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Floor Joists |
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Design Loads
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Snow Loads
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In general, sizing any piece of lumber requires making sure the four main properties
for the species and grade of wood aren’t 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 they’re 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
it’s very important to know all the details.
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 people’s 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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