A shed dormer is a feature that can add more space and a lot of light to any upstairs loft or room. This type of dormer rests the entire load on the existing bottom end frame, which saves a lot of time in underlying construction. While other styles of dormer rely on having to sister in a new framework, the shed dormer does not. A shed dormer has a flat pitched roof, not an angled gable.
Building a dormer does require some skill with construction, but with some simple tools, the project is relatively easy.
Planning and Design
Before you open up your roof, do a thorough inspection of the rafters and the stick construction that supports the roof itself. Take careful measurements of the area over which you want to install the dormer, and transfer those measurements to a graph plan.
You should avoid interfering with the joist structure of the main house frame as much as possible, so the actual size of the dormer will depend on how the upper frame was built. If you can remove just a few of the rafters, installing the dormer will be much easier.
Some shed dormers go all the way to the roofline, but typically, they are installed a few feet below the rooftop. A shed dormer is the easiest to install and to seal correctly, as the new dormer frame is sitting on the original house frame.
Step 1 - Mark an outline of the opening on the outer part of the roof, as well as on the inside of the ceiling. This is where your measurements really come into play. Measure three times, cut once!
Step 2 - Remove the shingles from the area that will be taken out for the dormer. Drill pilot holes at each of the 4 corners.
Lower 4 pieces of string through each of the holes, and staple the top end to the roof.
Step 3 - Attach a plumb bob to the 4 corners. Mark the location of the plumb line point for each corner. Remove the string when you are done.
Step 4 - Drill through the ceiling at each of the 4 points. On top of the roof, place duct tape and plastic to keep the weather from coming through the holes.
Step 5 - On the inside of the room, cut the rectangle that is described by the 4 drilled holes. You should now have a clear view of the rafters.
Step 6 - Construct the framework for the dormer using the same size of dimensional lumber as your rafters.
Step 7 - Cut the upper roof from each of the four points.
Step 8 - Remove any of the rafters necessary. If you remove more than 2 in the span, you will need to sister in more rafters on each side for stability.
Step 9 - Install the dormer side frames to the existing rafters. You may need to put in a header at the top, and perhaps a footer at the bottom during installation.
Step 10 - Install cross members at the lower end of the dormer. The bottom frame should be ½ an inch larger than your window.
Step 11 - Install the outside plywood sheeting around the new dormer.
Step 12 - Install the ceiling rafters and plywood on top of the shed dormer.
Step 13 - Install the roofing material and skirting. It is a good idea to use a high-quality sealant to run the edges of the new dormer skirts.
Step 14 - Install the window. Seal around the edges with a window silicone.