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Floor Joists (2)

For many people building decks, standard span tables are often pressed into service. In fact, there are a few on-line calculators that will give you the same results without having to learn how to use standard tables. Here's a span calculator published by WoodWorksÒ. However, before you decided to use standard span tables/calculators, be aware that they have their limitations, especially when designing for outdoor structures.

So why would someone pay good money for span tables on BestDeckSite when they can look it up for free? Here are three good reasons. First, span tables/calculators don't factor in a reduction in strength for outdoor use. Second, span tables/calculators are inherently overly generous especially for longer span lengths. And third, span tables/calculators assume the loads are uniform and therefore, can't be used to design decks in areas that get snow.

When lumber gets wet, it gets weaker. For wood used outdoors, the moisture content of wood can easily exceed the 19% limit set by design standards. When this happens, we need to reduce both the fiber strength (Fb) by 15% and the modulus of elasticity (E) by 10%. You can learn all you'd like about these properties of wood in the floor joist and overview sizing sections, but suffice it to say the maximum allowable span lengths for lumber used outdoors is significantly less.

In addition, standard span tables sometimes suggest span lengths that result in a springy floor even when the lumber stays dry. A study referenced in the NDS indicated that the number of people that found floors to be too springy increased to the degree that the recommend joist length from span tables exceeded a 10-foot span. On BestDeckSite, we used a more rigid deflection limit of (L/480) while developing our tables to avoid this well know deficiency in standard span tables.

If you live in a part of the country that gets snow, span tables/calculators don't work at all. Most building codes today require that decks built in cold parts of the country be designed for drifting and sliding snow load conditions. This makes good sense. After all, windy conditions and snow sliding off the roof of the house tends to deposit the snow in a wedge shaped pile next to the house. Since regular span tables/calculators assume that the loads are level/uniform, they can't be used for tapered snow loads.

Not only have BestDeckSite joist tables been specifically designed to help you easily and correctly size lumber for outdoor use, but they'll also tell you when you're selecting a joist length that requires a special top flange hanger. This can happen when really long floor joist lengths are used. Since the capacity of any hanger is limited, our joist tables will tell you when you're exceeding the strength of standard hangers and, as a result, may want to consider a different configuration.

The bottom line is that span calculators only give rough estimates for lumber used outdoors and then only for non-snow loaded decks. On BestDeckSite, our joist tables factor in for "wet service" and other code requirements, have a stiffer live load deflection limit of (L/480), and work on both snow loaded and non-snow loaded decks. There's no figuring to do. Just look up the allowable span for your lumber size. Even if you're not sure how much snow to design for we've got snow load maps for the United States to help you out with that too.