What causes snow to buckle on a metal roof?

By The Metal Roof Experts at S-5!

Learn how to reduce the risk of snow buckling on a metal roof even when proper snow guards are in place.

Even when a snow retention system is properly installed, under certain conditions, snow can still slide off a metal roof unexpectedly. One of the key reasons for this is a phenomenon known as compression buckling.  

When a blanket of snow lacks sufficient depth and solidification, it may not have enough compressive strength to remain in place. Instead, the snow can buckle and fall over the snow guards, potentially falling off the roof. The amount of snow can be minimal, but it can also still pose a safety risk for your customers.

So, what causes a snow blanket to buckle, and what can you do to prevent it? The Metal Roof Experts at S-5!® break down the factors at play and the best strategies to keep snow on the roof.  

What is a buckling blanket of snow?

Compression buckling is more common when snow blankets are thin. Especially when the blanket of snow on the roof is dense and wet (not solidified), it may tend to undergo compression buckling. Thinner accumulations of snow on steeper slopes or longer rafter lengths increase the potential for the snow to buckle at some point above your snow guard assembly. This phenomenon is similar to a slender, unsupported steel column buckling under compression loads.  

During the buckling process, a loop of snow pushes upward from the metal roof’s surface as the heavy mass compresses toward the eave where the snow is restrained by snow guards. This loop can fold over the blanket and your snow retention device(s) in the downslope direction, often breaking free and, in some cases, falling off your roof.

Because this process normally occurs when snow blankets are relatively thin, the amounts of released snow tend to be somewhat minimal, but the falling snow may still pose a nuisance or hazard.

Why don’t snow guards at the eave restrain the buckling snow?

Any snow retention device or system relies upon the compressive strength of a snow blanket to resist the migration and release of the blanket. When the snow blanket increases in both thickness and solidification, it gains mass and compressive strength. This process is related to both the weight of the snow and the vector (angle) of the roof.

Our S-5! Snow Guard Calculator is designed to calculate the required frequency (population) of snow guards based on the design snow load and the allowable loads of the snow retention system. This calculation relies on the adequate compressive strength of the snow blanket. The calculator cannot take into account the buckling nature of a thin snow blanket due to a lack of adequate compressive strength.

The height of your snow retention device plays no role in the probability of a snow blanket buckling.

What are the factors involved in creating a buckling blanket of snow?

Let’s look at the variables influencing snow buckling:

• Slope: Buckling is more common with steeper slopes as the vector forces increase.
• Depth of snow blanket: A snowbank compresses from its own weight, hence thinner blankets generally have less compressive strength, making them more likely to buckle.
• Length of snow blanket: Longer blankets (from eave-to-ridge) have an increased vector force, corresponding to an increased potential for buckling.
• Moisture: Greater moisture content (when frozen) generally increases weight, vector force and compressive strength.
• Temperature: When moisture is unfrozen, it lacks compressive strength. So, warmer temperatures can increase the risk of buckling.

So, how can you decrease the likelihood of snow buckling?

When the snow blanket increases in depth, it is less likely to buckle. Unfortunately, increasing the snow depth (the sectional size of the snow blanket) is impossible to control.

Our extensive experience shows the most effective way to mitigate a buckling blanket of snow is to add a second row of snow retention above the first – even when calculations show it’s not required.

Ideally, the second row should be situated approximately eight-to-ten feet above the first row. This strategy reduces the unsupported length of the blanket, not only reducing its tendency to buckle, but if the snow does buckle over the upper row, the loop is still contained above the lower row.

Ideas to hold onto

A buckling blanket of snow may occur when the snow blanket on your metal roof is thin and quite wet (lacking in body and compressive strength). Under these circumstances, the snow blanket can form a loop, fold over your snow retention device(s) and fall off your roof.

Multiple factors can affect the probability of the snow buckling, including your roof’s slope, the depth and length of the snow blanket, moisture content and temperature. As the thickness of the snow blanket increases, its weight and compressive strength also increase, diminishing the potential for buckling.

The best method to restrain a buckling blanket from escaping your roof is a second row of snow guards located eight-to-ten feet above the first row.

Original article and images source: S-5!