Connect a Flat Roof to a Hip Roof

What You'll Need
Framing studs and roofing joists
Plywood roof sheathing
Framing screws
Roofing nails
Polyethylene sheets
Waterproof roofing membrane
Roof cement
Wood screws
Joist hangers
Roof drain and gutters
Rigid foam insulation

A flat roof is a cheap and easy to build framing solutions often employed for utilitarian structures such as patios, carports, garages and sheds. A hip roof, on the other hand, is even more complex than the conventional gabled roof with a single peak. A hip roof features a polygonal king board with roof sections sloping down to the fascia on every side. There is no gable in such a construction. The architectural effect greatly increases the perceived weight of the roof. Gables can be used around dormers and foyers to highlight those areas. Hipped roofs often have to be custom-built on site. However, connecting the flat roof of a minor addition is not as complicated.

Step 1 – Plan and Obtain Materials

Draw a blueprint of the proposed roof connection. Calculate your materials requirements in square feet of roofing materials and linear feet of framing materials. Choose a day with fair, dry weather to work.

Step 2 – Determine Attachment Point

The elevation of the flat roof addition will determine the way you attach it to the hip roof. It is easiest to attach the flat roof below the fascia of the hip roof, sloped away from the existing structure. If necessary, you can attach the flat roof at the level of the fascia, but you will need to add a drainage gutter.

Step 3 – Build Frame

Attach ledger boards to the joists at the site of the junction if you are connecting the flat roof below the hip roof fascia. Otherwise, remove the soffit and fascia from the hip roof on that side. Tie the addition’s joist headers to the existing walls top plates, leaving a trough to run the gutter through later. Place roof joists 16 inches on center across the span of the addition.

Step 4 – Cover Frame

Screw plywood decking to the top of the roof joists along the flat roof addition. Next, cover everything with polyethylene sheeting or some other type of vapor barrier. Install the roof drain. Local building codes govern requirements for roof drains depending on the square roofing of the addition and its distance from the existing structure.

Step 5 – Finish Roof

Build a parapet at the end of the flat roof. Install rigid foam insulation across the roof surface. Cut it into thin strips and fasten it diagonally across the corners below the parapet. Lay down the roof membrane across the surface of the flat roof. Roll it up and over the parapet. Use sheet metal coping bent over the surface of the parapet to hold the membrane in place. Slant the coping slightly inward so that rainwater drains onto the roof membrane. Finish installing the gutter system at this point. Close every junction and seam with metal flashing and roof sealant.