This table is simply a sample and may not be valid for your region. There are several beam span tables for all the species of wood. The table below shows an excerpt from a maximum span for built-up Douglas fir floor beams table for supporting not more than one floor. There are many beam span tables for all the different lumber species and also for the number of floors that are ultimately supported by this beam. The question now is how big does that center floor beam need to be? Once again, it is not necessary to do tricky wood beam calculations but rather, simply look up the answer in a beam span table. The picture below shows what the framing for this floor would look like in a three-dimensional perspective. The floor joists are still 12' long but now you can see a floor beam running horizontally across the middle of the house (supported by the lower concrete foundation wall). See the picture below for a plan view of how this will look. So we will have to place wood floor beams (or likewise, wood ceiling beams) across the width of the house to support the floor joists. Now at 24 feet, we are beyond the span capabilities of our table in our previous example. ![]() It would be possible to still frame the floor in the same way as above but just run the floor joists in the opposite direction (looking for a lumber size to span 13') but for our example, we are going to keep the floor joists running in the same direction. So the house dimensions will now be 24' X 13'. Let's expand our house beyond that 17'2" span capability to 24 feet wide. The widest span in the floor joist span table in Part 2 of this tutorial module showed that floor joists can span 17'2" if they are 2 X 12s spaced 12" o.c. For now, we'll look into supporting the floor joists with a floor joist beam. The wall could be either a properly sized structural concrete or concrete block wall or a wood framed wall. This support could take the form of a structural wall. Once we go beyond the allowable spans for floor joists (as shown in the floor joist span tables), we will need some kind of support under these floor joists. Using Floor Beam Span TablesĬontinuing on from Part 2: Wood Joist Span Tables of Residential Structural Design, we were just about to make our example house wider. Or see our Design Your Own House tutorial site map. Or even go back as far as the residential structural design page which explains basic house structure. If you are just starting out, you might want to go to the joist page since we will expand on the house design example introduced there. This site also has information on learning how to read joist tables and a joist calculator. You'll find a beam span calculator towards the bottom of this page. On this page we will explain how to read and design with floor beam span tables. 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.Designing with Floor Beam Span Tables Part 3 of Residential Structural Design 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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