Roof construction starts with a structural framing system made of wood or metal. The frame is constructed in such a way that the weight of the roof is distributed safely to the building's walls, then down to the ground. Builders rely on four basic roof construction techniques, ranging from simple shed roofs to complex intersecting structures.
A shed roof resembles a lean-to, with a single sloping platform that overhangs the walls. Construction of a shed roof starts by installing a double layer of rafters at the top of each wall. Parallel joists are connected to these rafters, and are installed so that they slope in the same direction as the room. At the high end of the roof, the joists are fastened to a horizontal support beam known as a "ridge board." This technique is one of the easiest and most affordable ways to construct a roof.
Gable roofs form a triangular shape that resembles an open book. They have a single peak, with the remaining two walls built up vertically to the underside of the gabled structure. These roofs are constructed using gable ended trusses to span the entire length of the building. The trusses rest of the end walls at either side, where they distribute the weight of the roof down to the ground. Every third truss is reinforced using a horizontal member known as a collar beam. Trusses in gable roofs are traditionally installed at 24 inches on centre, or at shorter spans as determined by the roof's designer.
A hip roof resembles two intersecting gable roofs, which form a pyramid shape. Sometimes the roof is designed with a flat top, while at other times it forms a single high-peaked point. To construct this roof, equally sizes of framing members known as common trusses are installed along the central portion of the structure Unequal girder trusses are then installed at each end to form the sloped hip portions of the roof. Small sections are typically built using hip rafters, which consist of single beams rather than full trusses.
Many roofs are constructed using a combination of two gabled or hip structures that intersect. Because of the complexity involved in constructed these intersecting roofs, they are generally designed by a structural engineer or architect. The point where the two roofs meet is called a valley. Specially-designed beams known as valley rafters are used to frame out the intersecting area. Traditional common or girder trusses form the main roof structure, while shorter rafters known as valley jacks fill in the smaller intersecting spaces.