Connecting new and old roofs is a challenging task, requiring a lot of labour, helpers, time and money. It also requires construction expertise. If you are not experienced in home building, hire a professional. It is a good idea to have your plans drawn or approved by an architect, since the connection will involve structural elements of the house. The new roof can be either gable or hip; the connection techniques are basically the same.
Remove shingles and other roofing material where the new roof will join, down to the deck board of the old roof. Then set one new truss in place against the old roof and use it to snap chalk lines to mark the location on the old roof for diagonal 2-inch-by-6-inch valley boards, which will hold jack rafters to tie the roofs together. Nail that truss in place to the wall caps and ceiling joists on the new addition. Use a level to be sure the truss is plumb. Nail the valley boards to the old roof, leaving space at the top for a ridge board.
Set additional trusses in place 24 inches apart. Lift them to the roof upside down. Have one person on each side with notched 2-by-4-inch boards to hold the truss and another person in the centre, then walk the truss down to its spot. Workers on the roof then can lift it and nail it into place. Continue this until all trusses are set. The final truss may have less than 24 inches separation to conform to 4-foot-by-8-foot sheathing panels. When all trusses are in place, nail a ridge board from the end truss to the old roof in slots atop the trusses. Make sure all trusses are plumb and secure.
Make jack rafters to tie the ridge board to the valley boards. These will have to be cut at angles and varying lengths down the slope of the old roof; consult a rafter table online or at a building supply store to get these angles. They should be 24 inches apart. Once all trusses and rafters are set, cover the new roof with OSB decking. Nail metal flashing in the valleys where the roofs meet, and metal drip edge along the eaves. Lay roofing paper over the decking, overlapping seams, then nail in shingles; bend shingles at the valleys to cover them and tie in with old roof shingles.
Tie a new roof to an existing gable differently. If your addition butts up to the end of the house, either on the same roof plane or lower, e.g., a one-story addition on a two-story house end, start your framing by nailing a new truss either to the last truss in the old roof or to the wall of the house. Remove siding to open the framing if nailing to the wall. Nail the truss to joists or studs. Then install other trusses and finish the roof the same as with a perpendicular new roof.
Finish the new gable end with house siding and trim. A hip roof on a new addition will tie to the old roof the same, but the end framing will be different. Consult framing tables or a building supply store for specific materials and cut angles.
Lifting trusses requires at least four and preferably six to eight people.
Be careful working on a roof. Workers on the ground should wear hard hats.