How to Size an Attic Roof Truss

Written by mark morris
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How to Size an Attic Roof Truss
The height of the peak and the width of the lower cord determine attic space. (A new home being built with wood, trusses and supports image by Paul Hill from

When sizing trusses for attic space, care must be taken to allow adequate overhead space. In addition, the trusses must be built from heavier material, due to the open nature of attic trusses, which must eliminate webbing, or internal bracing, allowing for the maximum usable space.

Skill level:

Things you need

  • Scale ruler
  • Plan

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  1. 1

    Measure the plan dimensions on the end of the roof under the gable, from the top outside of one wall to the opposite wall. Increase the bottom horizontal, or lower cord, long enough to overlap these walls on either side by at least 12 inches, effectively adding 24 inches to the piece. Use a scale ruler to make the calculations simple. Check the scale in the plan legend and use the edge of the ruler with the corresponding fraction for accurate measurements.

  2. 2

    Calculate the height of the roof peak. If there is less than 6 feet from the top of the bottom horizontal piece, or lower cord, to the inside of the roof peak, increase the height of the roof peak in your plans to create that much height.

  3. 3

    Calculate the angle for your truss piece ends. Use an angle finder, with one leg aligned to each face of the roof. Divide the number by two. Measure from the peak, or adjusted peak height, to the lower ends of the roof. This is your upper cord, or rafter length. Cut your upper cord pieces to this length, with both ends cut to the angle from the peak.

  4. 4

    Use lumber one dimensional size up from the original plan to compensate for the open structure. If the plan calls for 2-by-4 lumber, use 2-by-6. If the plan calls for 2-by-6, use 2-by-8, and so on. Cut your lower cord to the dimension calculated in step one, with the angle from step three at each end, one right and one left, to create a long trapezoid.

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