Connecting a flat roof to a hip roof is moderately challenging because of the varying angles and slopes involved. However, the addition can be realised with a little prudence and planning.
A hip roofed structure has uniform height facades; the fascia, or face board at the roof's edge, continues around the entire building, without varying elevation. The form of the hip roof is very attractive, especially in comparison to the triangular ends of a gable roof. However, the connection of another roof can complicate the geometry of the simple hip roof.
The addition of a flat roof to the hip roof is very appropriate and appealing. The flat roof has a continuous fascia, similar to the hip roof, and the low profile can make an attractive accent to the existing home. With care, a flat roof can be the perfect extension of a hip roofed structure.
Sketch, on a sheet of paper, the existing structure with hip roof and the proposed structure with a flat roof.
To avoid problems with shedding water, the flat roof must not be above the fascia of the hip roof. If the flat roof meets the fascia of the hip roof, a heavy gutter system should be used at the intersection of the two roofs. If the flat roof intersects with the existing structure's exterior wall, the slight slope of the flat roof should fall away from the existing structure.
The intersection of the two roof slopes can cause water to back up, possibly leading to roof leaks. Make sure there is adequate drainage using gutter or roof drain assemblies.
Bolt a ledger board, or joist support, to the existing exterior wall at the elevation of the flat roof, unless the flat roof is to meet the fascia of the existing hip roof. If the roofs meet at the existing fascia, remove the soffit, or underside of the roof eave, and fascia board from the area where the two structures meet, and tie the addition's walls and joist headers to the top plates of the existing structure. At the connection allow a trough for a gutter to be installed, collecting water from the slope of the hip roof.
Place the addition's roof joists at 16 inches on centre, running perpendicular to the existing facade. The joists should be level, creating a horizontal plane. Screw the plywood decking to the roof joists, and cover the assembly with polythene sheeting.
If a roof drain is to be used, install the roof drain, following the instructions provided with the drain. Roof drains should be used if the addition's footprint is greater than 300 square feet and extends more than 16 feet beyond the existing structure's footprint.
Build up a 4 to 6 inch parapet, parallel with the joists, above the joists and decking. This parapet will hide the slight elevation change of the low-slope rigid foam insulation.
Place and fix the low-slope rigid foam insulation. Cut cant strips from excess rigid insulation, and place the cant strips at the intersection of the parapet and roof. Install the EPDM roof membrane. Follow the installation instructions provided by the EPDM's manufacturer. Seal any openings and seams with bonding tape, and flash any openings or junctions between surfaces. Cover the EPDM with ballast aggregate, and install the coping, if desired. The ballast holds down the roof membrane, and the coping defines the roof's edge with a continuous band of stone or concrete.
A flat roof is not really flat. The roof has a shallow slope--approximately 1/4 -1/2 inch of drop per 12 inches of run. The rigid insulation creates this slight slope.
Flat roofs will leak if the roof membrane was installed incorrectly or water stands indefinitely on the roof's surface. Clean debris from the roof surface, flash all openings and surface intersections, and take care when installing the roof.