Building a Shed Roof Over an Existing Roof

What You'll Need
Tape measure
Screwdriver or screw gun
Metal cutters
Sheet metal (how much depends on the size of your roof)
Roofing nails
Rubber gasket tape
Long screws (at least 3 inches)
Purlins (the length is determined by the size of your roof)

Building a shed roof over an existing roof is a good way to extend your existing roof's life as well as provide a cushion of insulation against the elements. The basic process is using sheet metal to build another roof on top of the roof that's already there. With the proper tools and a little know-how, anybody can complete this project. Just make sure to observe safety rules and standards, especially since you'll be working high above the ground. 

Step 1 - Attach the Purlins

The purlins are the framework that the shed roof is going to rest on. They will need to be attached to the roof of your house to provide a secure base. Line up as many as you need along the edge of your roof, making sure each purlin is even and pressed tightly against the next. Screw in the screws at 6 inch intervals until they're secure. Once you have the base purlins installed, place the remaining ones that will travel up your roof. Use the saw, if needed, to cut them to the proper length. You will generally want a purlin at either side of each piece of sheet metal, as well as an additional one in the middle, just for the added support. Once you have them placed, screw them in securely.

Step 2 - Attach the Sheet Metal

Once you have the purlins attached, you want to lay the first piece of sheet metal down. Attach a strip of rubber gasket tape that stretches the entire length of the piece to the furthermost purlin. This will provide a protective layer and prevent water and the elements from getting inside your new roof. Once the tape is laid out and the adhesive is exposed, lay the piece of sheet metal down on top of it, making sure it is completely even with the purlin. Hammer in the roofing nails every few inches through the sheet metal into the purlin to make sure it is absolutely secure. 

Step 3 - Finish the Layering

Once the initial sheet metal is in place, you will need to continue the same steps until your roof is completely covered. Place another strip of rubber gasket tape to the outermost purlin underneath the first piece of sheet metal you placed and expose the adhesive surface. Press down the first piece of sheet metal until it's secure. Then apply another piece of gasket tape to the top of the sheet metal and lay the next piece down on top of it. Once you have that piece securely fastened, pound in the same amount of nails. Continue this until your roof is entirely covered. When you get to the final piece, use the metal cutters, if necessary, to cut it to the proper width. If you want some additional insurance against water infiltration, apply a thin bead of caulk to where each piece of sheet metal overlaps the other.