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Floor Joists

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Design Loads

Snow Loads
Snow Loads

In general, sizing any piece of lumber requires making sure the four main properties for the species and grade of wood arenít exceeded. These four properties are fiber strength (tearing), elasticity (bending), shear stress (cracking), and grain compression (crushing). When it comes to sizing floor joists, traditionally builders do this by first looking up the two most critical design properties for the intended wood (fiber strength and the modulus of elasticity) and then use these two values in what are referred to as "span tables". Typical, floor joist "span tables" list maximum distances a particular floor joist can span based upon fiber strength, elasticity and lumber size.

All too often, the limitations and correct use of span tables are not well understood. Span tables are tailored for sizing lumber that is used indoors, where it stays dry and only carries uniform live and dead loads, so if you're building a house, not knowing all the ins-and-outs of span tables isn't an issue. Carpenters building houses just look up the required sizes in the tables and theyíre ready to go. However, when the task at hand is sizing lumber for a deck or gazebo where the lumber gets wet, must carry a combination of loads, and/or will be subjected to non-uniform snow loads, then itís very important to know all the details.

Check Point Check Point - The 2003 International Building Code (IBC) requires that decks in colder parts of the country be built to carry drifting and sliding snow loads. In other words, you must design for both the amount of drifting snow that could pile up in a wedge shape along side the house and the loads from snow sliding off the roof. Drifting snow loads can easily double the loading on the deck framing right next to the house. The fact that the loads are not uniform, makes traditional span tables useless. Take the guess work out of sizing your framing by getting a password today and gain immediate access to on-line calculators that will help you size your lumber for your local conditions.

Even if you live in a warmer climate, regular joist span tables were written to keep the deflection less than 1/360th of the total span. In most cases, this makes for a nice stiff floor. However, according to the NDS Commentary, a "comprehensive study of homeowner responses to levels of floor performance" was conducted in Canada that showed the number of people that found floors to be too springy increased to the degree that the joists exceeded a 10-foot span. They went on to say that a computer model that did a better job of sizing lumber according to peopleís preferences limited deflection at 12 feet by 72% and at 20 feet by 38% of what it was at 10 feet. What this means is that for longer lengths, span tables tend to be overly generous.

Now I know what you may be thinking, "Ugh, this is way too complicated. Do I have to know how to use span tables and understand the details of non-uniform loading?" Happily, the answer is no. BestDeckSite provides and on-line Joist Calculator that takes all these factors into consideration in helping you to easily size your joists. There's no need to know all the ins-and-outs of sizing lumber.

The content under the "How-To" menu is a small sampling of all the material covered on BestDeckSite. For immediate access to in-depth information on sizing joists, bracing for taller decks, cantilevered joists, and framing openings around trees, as well as, comprehensive coverage of all aspects of gazebo and deck building, get a password and log-in now.

     Floor Joists

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