How to Build a Reverse Gable Roof

What You'll Need
Circular saw
Hammer or nail gun

A gable roof or A-frame looks like an inverted V when viewed on end. This type of construction is popular because it allows rain, snow and debris to just slide off the roof and not build up. A reverse gable roof is also known in some areas as a dormer window roof. In a reverse gable roof, the reverse gable sits perpendicular to the main roof line. Below you will find the steps and tools you need to create a reverse gable roof.

Step 1: Determine Roof Pitch

Before you can begin building the roof over your dormer, you need to determine what the pitch of your main roof is, so they will match. The easiest was to do this is to divide the rise (vertical distance from wall line to peak) by the run (length of roof joists from peak to edge of eave). Once you have this number, you can calculate the measurements for the joists using algebra and simple variable replacements techniques, since you should already have the dormer itself built.

Step 2: Measure and Cut

Once you have your roof dimensions, it’s time to measure the wood for the joists and ridge peak and cut them. Be sure to cut at angles to allow the ends to mate up properly.

Step 3: Assemble Trusses

Once you have all your wood cut, it’s time to start putting it all together. Lay out your roof base, with the short edge down. Then set in your diagonals and secure them to the base and at the peak per local building codes. Do this for as many trusses as you will need in order to maintain an eighteen to twenty-four inch joists separation.

Step 4: Assemble the Roof

Once the trusses are complete, it’s time to assemble them on the roof. Start at the outer edge and work inward, maintaining spacing between trusses and joists as per local codes. Secure these trusses to the walls in the manner specified by code. As you approach the meeting point between dormer and main roof, you will most likely need to trim the trusses to fit properly. You will also need to remove some of the surrounding roofing material in order to properly mate the two roofs together.

Once all of the trusses have been installed and properly secured, it’s time to install the peak spine, or ridge. Again, secure this to the truss peaks in the manner described by your local building codes.

Step 5: Lay Roofing Underlayment

In most jurisdictions, the material used to provide a roofing base is ¾-inch AC and fire rated plywood. Cut to fit and secure the to the roof trusses with wood screws or nails. Once the plywood is down and secure, it’s time to lay the water barrier tar impregnated felt or paper. Start at the bottom and unroll lengthwise, securing with plenty of staples. As you move up the roof, overlap the previous layer by approximately three inches to provide protection against water leaks.  Once this barrier is down, you can move on to laying down your roofing material of choice.

Remember to thoroughly consult local building codes prior to beginning this project.