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Rafters are loading bearing structural members that are used in roof construction. 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. By implication, rafters receive the load from the roof covering/sheeting, accessories, and other services that may be attached to it. They are also important members for resisting upward wind pressure on roofs. Rafters can be constructed using steel or timber. Furthermore, they are constructed in series and laid parallel to each other at usually a constant spacing. 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. Figure 1: Typical components of a hipped roofUnder heavy gravity loads, rafters have a tendency to flatten outwards on the walls. This is generally a case of structural failure of rafters, and can lead to the collapse of the walls if the spans are longer and the walls are thinner. To overcome this problem, coupled rafters have been used, which are two opposing rafters joined together by a horizontal tie beam. However, such roofs were structurally unstable, and since they lacked longitudinal support, they were prone to racking, or horizontal movement-induced collapse. Timber roof trusses were developed later, during the Middle Ages. A cross-braced timber roof truss creates a stable, rigid unit. It should, in theory, balance all lateral forces against one another and only thrust directly downwards on the supporting walls. Figure 2: Typical trussed rafter sectionThe functions of rafters in a roof are as follows;
Types of RaftersRafters are usually constructed using timber or steel. Steel rafters are popular in the construction of portal frames, where they are directly supported by steel columns and stanchions. Timber rafters are more popular in the construction of residential homes, small offices, or other smaller structures. Steel RaftersSteel rafters in portal frames are usually subjected to significant bending moment and shear forces from the dead and imposed loads from the roof. In order to increase the rigidity of the rafters, haunches are introduced at the eaves and at the apex. Lateral stability of the rafters are enhanced by purlins or cross-bracings. In some cases steel curved rafters are used in the design of industrial steel structures. Figure 4: Steel portal frame consisting of stanchions and raftersTimber RaftersTimber rafters are popular options in timber/wooden roof structures. Rafters are part of a basic wood framing system and are made of wood lumber. The common rafters form the sloped sides of the triangle on a traditional gable roof, which has a triangular shape. Many common, or general purpose, rafters make up each roof. 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. Timber rafters rely on nails or screws for connection. Trussed rafters are generally employed for large scale timber roof construction, but direct timber rafters are more efficient for small scale constructions. Hence, timber rafters are usually constructed in form of A-shape, consisting basically of the rafters, rafter ties, and ridge board/hips. The rafters and external walls are then connected with ceiling joists. As a result, the area in the roof is left as a vaulted ceiling that may be finished with insulation and drywall. It could also be left as open space in an attic. Figure 5: Typical components of a roof rafterFigure 6: Well constructed roof rafterWhile the most basic gable roof can be built with just one type of timber roof rafter, the most sophisticated roof designs can incorporate different types of timber rafter. The types of timber rafters often employed are; Figure 7: Plan view of roof framing membersPrincipal rafterPrincipal rafters are the largest form of rafter found at the ends of a roof structure in a timber-framed roof. They are commonly used to carry a purlin and sit directly on a tie beam. 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. The principal rafters, when combined with the principal purlins, constitute a very stable roof construction system. Common rafterThe basic gable roof is constructed using a common rafter. This style of rafter starts at an outside wall and extends all the way to the roof’s ridge board or peak. The common roof rafter is used to calculate the roof’s height and where the ridge board should be installed. The roof is now ready for the next type of rafter once the ridge board has been installed. Smaller rafters located in between the principals at both ends. Figure 8: The components of a hip and valley roofHip rafterThese are rafters that runs diagonally between the roof ridge and the top of the wall plate, forming a hipped roof. A hip rafter connects to the ridge at a 45-degree angle, as opposed to ordinary roof rafters, which run perpendicular to the peak of the roof. Traditional stick-framing techniques can be used to make these rafters, or they can be integrated in a pre-engineered steel or timber truss system. Valley rafterThe valley rafter is the rafter in the valley line that joins the ridge to the wall plate along the meeting line of two sloped sides of a roof that are perpendicular to each other in a roof framing system. To put it another way, it’s the main rafter at the bottom of a hip and valley roof. Jack rafterA jack rafter is any rafter that is shorter than the whole length of the sloping roof, such as one that begins or ends at a hip or valley. They extend up from the top of the wall plate at a right angle (90°) to abut into an existing hip rafter. A jack rafter is one that has been shortened by falling on a hip rafter or being interrupted by a dormer window. Figure 9: Valley jack rafterFigure 10: Hip jack rafterBarge rafterThis is the outermost rafter on a gable end and is occasionally utilised to form a roof overhang. It is one of the two rafters that support the portion of a gable roof that extends beyond the gable wall. Rafter Design and SpacingRafters are designed to be structurally stable under gravity and horizontal loads, without undergoing excessive deflection or failure. The design of rafters often involves selecting the appropriate timber class, dimensions, and spacing that will safely support the roof load for a given roof span. In most cases, tables are available for the selection of rafters. Figure 11: Rafter design definitionsDefinitionsRafter span Rafter run Roof span Roof slope Rafter Design TablesUsing regularised UK timber dimensions, it is possible to determine the maximum span of timber rafters subjected to different values of live loads according to BS 5268-7.5. Table 1: Maximum Clear Span of Rafter when slope is more than 15 degrees but less than 22.5 degrees (Timber class: C16; Imposed load = 0.5 – 0.75 kN/m2)
Table 2: Maximum Clear Span of Rafter (Timber class: C16; Imposed load = 0.75 – 1.00 kN/m2)
Table 3: Maximum Clear Span of Rafter when slope is more than 22.5 degrees but less than 30 degrees (Timber class: C24; Imposed load = 0.5 – 0.75 kN/m2)
Table 4: Maximum Clear Span of Rafter when slope is more than 22.5 degrees but less than 30 degrees (Timber class: C24; Imposed load = 0.75 – 1.0 kN/m2)
Assumptions in the Preparation of the Table
Installation of RaftersThe following steps may be followed in the installation of rafters;
error: Which rafters extend diagonally from the corners of the top plates to the ridge board?Valley rafters extend diagonally from the top plates to the ridge board along the lines where two roofs intersect. Jack rafters, also called jacks, never extend the full distance from the top plate to the ridge board. There are three kinds of jack rafters. Hip jack rafters extend from the top plate to a hip rafter.
What part of the rafter laid diagonally from the corners of a plate to the ridge?
Which kind of rafter extends at right angles from the plate to the ridge?Common Rafter:
A rafter that runs at right angles from the top of the wall plate to the roof ridge. These are the inclined members supporting the battens to support roof coverings.
What are the types of rafters?The seven types of roof rafters are often used in combination. There are basically seven different types of roof rafter designs used to create a roof: common, hip, hip jack, valley, valley jack, cripple jack and flying hip.
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