Wooden trusses are triangulated structures that are used to carry the load of a house's roof to the outside walls. These trusses have mostly replaced traditional rafters in homes. While they're extremely strong and specially engineered though, roof trusses may still need bracing in order to support the complete weight of a given structure.
Locate the Weak Points
Not all wooden trusses will need bracing. A small roof such as a shed or garage may not bear a great deal of weight, and so the trusses will be fine on their own. However, if a roof truss needs to be cut for some reason (say to accommodate a chimney or other structure) then the place where that truss is cut is a weak point that will need bracing. Also, if a roof is overspanned, then bracing may become necessary. Overspanned roofs are those where the trusses reach out too far without enough support and they can't hold the weight of the roof. Typically it takes a roof collapse, or an inspection by an engineer to determine if trusses are in fact overspanned.
Wooden trusses have three parts: a single shaft called the bottom chord that goes straight across the base like a rafter, and two others that lean against each other at an angle to form a triangle. Inside this triangle is a straight shaft from the center of the bottom chord to the top point of the triangle, with angled shafts going to support the two "walls" of the triangle. These are all called the web, and it's the shafts in the web that are braced. As a general rule if a web member is six feet in length or longer it should be braced to avoid buckling. A wooden brace, commonly a 1 inch by 4 inch board, is then placed beneath the web member and anchored to the bottom chord. The bottom chord is the wooden board that runs beneath the wooden truss like a ceiling rafter, and it supports the truss.
In order to secure a brace to the bottom chord, it may be necessary to use a gusset plate. These plates are made of steel or plywood. A gusset plate will screw into a brace, and then it will also screw into the bottom chord to provide a strong connection. Using these plates negates the need for nailing through the bottom chord, and the guess work of trying to figure out where the nail and/or screw will come out. It also means that pilot holes won't be drilled through the supporting bottom cord, which may weaken the structure.
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