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Dormer Window Installation

A dormer uses a window built into the gabled extension protruding out of the roof. These windows can be inaccessible, and for aesthetic purposes only, or can be a clever way to add light and extra space to an upstairs room. Because of their high location, installing one of these windows can be a little tricky.

The installation of a dormer window requires a series of steps that must be carefully followed to ensure proper installation and prevention of leaks.

  1. The first step is to select your window style. Decide what purpose you want the window to serve. If you are hoping for ventilation, choose a window that opens, but still looks attractive from the outside.
  2. Measure your opening at the top, center and bottom. If you have more than an inch discrepancy at any point, you will need to nail in wood filler strips.
  3. Windows can be one of the biggest culprits for leakage and dry rot issues. The standard trim and housewrap often times isn’t adequate protection. It is recommended to put a layer of waterproof membrane or builder’s felt over the existing housewrap for an extra layer of protection.
  4. Metal flashing can be installed over the membrane for even more protection and to deflect water away from the window.
  5. Make sure the window’s nailing fins are folded out perpendicular to the sides of the window frame. Carefully tilt the window into place, and make sure the gaps between the frame and the studs are equal on both sides. Gently tack into place in the corners, but don’t drive the nails all the way in.
  6. Use a level to check both sides, the top, and the bottom of the window. Square the window by measuring diagonally from corner to corner, making sure the measurements are within 1/16 inch. You may need to adjust the window a couple of times to reach this point. Once the window is level and square, finish nailing it in through the nail fins.
  7. Put another layer of waterproof membrane over the nail fins.
  8. Place a thick bead of exterior caulking over the window frame, and then press metal flashing into place. Flashing should be provided by the window manufacturer, or you can purchase it separately.
  9. From inside the house, caulk the gap between the studs and the frame with expanding polyurethane foam. Fiberglass insulation can be used as well, just seal it in place with a piece of aluminum tape.
  10. Trim both inside and out with wood or composite trim material. Add primer and paint for a final waterproof seal.

Installing a dormer window does require a certain amount of skill, and if you are up to tackling the project yourself, you can find more detailed examples at thisoldhouse.com. This is actually a pretty quick and simple project for a professional, and if you’re interested in requesting a free estimate, check out these certified contractors located in your area.

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