The common roof rafter is used to calculate the roof’s height and where the ridge board should be installed. This style of rafter starts at an outside wall and extends all the way to the roof’s ridge board or peak. The basic gable roof is constructed using a common rafter. The principal rafters, when combined with the principal purlins, constitute a very stable roof construction system. ![]() Principal rafters run from the roof’s ridge to the wall plate they’re a little heavier than ordinary rafters, and they’re usually framed into a tie beam at a corner post, story post, or chimney post. They are commonly used to carry a purlin and sit directly on a tie beam. Principal rafters are the largest form of rafter found at the ends of a roof structure in a timber-framed roof. It could also be left as open space in an attic.įigure 7: Plan view of roof framing members Principal rafter As a result, the area in the roof is left as a vaulted ceiling that may be finished with insulation and drywall. The rafters and external walls are then connected with ceiling joists. ![]() Hence, timber rafters are usually constructed in form of A-shape, consisting basically of the rafters, rafter ties, and ridge board/hips. Trussed rafters are generally employed for large scale timber roof construction, but direct timber rafters are more efficient for small scale constructions. Timber rafters rely on nails or screws for connection. The number of common rafter units needed for each project is mostly determined by the roof’s size and scope, as well as the distance each unit rafter must span. Many common, or general purpose, rafters make up each roof. The common rafters form the sloped sides of the triangle on a traditional gable roof, which has a triangular shape. Rafters are part of a basic wood framing system and are made of wood lumber. Timber rafters are popular options in timber/wooden roof structures. The spacing of rafters can be determined from the design requirements, type of roof sheeting to employed, type and spacing of purlins, availability of materials, etc.įigure 4: Steel portal frame consisting of stanchions and rafters Timber Rafters Furthermore, they are constructed in series and laid parallel to each other at usually a constant spacing. Rafters can be constructed using steel or timber. They are also important members for resisting upward wind pressure on roofs. They typically run from the ridge board or hip of the roof at a sloping angle to the roof wall plate, columns, or roof beams, depending on the support system adopted for the entire roof structure.īy implication, rafters receive the load from the roof covering/sheeting, accessories, and other services that may be attached to it. ![]() FIGURE R802.5.1 BRACED RAFTER CONSTRUCTION TABLE R802.5.Rafters are loading bearing structural members that are used in roof construction. ![]() H R = Height of roof ridge measured vertically above the top of the rafter support walls. H C = Height of ceiling joists or rafter ties measured vertically above the top of the rafter support walls. Note: Where ceiling joist s run perpendicular to the rafter, rafter ties shall be installed in accordance with Section R802.3.1. (Uninhabitable attics with limited storage, live load = 20 psf, L/Δ = 240) CEILING JOIST SPACING (inches)įor SI: 1 inch = 25.4 mm, 1 foot = 305 mm, 1 degree = 0.018 rad. (Uninhabitable attics without storage, live load= 10 psf, L/Δ = 240) CEILING JOIST SPACING (inches)Ĭheck sources for availability of lumber in lengths greater than 20 feet.įor SI: 1 inch= 25.4 mm, 1 foot= 304.8 mm, 1 pound per square foot= 0.0479 kPa.Ī. Shall be in accordance with Tables R802.4(1) and R802.4(2).įor other grades and species and for other loading conditions,ĬEILING JOIST SPANS FOR COMMON LUMBER SPECIES
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