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. You can break up a long span of rafters by installing a purlin and bracing on the underside. This span table is to the format and methodology given in BS 5268-7.5. When calculating the span of joists or rafters, use the horizontal distance between vertical supports. Click on a span to generate a full calculation (you can adjust the exact span and slope). 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. The tabulated rafter spans assume that ceiling joists are located at the bottom of the attic space or that some other method of resisting the outward push of the rafters on the bearing walls, such as rafter ties, is provided at that location. 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. As the load increases, however, the spans decrease. Weigh all of the elements when designing wood structures. Having said that, though, Table R507.6 of the 2021 IRC limits a 16’ Southern pine to 3’-4 and DF-L, Hem-Fir, and SPF to 3’-3 for 14’ or longer spans with live loads of 40PSF. Some areas have wind and seismic conditions. Note: The 140 × 70 mm rafter size permitted to span 2.9 m could also have been used (since 2.9 × 1.1 multipler 3.19 m), but this is not a stock size so was not chosen. This indicates a 190 × 45 mm rafter size. For example, some localities get large amounts of snow, whereas others get none. Read the rafter size in the left column of Table 10.1 for the 3.3 m span chosen. Before determining the maximum spans for joists and rafters, you must know the load requirements for your region.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |