How to Form a Dormer Window

Written by tammy domeier | 13/05/2017
How to Form a Dormer Window
Dormer windows increase living space and add aesthetic interest. (Rather twee dormer bungalow with an attractive front garden image by hazel proudlove from

Dormer windows extend the living space of an attic without increasing the height of the roofline. You can incorporate dormer windows into the design of the house during the initial roof framing, or you can retrofit dormer windows onto an existing roof. Gable dormers and shed dormers are the two most common dormer types. Gable dormers run perpendicular to the roofline while shed dormers run parallel to the roofline. Forming gable dormers on an existing roof is a common but complex project.

Read the blueprints to determine the length of the rough opening for the dormer window.

Measure the exterior area of the roof that you will use to form your dormer window using a tape measure. Mark the area with chalk, cut through the roof shingles using a circular saw and remove the roof shingles.

Read the blueprint to determine the dimensions of the dormer window and mark the location of the front dormer wall using chalk. Place a builder's level on the outside of the dormer window line and mark the rafters with chalk. Cut off the rafters using a circular saw, deducting an additional 1 1/2 inches to accommodate the header.

Cut new rafters the same length of the old rafters and nail the rafters underneath the roof sheathing using 16d common nails.

Measure the length of the double headers, or the boards that support the top and bottom of the dormer window, using the blueprints as reference. Cut the lumber using a circular saw and nail the headers in place using 16d common nails. Use a builder's level to plumb the corners and brace the headers by nailing 2-by-4 lumber around the headers.

Nail a corner stud into the roof for insertion of the side wall, cut the end of a 2 by 4 using a circular saw to form the top of the side wall and nail the top of the side wall to the corner stud. Cut 2 by 4 lumber to form the bottom plate, which rests against the roofline. Mark the length of the bottom plate every 16 inches and nail a stud through the bottom plate onto the roofline using 16d nails. Add another bottom plate to the other side of the dormer window. (See References 2)

Use the roof slope in your blueprints to make a ridge using 2-by-4 lumber that will run down the middle of the dormer to the roofline. Use the blueprints to determine the length and slope of the common rafters. Nail the rafters to the ridge using 16d nails.

Install the 2-by-4 overhangs for the wall sheathings by nailing 8d nails every 6 inches along the sheathing. Add roof trim and roof edge mouldings according to your blueprints. Complete the interior framing by nailing the rafter ties using 16d nails.

Things you need

  • Blueprints
  • Tape measure
  • 2-by-4 lumber
  • Chalk
  • Circular saw
  • Builder's level
  • Studs
  • 16d nails
  • 8d nails
  • Roof trim
  • Roof edge mouldings

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