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How to Calculate Support for Attic Trusses

About 80 percent of new houses today are built with trusses, manufactured in factories using computers and special equipment to make precise cuts and fasten wooden members with strong steel plates called gussets. All trusses are formed in patterns of triangles, and all have some common components: a joist or horizontal chord that goes from wall to wall, two or more angled rafter chords that make the slope of the roof, and internal braces called webs. There are two basic types of attic truss: one is suitable for creating a room, the other simply for adding storage space.

Things You'll Need

  • Span tables
  • Scientific calculator
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    • 1

      Determine the basic loads on the roof: dead load, or the weight of the trusses and roofing material; live load, which is external weight, primarily snow and ice accumulation; and wind load, or force of wind against a roof. Refer to government and roofing industry tables for loads per square foot, which vary according to the type of material and the climate conditions. Include in dead load the weight of any room finishing, which is essentially hanging on the frame and trussing.

    • 2

      Decide the stiffness required -- the allowable deflection or amount of bend or spring a truss can have; all wood will bend slightly under pressure, some varieties more than others. Check building codes, which generally have regulations on allowable deflections based on the use of the space; habitable attic rooms generally must deflect no more than 1/3 inch in 10 feet under a load of 30 lbs. per square foot.

    • 3

      Refer to span tables from the American Wood Council and similar industry and educational sources, which show the allowable spans for various types of wood, different dimensions of truss lumber and spacing of trusses. Check organizations like the Structural Building Components Association for similar details to calculate the support needed for a specific truss style.

    • 4

      Check the Internet for free scientific and roof design calculation programs, which use basic roof dimensions and loads to calculate support details for attic trusses. Gather all pertinent information, such as the width and length of the roof, the pitch or slope of the sides and the planned use of the space, then supply the details to a truss manufacturer, who will design a truss to those specifications.