Tie Pergola Into Existing Roof . Adding a gable roof to an existing roof. The panels should overlap each other slightly. Tying a patio roof into existing house. Techos de casas, Porches de from www.pinterest.com Now, you can install a valley board to the existing roof. Gable roof patio cover attached to addition build house attaching open in how attach a an tying into existing over deck porch fastening the hundt covers and decks. Check with the local city’s codes department before adding the roof.