![]() Quite often, the distance a beam can span is also determined by the spacing of joists or rafters it will support, and vice versa. Note that these are normally only required to trussed rafter roofs and not for traditional purlin and rafter roof construction. SPAN TABLES FOR JOISTS AND RAFTERS 3 AMERICAN WOOD COUNCIL Table 9.1 Required compression perpendicular to grain design values (F c) in pounds per square inch for simple span joists and rafters with uniform load. The distance a joist, rafter, or beam can span depends on the wood species, grade of lumber, loads, and how they are used. This is the right way to fix truss clips connecting the rafter and wall plate. Vertical strapping at eaves pitched roofsĪt eaves level, vertical straps 1m long should be fixed as shown in the diagram opposite at a maximum of 2m centres. Effective strapping at gable wallĮach strap should be fixed as shown in the diagram opposite with noggins between the rafters to support the metal strap along its whole length. For wider spacing, Fb values shall be reduced 13. These Fb values are for use where repetitive members are spaced not more than 24 inches. ![]() Gable walls need to be strapped to the roof at the maximum 2m centres using 30 x 5m galvanised mild steel straps. TABLE W-2 DESIGN VALUES FOR JOISTS AND RAFTERS - MECHANICALLY GRADED LUMBER1,2. Understanding these tables can be daunting and complex, but with the right research and guidance, anyone can learn how to use them. This diagram illustrates the typical arrangement of a rafter and purlin roof and shows the dimensions referred to in the tables above. Span tables help ensure that the rafters are sized correctly for the job, preventing issues like sagging and structural failure down the line. ![]() Roof pitch between 30 and 45 degrees strength class C24 Size of purlins in mm
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