Any roof construction is a challenge. It involves working on top of a structure and lifting and moving heavy components. If you are not an experienced home builder, leave the job to a professional. To tie a hip roof into an existing roof, get a roofing table that will explain rafter angles and cuts. You will need a lot of them for a hip roof, which slopes on three sides rather than two as on a pitched or gable roof. Also, recruit helpers. This job requires at least four workers and preferably six to eight.
Prepare the surface for connecting a new hip roof to an existing roof by removing shingles and other roofing, down to the decking of the old roof. Set up one prefabricated roof truss abutting the old roof as a guide for marking valley boards. Snap a diagonal line from the truss peak to the bottom of the existing roof on both sides. Lay the truss down and nail in 2-inch-by-6-inch valley boards along those diagonal lines, leaving an opening at the top for a ridge board. Seal the outside edges of the valley boards with roofing cement.
Install prefabricated trusses out to where the hip slope will begin; this will vary with addition length and width and the pitch of your roof. Check roofing tables online or at building supply stores to calculate these dimensions. When your trusses are set, nail a ridge board along slots in the tops to connect to the existing roof between the valley boards. Nail the ridge board to the valley boards from the sides. Cut and nail jack rafters to connect the ridge and valley boards; these will vary in angle and length from the ridge to the old roof.
Begin your hip roof framing with three rafters: one in the centre from the ridge board to the roof end and two others diagonally from the last truss to the corners of the roof end. Then cut and nail jack rafters on either side of the centre rafter to the outside walls; these should be cut to extend beyond the walls and be notched to fit over the wall caps. Nail centre rafters first and nail them in pairs, one on each side of the roof to keep common rafters plumb. Then cut and nail jack rafters parallel to the centre common rafter, from the top to the end wall. Extend them over the wall and notch them as with the side rafters.
Cut short joists to nail from the end wall to the length of the rafter overhang. This should be the same dimension as the overhang on the sides. Then deck the hip roof with oriented strand board, and nail metal flashing to the valleys and peaks and metal drip edge along the sides. Cover the decking with roofing paper, overlapped at the seams, then nail on shingles. Bend shingles at the valleys to cover them and tie in with shingles on the old roof.
Finish with fascia or facing boards around the edges of the hip roof and soffit boards covering the overhang. These are optional; most hip roofs will have them but they are not essential.
Raise prefabricated roof trusses with a team: one person on each side with a 2-by-4 notched to hold the truss upside down, one in the centre to lift it and one or two on the roof. Walk the truss down the walls to its spot, then let the roof workers lift it upright.
Use caution working on roofs. Workers on the ground should wear hard hats.