In simple language, the ridge of a roof is the peak where two opposing roof planes meet. Getting a little more technical, the National Roofing Contractors Association defines the ridge as the "highest point on a roof, represented by a horizontal line where two roof areas intersect, running the length of the area." So the ridge is basically the peak of a roof, but ridge also refers to the board or beam that is used in building the ridge.
Ridge Boards and Beams
In traditional house framing, also called "stick framing," a basic roof frame consists of opposing pairs of sloping rafters that meet at their top ends at a ridge board or ridge beam. The top end of each rafter is cut at an angle so that it meets flush with one broadside of the ridge board, and the ridge is sandwiched between the rafters.
The rafters are nailed to the ridge or are fastened with metal framing connectors. In this way, the ridge board provides for a structural connection for the rafters, adds lateral (side-to-side) stability, and forms a rigid spine for the roof peak. The bottom ends of each pair of opposing rafters are tied together by a horizontal board called a joist; together, the joists usually form the ceiling frame of the top floor of the house (also the attic floor). The rafters, ridge, and joists form a triangular assembly that has considerable structural strength with an open area in the middle, creating the attic space.
Ridge boards commonly are made of 1x8 or 2x8 or larger lumber. The terms ridge board and ridge beam are used interchangeably in standard construction.
Timber Frame Ridge Beams
Timber framing is a traditional method of framing that uses heavy timbers instead of standard board lumber. A timber frame roof has rafters and a ridge beam, as with stick framing, but the ridge is a large structural beam that the rafters sit on top of or sometimes fit into notches cut into the beam. With this design, the ridge beam bears much more load (weight) than the ridge board on a standard stick-framed roof. Ridges in timber framing are typically called ridge beams rather than ridge boards.
The most classic type of house roof is a gable roof, which has two roof planes and a triangle-topped wall at either end, known as a gable wall. A variation on this is the hip roof, which has no gable walls. Instead, it has an additional roof plane at each end of the central, horizontal ridge. In addition to the horizontal ridge, a hip roof has sloping ridges or peaks where adjacent roof planes meet. This ridge is created by what is called a hip rafter. It works just like a ridge board but is set at an angle.
Many modern homes have roof frames made with prefabricated trusses rather than stick framing. Trusses are triangular wood frames built in a factory as complete units and shipped to the job site for installation. Trusses are set onto the walls of a house and are tied together with horizontal boards called purlins. Since each truss is a complete frame, trusses do not use a common ridge board, and the purlins (along with the roof decking) provide lateral support. Once the decking and roofing are installed, a truss roof looks just like a stick-framed roof.