How to design flat trusses

Written by bob haring
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Trusses make roofing much easier. For many years, roofs were framed by cutting and installing rafters on the job site. This required careful calculations and some careful handling. Prefabricated trusses changed that. Trusses are made in factories, where machines cut boards precisely and other machines fasten elements together with steel plates embedded into the boards with long points. Trusses are delivered to the job site ready to install. They are lighter and easier to handle that rafters and make roof framing much simpler and faster.

Skill level:

Things you need

  • Tape measure
  • Truss design catalogue

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  1. 1

    Measure the building to be roofed from side to side and front to back. The width from outer wall to outer wall will determine the length of the truss. The length from front to back will determine how many trusses will be needed; they are spaced 24 inches apart, so divide the length by 24 to get that number. Pick a style of truss depending on the type of flat roof wanted.

  2. 2

    Build a basic flat roof with simple girders, called parallel chord trusses. These have top and bottom plates with upright braces spaced evenly across the truss from side to side and diagonal braces between the uprights to connect top and bottom plates. Space these braces depending on the height desired for the truss; that will be determined by the span or width across the roof. Pick truss lumber dimensions based on the load of the roof.

  3. 3

    Make flat top trusses if a flat roof is wanted but needs to have space inside it for heating and cooling ducts or other utilities. Flat tops have a bottom beam from wall to wall, with angled rafters on either end and a flat top beam between them. Place upright braces in the centre of the truss and where the rafters and top plate join. Install angled braces between the top of the centre support and the bottoms of the two side uprights. Pick lumber and height based on roof load and the need for access.

  4. 4

    Vary the flat roof truss design to provide better drainage with a low profile style. Make these trusses basically the same as parallel chord style, except increase the height of the vertical supports gradually from wall to centre to make a very slight peak. Decide the pitch or slope depending on use and personal preference. Make it very slight, just enough to provide some drainage, or a little higher to conform to an abutting building. Choose lumber based on roof load.

Tips and warnings

  • Tables are available at roof and building supply stores to calculate roof load, the weight of the roofing material, the force of the wind and other weather factors.

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