how to frame a gable roof addition
Steps. 1  1. Nail top plates to all the outside walls. 2  2. Measure and cut ceiling joists from 2-by-4 lumber. 3  3. Cut a ridge board form 2-by-6 lumber.  4. Lay planks across the ceiling 1  joists. 5. Calculate the rise of the gable roof.  6. Measure and cut the rafter boards from 2-by-4 1  lumber. 7. Make a collar tie for each pair of rafters.