How to Add a Pitched Roof to a Flat Roof

Adding a pitched roof to a flat roof is a challenging task that may require the service of professionals. First determine if the house walls will support the additional weight of a pitched roof; this may involve an architect or structural engineer. Check with local building authorities for regulations or building codes that may affect the roof design. A house built for a flat roof probably will use a low-pitched roof, but if you want to add living space or attic storage it may require a steeper pitch.

Things You'll Need

• Pry bar
• Shovel
• Construction dumpster
• Roof trusses
• 2-by-4 boards
• Hammer
• Circular saw
• 3-inch framing nails
• Oriented strand board (OSB)
• Metal drip edge
• Roofing nails
• Roofing paper
• Shingles
• Roof vents
• House siding and trim

Instructions

1. Measure the roof's width and length, decide the pitch for the new roof and order prefabricated roof trusses to those dimensions. Determine the number by dividing the length by 24 -- trusses are spaced 24 inches apart. If this is an uneven number, add one truss. Decide what amount of overhang you want for the eaves because that affects their width. A roofing company or building supply outlet will custom-make trusses to fit the house.

2. Remove old roofing with a pry bar and shovel. If the flat roof is "built up," formed of alternating layers of roofing paper, asphalt compound and gravel, it will be messy. Do not have to remove wooden decking unless necessary to get rid of the roof material. You can leave insulation in place or remove it. If you are adding living space or storage, install plywood or oriented strand board decking on the joists. Examine the wall caps to make sure they are solid; replace any that are damaged with new 2-by 4 boards.

3. Mark both side wall caps in 24 inch intervals starting from the back; the last space may be slightly less than 24 inches. There will be two end trusses identified by extra bracing; raise one into place at the back of the house, flush with the outside of the wall. Nail the ends to the wall caps, set it plumb, then brace it with 2-by 4 boards, nailed to the truss and stakes in the ground. Lift other trusses one at a time; have one person on each side of the house with 2-by 4 boards notched at the ends to fit the truss. Lift the truss upside down with these, then have workers on the roof set it upright and nail it.

4. Install a 2-by 4 ridge board once all trusses are in place. This board will fit into slots built into the trusses. Splice two boards if necessary to fit the length of the house. Nail a 2-by 4 board lengthwise across the joining ridge boards; make the splice between trusses, so each board is firmly supported. Check all trusses to make them plumb, then nail into place at the wall caps and ridge board.

5. Nail 4-by 8-foot panels of OSB over the trusses, cutting with a circular saw as needed to fit both width and length. Nail metal drip edge along the ends and eaves. Put roofing paper over the OSB, allowing generous overlaps at seams, then nail in shingles, starting at the bottoms of the two roof sides. Nail cap or ridge shingles over the roof peak.

6. Put OSB on the gable ends, cutting to fit. Cut vent openings near the top of each gable peak and install metal or vinyl vents; this is essential to keep the newly-roofed area ventilated. Install house siding on the ends of the gable, working up from the bottom. Finish by cutting and nailing on fascia or trim boards along the edges of the gable.

Tips & Warnings

• Workers on the ground need hard hats.

References

• Hometime: Roof basics
• Online Tips: Install roof trusses
• Reliable Remodeler: Frame a roof

Resources

• Read this Article in Spanish