LMCURBS®

Division of Longview Mechanical Contractors

1-800-284-1412

Designing Snow Retention Attachment

The primary force acting against a panel’s pinned point is a blanket of snow. This is also the force for which any snow retention system should be designed. To ensure that an intended snow retention system functions as expected, it is important to know the types of loads created by a blanket of snow. Fortunately, calculating such forces is not a mystery. The diagonal (or vector) load which results from a blanket of snow on a roof can be calculated from the vertical load, the roof angle and the length of the roof plane. The steeper the roof angle, the more exaggerated the vector load. Similarly, the longer the roof plane, the more area for snow to accumulate and therefore, the greater the load. The exact load (per square foot) can be computed mathematically by utilizing the following equation: Vector Force = Vertical Load x Sine Roof Angle. For the Vector Force to be expressed in pounds per linear foot of eave, multiply the vertical load (expressed in pounds per square foot) by the length of roof (from eave to ridge): Vector Force (pounds/linear foot of eave) + Vertical Load (pounds per square foot) x Distance (linear feet from the eave to ridge) x Sine Roof Angle. This equation does not consider any frictional coefficient of snow to roof for reasons stated previously. However, some friction normally does exist due to the clinging of snow to the panel seam area which is usually an irregular geometry. The figure usually used for vertical load is the design snow load for the roof. Most often (when permitted by code), ground snow is reduced for roof snow by 20% or more because a significant amount of snow is normally blown from the roof and does not accumulate the same way as it does on the ground. To illustrate this calculation, consider the following example. A building has a 5:12 slope roof, which is 43’6" from eave to ridge The design (roof) snow load is 32 p.s.f.:

Vector force = 32 x 43.5 x .826 = 532.6 pounds per lineal foot of eave. If snow guard attachment is made every 16" (1.33’) along the eave, each attachment must be capable of resisting 532.6 x 1.33, or 710 pounds, plus any desired safety factor.

Using all the same criteria, but changing the slope to 8:12 would result with a force of 773.35 lbs. per linear foot of eave, an increase of more than 45%. Choice of safety factors depends upon importance of use, potential for installation error, accuracy and appropriateness of calculation and any applicable test results, experience, and designer discretion.

Disclaimer: Note that the set screws must be torqued to 115" lbs. with a screw gun and checked periodically during installation. LMCurbs is under no obligation if a failure occurs from an improper design or installation. This information is not meant to fit every application. For severe applications, call 1-800-284-1412 to discuss your particular situation.