Buckling of Snow
Caution! Any snow retention device or system relies upon the compressive strength of a snow blanket to resist and retard the migration and/or release of the blanket. The Snow Retention Calculator is designed to calculate the required frequency of snow retention based on the design snow load and the allowable loads of the snow retention system. It relies on the adequate compressive strength of the snow blanket. The calculator does not take into account the buckling nature of a thin snow blanket that may lack adequate compressive strength.
In rare cases involving thinner accumulations of snow on steeper slopes, or longer rafter lengths, there is a potential for the snow blanket to “buckle” at some point above the snow guard assembly. This phenomenon is similar to a slender, unsupported steel column that buckles under compression loads. Especially when the blanket of snow is dense and wet (not solidified), it may tend to undergo compression buckling, pushing a “loop” upward from the roof surface as the blanket compresses toward the eave where restrained by snow guards. This loop, can in some cases fold over the snow retention device(s) in the downslope direction, breaking free and in some cases fall from the roof. Since this normally occurs when snow blankets are relatively thin, the amounts of released snow tend to be somewhat minimal, but may still pose a hazard or nuisance. As the snow blanket increases in both and thickness and solidification, its compressive strength also increases, diminishing this buckling tendency and therefore minimizing this potential.
While this aspect of snow retention is more “art” than science, our experience has shown that the most effective approach for mitigating this “buckling blanket” tendency is to add a second row some distance above the first even when calculations show that it is not required. This (first) reduces the unsupported length of the blanket, and second, If the blanket buckles above the upper row, the “loop” is usually still contained above the lower row. If the job requires one row of assembly with a clamp on every other seam, than the addition of a second row should require the same amount of assembly as the first row.
Conditions that cause it and how to reduce it
Moist snow may undergo compression buckling where it is stopped by the snow retention device. This can cause the blanket to fold up and over the rail where pieces of it then break away.
When the blanket of snow on the roof is dense and wet (not solidified), it may tend to undergo compression buckling, pushing a “loop” upward from the roof surface as the heavy mass compresses toward the eave where restrained by snow guards. This loop, can fold over the blanket and the snow retention device(s) in the downslope direction, often breaking and in many cases falls from the roof.
Compression buckling tends to happen more when blankets are thin, as when the thickness of the blanket increases – it gains compressive strength. This tendency is related to both the vector force (and mass) and the compressive strength of the snow. Hence the following variables are at play:
- Slope: Buckling is more common with steeper slopes as vector forces increase
- Thickness of snow blanket: Thicker blankets generally have greater compressive strength and lower mass, and are therefore less likely to buckle
- Moisture: Greater moisture content increases mass, which in turn increases the likelihood of buckling
- Temperature: Warmer temperatures decrease compressive strength, and this increases the probability of buckling
- Length of snow blanket: Longer blankets have an increased vector and a correspondingly increased probability of buckling
Contrary to popular myths, the height of the snow retention device plays no role in the probability of a snow blanket to buckle, and heavier accumulations decrease probability, not increase it.
Surprisingly, a thin blanket of snow poses a greater risk of buckling than do thicker blankets.
Commentary – Solution
This is analogous to compression buckling of a steel column, except that the properties of the snow are less predictable. The problem is noted when the snow blanket is minimal in thickness and very wet (lacking in body and compressive strength). Two things decrease the likelihood of the snow buckling:
- Reducing the unsupported length of the column (snow blanket) by adding rows of snow retention
- Increasing the sectional size of the column (snow blanket), i.e. increasing the snow depth, which is impossible to control. When the snow blanket increases in depth, it is less likely to buckle.
Our experience has shown us that the most effective approach for adding rows of snow retention is to add a second row some 8-10’ more or less above the first. If the blanket buckles above the upper row, the “loop” is usually still contained above the lower row.