Sizing Roof Rafters, Floor Joists, & Beams

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Lumber Span Tables: Roof Rafters, Floor Joists, Beams

Need help sizing roof rafters, floor joists, or beams? In this section, we cover how to size floor joists, beams and roof rafters on three different levels. The first level is titled "General Guidelines" and is for those people that don’t want to have to make any calculations or look up numbers in lumber span tables. The second level is for individuals that want to size beams, roof rafters, or floor joists outside of the parameters of the General Guidelines. At this level, we provide simplified tables and show you how to use lumber span tables. Finally, the third level is for the more studious type. Here we provide detailed, engineering calculations to help you get the size right for beams, floor joists, or roof rafters under various loading conditions.

Some of the interesting topics discussed in this section deal with the relationship between the length of floor joists and beam size. Specifically, you’ll see how to balance the length of floor joists, beam size, and pier spacing to come up with a workable combination for your soil type and overall deck size. Further discussion includes how to determine the load roof rafters place on wall headers/beams and how to size them accordingly. Along the way, you'll find an explanation of why lumber span tables, while being technically accurate, sometimes result in sizing floor joists that are too springy.

Roof Rafters and Lateral Bracing

Along with helping make sure your floor joists and roof rafters are strong and stiff enough, this section also looks in-depth at designing open cathedral roofs. These roofs are completely exposed on the underside with only the roof rafters showing. The ceiling joists and collar ties that add clutter and reduce headroom have been removed.

Now this may not sound particularly difficult to accomplish until you consider the fact that the ceiling joists/collar ties are what keep the roof rafters from splaying out the tops of the walls when the roof is loaded by heavy snow or a strong wind. This section on roof rafters, beams, and floor joists, will show you how to size your roof rafters, determine the number of fasteners at critical connections, and provide hidden reinforcement that will allow you to have an uncluttered, open, and tall cathedral ceiling that’s very strong. Want to know how to use lumber span tables, size beams, roof rafters and floor joists? You've come to the right place.