Reinforcing your joists increases load-bearing capacity.
The wooden joists or beams over your garage help hold your roof in place. If you have an attic over your garage, these joists also bear the load of your attic flooring and any materials you store in the attic. If you need a storage area above your garage, strong ceiling joists are necessary to support the added weight. If the materials you intend to store are particularly heavy, 2-inch by 6-inch ceiling joists may require additional support.
Strength
The strength of your 2-inch by 6-inch ceiling joists depends on the length of each joist and whether or not your builder installed additional ceiling support. When additional support is not a factor, longer ceiling joists will always be weaker than shorter ones because the distance between the supporting structure on either side is greater. Each joist's weight also helps determine its strength. All beams must support their own weight in addition to any weight you place upon them when building an attic floor or storing belongings. Longer joists weigh more and suffer greater strain from the very beginning - making them weaker than shorter joists.
Location
A ceiling joist supports weight differently, depending on where that weight is placed. Your 2-inch by 6-inch garage joists, for example, are stronger on either end than in the center. This is because a supporting structure holds each end of the joist aloft. The center of each joist is the further point from the supporting structure on either side, making it the weakest point of the beam. Thus, it is best to place heavier loads closer to the wall than in the center of the attic floor. Doing so places the least strain on the joists.
Considerations
Although you can construct a garage using thicker, stronger joists, 2-inch by 6-inch joists are sufficient for most garages - even if you intend to build an attic storage area above your garage. Fine Homebuilding notes that 2-inch by 6-inch garage joists will support a weight of up to 50 pounds per square foot. If you overload your garage's ceiling joists, the ceiling may appear to sag from the strain. While your 2-inch by 6-inch joists can collapse from too much weight, such a scenario is uncommon.
Strengthening Your Joists
Your 2-inch by 6-inch joists will bear a heavy load, but you can add additional support to further strengthen each beam and give yourself peace of mind. Installing additional joists alongside existing joists - a process known as sistering - increases the weight your existing joists will support. If you select this option, slightly wider joists, such as 2-inch by 8-inch joists, give ceiling even greater strength and support than adding more 2-inch by 6-inch beams.
New Construction
If you have not yet constructed your garage, consider doing so with I-joists rather than 2-inch by 6-inch joists. Rather than being beams, I-joists are shaped like a letter "I" and will generally support a heavier load than 2-inch by 6-inch beams. While you can replace your existing 2-inch by 6-inch joists with I-joists, it is easier to build the garage using I-joists from the beginning than to remove and replace existing beams.
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