Determining where to place snow retention rows is a common question. The first row is the easiest to answer, but it is really more of a question. How much snow do you really want to fall off the roof?
The Metal Construction Association “Metal Roof Design for Cold Climates” document has some commentary on pages 10 and 11 with respect to the first row of a snow retention system. This industry document recommends, “The first row of units or cross-members should be located close to (within 12-in. of) the eave end of the panels.”
The reason for this recommendation:
“This is because at some point, the snow bank that envelopes the snowguard will shear at the approximate location of the guard, and whatever portion of the bank is below the guard may fall from the roof.”
You will find several myths surrounding this topic, but the sighted “Metal Roof Design for Cold Climates” published by MCA is a good source of information to determine this and other cold climate designs. This document explains this question as well as many others regarding snow retention and metal roofing in cold climates.
Figure 11 “Guidline for spacing of multiple rows” is a helpful resource found on page 10 of the MCA “Metal Roof Design for Cold Climates.” As you can see from the Figure 11 layout, snow retention products are preferred near the eave, this is to take advantage of the compressive and shear strength of snow pack rather than being dispersed all over the roof surface.
Another issue that is often overlooked, and can be very critical, is the need for a second row, even when calculations say you don’t need one. This is explained in the sighted document and is caused by a lack of compressive strength in the snow. We at S-5! like to call it a “Buckling Blanket.”
A Buckling Blanket is 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 it is 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 some cases falling from the roof or impacting lower rows of assembly. A simple solution is to reduce the unsupported length of the snow blanket by adding rows of snow retention. For more information please see: Buckling of Snow.
The MCA “Metal Roof Design for Cold Climates” document has other useful information in it with respect to this issue. For more information about MCA please visit their website: www.metalconstruction.org or click here to download the PDF document entitled: MCA-Cold Climates