A bay window is an attractive feature on a house, but tying the bay window in to an existing roofline can be difficult. The geometry of the bay window is almost always different from the rest of the house. A typical bay window does not have the same head height or structure as a typical building -- it is a cantilevered space attached to a building's frame.
The most seamless method to tie a bay window roof to the main roof is to extend the main roof eaves beyond the bay window. The eave soffit should be at least six inches beyond the face of the bay window, and the bay window should be framed into the eave, without a gap or secondary roof. This method creates a continuous fascia on the structure, but extending the eaves adds a great deal of material cost, labour cost and bracing for the cantilevered eaves on either side of the bay window.
Match Bay Window and Main Roof Angles
A common method of tying a bay window to an existing roof is matching the bay window's roof with the main roof. The angle and roofing material of the two roofs should be the same. Furthermore, the bay window's roof should begin where the existing roof ends, without an elevation change or gap on the cornice of the existing roof.
Turret and Parapet
For bay windows that cannot tie into the existing roof, you can extend the framing of the bay window up, beyond the roofline, creating a turret. The bay window roof can be lower than the framing, creating a parapet, yet sloping to the existing roof, to create positive drainage. This technique will look appropriate on only a fraction of existing roofs and you will need to flash or cope the parapet wall, but it is a reasonable solution for some structures.
Tie Bay Window Roof to Main Structure Wall
The most common method of tying a bay window roof to an existing structure is terminating the bay window roof in to the existing structure's exterior wall, below the existing roof. This will not tie the roofs directly together, but it avoids complicated cuts and strange geometries, which might mar the structure's facade.