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How it Works: Roof Trusses

by Rob Munach

The roofs on most new houses in the United States are built with trusses. In this "How It Works" column, structural engineer Rob Munach explains how trusses carry their loads and how T-bracing and roof sheathing add strength. He identifies the different parts of a truss--top chord, bottom chord, web, and nail plates--and how each functions. An illustration shows the compression and tension ratings of a truss that has been modeled with a combined dead, live, and snow load of 40 lb. per sq. ft. Munach stresses that because trusses are built with no redundancy, cutting them can have consequences ranging from damage to interior finishes and materials to catastrophic roof failure. Another illustration shows how cutting the web of a truss increases the deflection potential of the bottom chord.