So you want to pitch your flat roof. It might be that you want to add attic space or simply change the look of a certain portion of your home. No matter what the reason, pitching a flat roof is a big job and requires special tools and knowledge to complete. Here are the basics of what is involved in pitching a flat roof.
Calculate what the roof span will be. Measure from top plate to top plate, add any overhang measured from the face board, and divide in half. This will tell you where the roof ridge will fall.
Determine the pitch and calculate the material costs. In most modern construction, roof trusses are manufactured then shipped to the building site with a pre-determined pitch. If you want a non-typical roof pitch, then you will need to build your own. Once the pitch has been established, calculate the surface area of the new roof and make up a materials list.
Remove the roofing material and tie in the roof trusses to the existing lumber with a crowbar and sawsall. You will need to expose wood, so make sure you do this when there is little or no rain, or cover your work area with construction tarps. Cut out any rot and analyze whether or not the existing flat roof joists and lumber can take the load of the new configuration. Make sure to bridge your roof trusses and space according to local building code.
Re-sheath the roof with plywood or OSB, depending on your local building code. If the sheathing on the old flat roof did not have any rot, you could re-use it to sheath the new, but keep in mind you will need approximately twice as much material. Use galvanized nails to attach the sheathing.
Replace the roofing material. You can use a wide variety of roofing materials, such as tile, slate, shingle, rubber composite or even cedar shake. Be sure to place tar paper beneath the roofing material and use the recommended fasteners for your type of roofing.