Built-up wood Beams (1) Where a beam is made up of individual pieces of lumber that are nailed together, the individual members shall be 38 mm or greater in thickness and installed on edge. With the purlin/bracing installed, the long rafter span would be broken into two shorter spans, thus allowing the rafter to carry additional load.īefore you decide on lumber sizing and span, consult your local building code officials to determine design loads for your area. The Ontario Building Code Built-up Wood Beams 9.23.8.3. You can break up a long span of rafters by installing a purlin and bracing on the underside. When calculating the span of joists or rafters, use the horizontal distance between vertical supports. The number 1 grade material is more expensive than the #2 grade, but if you can use fewer or smaller number 1 grade joists or rafters in your design, you may actually save money.ĭepending on the availability and cost of various size framing materials, it may be more economical to use larger joists or rafters in your design because you can typically use fewer of them. For example, you can span a #1 grade (no knots) southern yellow pine joist a greater distance than a #2 grade of the same species (with knots). The lengths and sizes of joists vary depending on the species and quality of the lumber used. To calculate maximum spans of additional species of lumber, use the Span Calculator or the Span Tables for Joists and Rafters on the American Wood Council website.Ĭonsiderations when using the Rafter and Joist Span Tables Contact your local building code officials to determine the building code for your area. Weigh all of the elements when designing wood structures. Some areas have wind and seismic conditions. For example, some localities get large amounts of snow, whereas others get none. Before determining the maximum spans for joists and rafters, you must know the load requirements for your region.
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