Extending a gable roof implies an addition to a house. There are few reasons to warrant the effort and expense to extend a gable roof a little distance without an addition. The toughest part of extending a gable roof will be matching the roof tiles and other components of the original roof, especially if it is an older house. It may involve ordering prefabricated roof trusses to match rafters in the original roof, but it is easier and better to use trusses rather than fabricate rafters on the site.
Remove trim boards and siding on the existing gable end with a prybar, down to the studs and original roof framing. Peel off roofing from the end back to the next truss or rafter. Order prefabricated trusses to match the pitch of the original roof; determine that by measuring the width of both old and new walls and the height from the base to the peak of the old roof, using tape measure. Match the dimension of the top ridge board, usually 5 by 10 cm (2 by 4 inch) or 5 by 15 cm (2 by 6 inch), when ordering new trusses.
Nail a first truss to the existing end of the house with hammer, matching it to the truss or rafter on the old roof. Width, height and pitch should be identical. Nail this truss to the cap boards on the side walls and the last truss or rafter on the old roof, using hammer. Cut the ridge board at the peak of the old roof with reciprocal saw. Cut it halfway between the outside truss and the next truss, and remove it to splice in new ridge board.
Mark the new walls using tape measure at 60 cm (24 inch) intervals for new trusses. Lift trusses into place one at a time. Use level to make sure they are plumb and nail them at the ends to the wall caps. Lift trusses with one person outside each wall using a notched 5 by 10 cm (2 by 4 inch) board to hold the truss upside down and walk it into position, where workers on the roof can raise it and nail it.
Install all trusses to the end of the new roof, checking each with a level for plumb. Brace the end truss with a 5 by 10 cm (2 by 4 inch) beam nailed between the truss and a joist. Place a ridge board, the same dimension as on the old roof, in the spaces allotted at the tops of the trusses. Use a splice to connect old and new. Cut 45 cm (18 inch) sections of board and nail them across the joint of old and new ridge board, with the new board sitting atop both the first new truss and the last old truss and the splice between old trusses. If one ridge board won't cover the new roof, make a second splice on the new roof board.
Cover the new roof with oriented strand board (OSB), nailed to the tops of the trusses with roofing nails. Cut it to fit with circular saw. Remove one section of shingles on the old roof. Lay roofing paper over the OSB, overlapping seams at least 10 cm (4 inches), and extending at least 15 cm (6 inches) over the old roof. Nail on shingles with roofing nails, starting at the old roof and working to the other end. Shingles are made to interlock and overlap.