallet jack to move the units to avoid injury to a worker.

Practicing safe handling techniques and wearing PPE equipment will protect the window as well as your workers.

Preparing the Rough Opening 

Before the window is placed in the rough opening, it is important to ensure that the opening is isolated. To ensure the window will fit properly, the opening needs to be large enough to allow the window to slide into position and be fitted in place with a few shims. The opening should be plumb, level and square to within a ¼ inch, unless the manufacturer requires greater tolerances (check the manufacturer’s installations instructions).


After the opening is ready, it is now time to protect the opening by properly installing the water resistive barrier (WRB) and isolating the opening. The WRB is the first line of defense for managing incidental water that may find its way past the exterior wall covering. When working with the WRB, always be sure to check the manufacturer’s installation instructions for the proper lapping, fastening and sealing of the product, particularly in and around the rough opening. Most manufacturers recommend that the WRB be installed in a water shedding fashion that starts at the bottom working its way upward, with each layer overlapping the layer below. After the WRB is installed, it should be carefully cut in a modified “I” manner. The resulting WRB flaps should then be wrapped around the rough framing at the sides and bottom of the opening, and then securely fastened on the interior side of the framing. The top flap of the WRB should be left loose to allow it to be tied into the head flashing later. With the WRB properly secured, the next step is to add a sill pan flashing at the base of the opening that overlays the WRB and covers the lower edge of the opening to a depth of at least 2 inches and up the sides of the opening at least 4 inches. By preparing the window opening in this manner, any water that may find its way behind the exterior cladding or around the window at the installation joints will be diverted onto the WRB and outward toward the building exterior.

Prepare the Window

While this may sound like a no brainer, nothing leads to more callbacks to the manufacturer (other than a failure in the product itself) than an improper installation. Even a window out of square by as little as a quarter inch can create problems such as hardware not properly lining up, gaps between sashes that allow air and water to pass through, and in some cases sashes that pop out of the opening due to the friction channels not properly engaging between the sash and frame.

In many cases, perceived product defects are actually a result of improper installation.  In one such case, two windows were mulled together in the field without using the proper mullion strip and sealant. This allowed water to leak through the gaps between the window frames, find its way behind the sill flashing, work its way down the wall sheathing, and become trapped inside the floor assembly. Eventually the water began to spread across the drywall ceiling of the room below the leaking window. Needless to say, not only did the window need to be reinstalled correctly, all the subsequent damage needed to be repaired as well … all because of the improper installation. So the next time you have a window to install, make sure you check with the manufacturer’s installation instruction and consult with your installer to make sure it’s done correctly.

Steve Orlowski

Sr. Director, Standards & Technical Activities, WDMA

Photos coutesy of Pella Corporation

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