This is a post written by Tim Underhill of the National Slate Association
Snow-retention, for obvious reasons, is regionally influenced. The overall picture is, historically at least, a lot of snow in the North, not so much in the South. Subsequently, up North, the goal of snow retention is, well … non-retention. Get the stuff off the roof as quickly as possible. How this is achieved, however, clues us in to the why of snow-retention elsewhere.
In areas accustomed to heavy snow-fall, the first two or three feet of the eave is likely to be a “snow-pan,” a flat or standing-seam sheet metal treatment. The main field of most roofs is above heated space. There, heat escaping from within the building envelope melts the snow, moving it down the roof. At the eave, however, especially if the soffit is extended, the temperature is ambient (no heated space beneath) and snow is likely to re-freeze and collect. The metal snow-pan absorbs radiant heat, and, providing little friction, enables the snow to continue its slide off the roof. Without this system, successive heavy snow-falls can create havoc.
The snow-pan identifies the problem area on a roof, where ice-dams would otherwise form, the why of retention systems. Large amounts of ice and snow on the eave will block run-off, directing it sideways under the slate and back up the roof.
Further South, shallow-pitched roofs are the norm and snow-retention comes into play. It is not desirable to dump the snow on the ground, since the ground is less likely to be frozen, and large deposits of water around a foundation can cause subsidence. Then where should the snow go? Above and within the building envelope would be a good choice, right where the heat is. Snow in this area can melt and leave the roof harmlessly, as water.
So, grab your favorite snow-guards: pig-tails, loop-the-loops, Mullanes, Bergers, Siegers, snow-rails, et al and go at it. The where is solved. The lowest course of snow-retention should be vertically within the building envelope.
A perfect wintry roof-scene (it should be a Christmas card) is a roof blanketed in snow (indicating effective snow-retention) and the bottom two feet of the slate roof exposed, where all that passes is melted snow.
There are some related considerations. Dormers should be regarded as very large snow-guards, since they hold snow in the right place. Except for aesthetic purposes, snow-guards below a dormer don’t do much. Snow-guards around valleys are good, so long as you know the valley was installed correctly, with a slater’s bend on the metal and a wide underlayment protection. The most often asked question though, is how many?
Typically, for a residential roof (figure a gable 14-square roof for an easy example), two rows of snow-guards, two slate courses apart, staggered, on 2-ft. centers, provide a large “entrapment” area. And that is the goal, to create the largest area around the snow-guards in which to trap the snow. This will then support the snow above, during the time it takes for the melting process. Densely installed snow-guards look impressive, but are less effective than those spread more widely.
For larger roofs, the arrangement should be doubled, tripled or more, to hold large areas of snow on the roof. A snow-slide from 30-feet up will ignore all retention efforts in its path. For this consideration, types of retention system become the question — snow-guards, snow rails, boards etc. — another article.
The how of installation gets a little thorny. The so-called retro-fit snow-guards (I’ll go out on an ice-floe here), should be sent back to where they came from. No snow-guard should be jammed under slate and hooked to a slate-nail. The nail is already being used. This is low-brow. It’s a cheap fix that creates bigger problems. Look along any snow-guard line installed in this manner and you will see slate being dragged off the roof.
Good snow-guard installation requires slate to be pulled, and the guards anchored independently into the decking. They are set in the center of the slate above, and should disturb that slate the minimum. If the shaft of the snow-guard is thin enough, it can sit in the key-way and cause little disruption. If it is thicker, the foot of the snow-guard should be recessed (notch each side of the keyway it sits on) to prevent any lift of the slate course above. In this case the keyway becomes a lot wider and I would recommend a copper bib beneath it. A lot of work for one snow-guard, to be sure, but at least well done.
There are numerous topics related to snow-retention: we all have our favorite snow-guards and understanding of the process; how to hang gutters to survive snow-slides; heat tape, etc. I always feel those in the harsher climates should get the most air-time. On which topic …
Go to the National Slate Association’s website, log in and join the conversation on our NSA blog where a snow-retention conversation is underway.
And, stay warm! —Tim Underhill, NSA Executive Director