To build a covered porch, assemble the materials and tools needed, including lumber, galvanized nails, a level and a saw. If attaching the porch roof to an existing house roof, install a ledger board with lag screws and seal it to keep water out. Paint, seal and cut the lumber into rafters. Erect porch posts, and mount the rafters between the porch posts and the ledger board. Check the angles of the rafters with the level, then install the roof.
Use aluminum flashing to seal out water by tucking it under the siding of the home and running the material over the top and down the front of the ledger board. After crafting the rafters to the appropriate length, mount the joist hangers before hanging the rafters.
To attach the rafters, secure them to the ledger board and then settle the joists. Install the rafters from the ends of the porch roof and work toward the center. Use support beams to connect the rafters to each other as well as the porch posts. Rafters can extend over the support beams but should not extend further than a third of the overall length of the rafter. To finish the roof, add fascia to the end of each rafter.