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.
SnoClip™ design has changed. SnoClip must now slide on from the end of the ColorGard crossmember. Be sure to slide SnoClip onto assembly before attaching crossmember to clamps.
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.
Find out what your own roof may be vulnerable to by using our Web-based calculator.
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.