Many older-style mobile homes were built with aluminum roofs. These surfaces become problematic as the age of the home increases and have to be re-coated or patched repeatedly. One way to improve the integrity of the mobile home's roof system is to increase the pitch of the roof. Since the mobile home is manufactured in a factory, and cannot be disassembled and modified like a traditional home, the best way to increase the pitch of the home's roof is to install a new roof using scissor trusses over the existing roof.
Planning for the Project
Order scissor trusses to match the existing roof pitch, and to create the new roof deck. Scissor trusses look like a bent regular truss. Whereas regular trusses have a one-piece, level, bottom chord and angled surfaces for the roof, scissor trusses have a two-piece, angled, bottom chord. Scissor trusses will fit over the top of the existing pitched roof, or domed aluminum roof.
Identify the existing wood framing that is in place around the perimeter of the mobile home at the roof level. Mobile homes are built with a wood structural member just under the metal siding at the building's upper corner, where the roof meets the siding. This framing member must be identified around the entire perimeter of the existing roof. If necessary, cut the existing metal siding with a saws-all and peel it back like a can of sardines in order to see the wood structure under the metal skin.
Build a solid, two-by-eight wood frame around the top edge of the mobile home. The new roof trusses must rest on a securely anchored wood frame when they are installed. This two-by-eight frame must be securely anchored to the existing wood frame of the mobile home that is under the existing metal siding, and must rise one to two inches above the edge of the existing roof. This step is the most critical step in the entire process.
Anchor the two-by-eight frame to the wood frame of the house with lag screws in addition to the nails which hold it in place. This wood structure will support the weight of the new roof and must be firmly attached to the existing home in order to distribute the weight of the new roof to the frame of the home.
Installing the Trusses
When the trussed are delivered, install the gable trusses at each end of the roof first. Hold the gable trusses in place by running multiple pieces of two-by-four framing down to the ground level. These will be removed after the roof trusses are connected and the rest of the roof framed in.
Lay out the trusses across the two-by-eight frame according to local building code. In most places, trusses are placed 24 inches apart. The trusses should be anchored to the perimeter frame and then to one another as they are installed.
Install the trusses across the length of the home. Each truss must be level, plumb, and square to the frame of the house. Use two-by-fours nailed across the trusses to hold them in place as they are installed. Toe-nail the trusses to the two-by-eight perimeter frame.
After the trusses are installed, lay and nail the half-inch OSB roof decking across the roof surfaces. Use the OSB to pull the trusses into alignment. The result will be a completely new roof installed over the top of the existing roof.
Finishing the New Roof
Frame in both of the gable ends of the new roof. A 16-by-20 roof vent should be installed in both ends in order to allow the space under the new roof to breathe. Cover the gable ends with 7/16 flake board. When framing and finishing in the soffit area, which is between the edge of the new roof and the sides of the mobile home, make sure to install adequate vents on these surfaces as well. The area between the new and old roof should be thoroughly vented to the exterior of the home to facilitate proper respiration.
Finish the gable ends of the new roof with vinyl siding to match the mobile home. Finish the soffit and fascia with vinyl soffit and aluminum fascia, respectively.
Apply new roofing to the new roof deck. The roof deck receives aluminum roof edge around the perimeter, then 15-lb felt over the entire roof surface, and then fiberglass or asphalt shingles. Install vented ridge cap across the roof's ridge line.
Extend any plumbing vent pipes through the new roof deck, and do not terminate them in the cavity created by the new roof deck. Use rubber-gasket, vent-pipe flashing to seal the vent pipes at the point where they protrude through the new roof.