How to Increase the Pitch by Building a Roof Over an Existing Roof

Installing new trusses over existing roof systems can save a deteriorating roof.

For older homes with aging roofs, if the roof decking under the shingles becomes spongy and soft, applying a new coat of shingles over the old ones will not solve the problem. Roof decks will decay as a natural result of age or if the attic area under the roof can is poorly ventilated. For these homes, the best solution may be installing a new roof at a greater pitch over the existing roof system.

Things You'll Need

• Scissor trusses, (the pitch of the bottom cords should match the pitch of the existing roof)
• Roof top truss delivery crane.
• 2-inch-by 8-inch dimension lumber / framing materials
• 2-inch by-6 inch dimension lumber / framing materials
• 2-inch by 4-inch dimension lumber / framing materials
• 1-inch by 6-inch trim boards
• 1-inch by-4 inch trim boards
• 1/2 inch OSB roof decking
• Roof deck plywood clips
• Aluminum roof edging
• 15 lb felt paper
• Roof shingles
• Air compressor
• Roof shingle nail gun
• Construction framing nailing gun
• Construction trim nailing gun
• Hammer
• Utility knife
• Pencil
• Tape measure
• Framing square
• Asst. framing tools
• Extension cords
• Compressor air hoses
• Ladders
• 7/16 inch flake board
• Vinyl siding supplies, including siding, j-channel, start strip, soffit materials
• Aluminum fascia materials and matching color nails
• Vented ridge cap
• Rubber gasket vent pipe flashing
• Gable vents.
• Builders 7 1/4 inch saw
• Contractor's saws-all
• Power miter saw
• Saw horses

Instructions

Preparing for the Project

1. Order scissor trusses which have the lower cord pitch which matches the existing roof pitch. Choose an upper cord pitch for the new roof deck. Scissor trusses look like a bent regular truss. Scissor trusses will fit over the top of the existing pitched roof.

2. Examine the soffit around the perimeter of the home and make plans to cut off the entire existing soffit. The soffit is the area of the roof which extends beyond the sidewalls. The new trusses will create a new roof line, so the existing soffit must be removed during the installation. You will want to visualize the tools and ladders required to complete this step, and gather the materials necessary before the trusses arrive.

3. Prior to the arrival of the trusses, remove any vinyl/metal/wood soffit material and disconnect any light fixtures mounted in the existing soffit.

Installing the Trusses

1. When the trusses are delivered, cut off the soffit around the house. Remove the roof deck, framing, existing rafter tails and any trim boards so that the edge of the existing roof is now flush with the unfinished wall cladding. Remove the top two or three levels of the home's siding. If the home has vinyl or aluminum siding, remove the top two courses. If the home has wood bevel or shingle siding, remove the top three courses of siding to expose the home's wall cladding.

2. When you cut off the existing roof's soffit, you exposed the ends of the existing rafters. Before installing the trusses, nail a new 2-inch by 6-inch dimension lumber to the exposed rafter tails along the entire length of the house so the new trusses will rest on this new structural member. The 2-inch by 6-inch dimension lumber must be level across the home to ensure the new trusses are level when installed.

3. Install the gable trusses at one end of the roof first. Frame the gable truss in by attaching the truss to the body of the house so that the siding is one, complete, unbroken plane. Apply wall cladding (usually 7/16 flakeboard) over the gable truss framing, overlapping onto the wall of the home to the mate the truss to the home. Hold the gable truss in place by running multiple pieces of 2-inch by 4-inch dimension lumber all the way down to the ground level. These will be removed after the rest of the roof framed in.

4. Lay out the trusses across the home's span so that each truss is installed directly above an existing roof rafter. Trusses are typically placed 24 inches apart but roof rafters are installed 16 inches on center. The new truss weight must be carried by the existing rafters and not land in between the rafters, resting on the roof deck.

5. Install the trusses across the length of the home plumb, and square to the frame of the house. Use 2-inch by 4-inch dimension lumber nailed across the truss' top cord to hold them in place as they are installed. Toe nail the trusses to the roof and into the existing rafters. After the trusses are installed, nail the 1/2 inch OSB roof decking across the upper cord to create the new roof surfaces. Use the OSB to pull the trusses into alignment. The result is a completely new roof installed over the top of the existing roof.

Finishing the New Roof and Siding

1. Frame in both the second gable end of the new roof as you finished the gable end on the first truss you installed. Install a 16-inch by 20-inch siding vent in the center of the new siding area on both gable ends to allow the space under the new roof to breathe. When framing and finishing in the soffit area, which is between the edge of the new roof and the sides of the home, install vents on these surfaces too. The area between the new and old roof must breathe to facilitate proper attic respiration.

2. Finish the gable ends of the new roof with vinyl/aluminum/wood siding to match the existing siding on the home. Finish the soffit and fascia with vinyl soffit and aluminum fascia respectively. If the siding on the home cannot be matched, select a siding that is intentionally different and install it as an accent.

3. Apply new roofing shingles by nailing them over the roofing felt according to the manufacturer's instructions. The roof deck is finished with aluminum roof edge around the perimeter, 15 lb. felt over the roof surface and then shingles over the felt paper.

4. Extend any plumbing vent pipes through the new roof deck. Do not terminate any existing vent pipes in the cavity between the roof decks. Use rubber gasket vent pipe flashing to seal the vent pipes at the point where they penetrate the new roof.

Tips & Warnings

• This is a complicated project. If you are unfamiliar with remodeling, and construction codes, consider hiring a qualified contractor to complete a project of this magnitude.

• When working with power equipment, make sure you wear the proper safety equipment, such as gloves, steel-toed boots, goggles and a hard hat.

References

• Roof 101: How to Install a Roof
• Ask the Builder: Scissor Truss Design
• SBC Industry: Truss Installation and Temporary Bracing

Resources

• Oak Ridge National Laboratry: Roof and Attic Insulation
• Solarbord: Install the Coolest Roof Systems