How to calculate snow load for roof trusses

Written by andrew rothmund
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How to calculate snow load for roof trusses
Even a thin layer of snow spread out over a large area can weigh more than you'd think. (Snow image by ljc from Fotolia.com)

Calculating the snow load for roof trusses means taking into consideration a variety of factors, including the load capacity of your trusses, the distance between them, and even the angle of the roof. While these measurements are usually done by an engineer or architect, they can be accurately measured by anybody, usually with the help of a manual or special online calculator. This will allow you to better work together with a professional in determining how to structure your roof, or in determining how much snow your existing roof can hold.

Skill level:
Moderate

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Things you need

  • Measuring tape
  • Calculator

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Instructions

  1. 1

    Determine the slope of your roof, either by using a roof slope calculator (see Resources), or by manually calculating the "rise" and "run" of your roof -- the amount of vertical rise divided by the amount of horizontal run in the same distance.

  2. 2

    Determine the actual snow load of your roof with a snow load calculator (see Resources). This will require you to know your geographical area's "ground snow load," which can be found using the map on the Kingspan Panels website (see Resources). Once you are finished with the calculations, you will have a snow load figure expressed in kg per square metres, or "kg/m2."

  3. 3

    Determine the number of roof trusses and the total area of your roof. You may have to refer to your home's plans, or go into the attic with a measuring tape.

  4. 4

    Divide the square metres of your roof by the number of trusses. Theoretically, each truss supports an equal share of the roof.

  5. 5

    Multiply the "kg/m2" number that you obtained in step 2 by the quotient that you obtained in step 4. This will give the theoretical snow load, in kilograms, that each truss can hold.

Tips and warnings

  • Keep in mind that the density of snow can vary greatly, depending on the weather conditions and location. Measuring and weighing a square metre of freshly fallen snow will allow you to better determine its density, and make your measurements more accurate.
  • Always hire or work with a professional architect or engineer when designing or altering structures. Improper measurements can lead to house failure, injury and death. Be careful, as always, when going into your attic or onto your roof.

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