If you are buying or building a new home, knowing the components involved in roof construction will make you a more discerning homeowner. The fundamental support for your roof comes from rafters and the roof deck, or substrate. Rafters are wooden beams designed to support the weight of your roof. The roof deck is installed over the rafters and acts as a unifying structure for the whole roof. Underlayment is placed over the roof deck and provides protection from water leaks. Roofing material is nailed over the underlayment. Both the roofing and the underlayment can be made from a variety of materials. Other elements found on roofs include dormers, valleys, skylights, and chimneys.
Read full article here: Roof Construction—Framing, Substrate, Underlayment and Shingles