This is a very important question to address as well as an important answer to understand! Before I answer, I’d like to ask a similar question. Will a dam hold back all of the water that could accumulate behind it?
On the surface, the simple answer to both questions is no. However, the answer is far from simple! A basic definition of a roof top snow device could be: “A device installed on a roof top or building facade with the intended purpose of managing the flow of snow and ice as it migrates off the roof.” Hold this thought for a moment and consider the definition of a dam.
Webster’s Dictionary defines a dam as:
“A barrier preventing the flow of water or of loose solid materials (such as soil or snow) a beaver dam or an ice dam especially. Especially, civil engineering: a barrier built across a watercourse for impounding water.”
The intended purpose behind snow retention devices and dams are similar. It is an attempt to manage the movement of water, snow, and ice beyond a given point. Typically, man-made dams have flow gates to manage the water levels behind them. Similar to snow retention devices, the objective is to allow water to drain around, or through the snowguards without letting the mass of snow release all at once.
So back to the question; “Can a snow guard manufacturer guarantee that their device will retain all snow and ice on a roof top?”
The answer is that snow guards are designed to manage the release of snow and ice from a roof top as the mass melts, or in some cases, crests and slowly drops off the roof. Consider these four variables:
The first variable to observe is the height of device or dam. For example, if water exceeds the height of a dam, it simply flows over. This doesn’t necessarily equate to snow retention all of the time. Snow is sometimes more liquid than solid; meaning the density changes with time and temperature.
In a perfect scenario snow would accumulate in a light powdery form, settle, and melt slowly at a temperature around 32 degrees. This process would happen overtime, thus allowing flowing water to run out under the snow retention device, similar to the same way the overflow gates manage the level of water behind a dam. However, Mother Nature rarely provides perfect scenarios.
I remember as a young kid in the 1970’s, winters in Vermont where snow would start falling in November, and stay throughout the winter into late March, early April. Snow retention devices that had been installed correctly functioned perfectly during those winters. As Snow accumulated, temperatures for the most part stayed below freezing. As spring approached temperatures slowly came up and the mass of snow gradually diminished. I also remember the winter of 2018-19, when there was little snow across the Midwest until April. Suddenly, all at once, a storm brought in up to 18” of wet heavy snow, that was followed by warming temperatures transitioning the heavy blizzard into a hard rain. It is difficult for a “pad style” snow guard to retain this volume and density of snow given the naturally occurring variable.
Picture a beaver dam or snow retention system that has a frozen mass retained behind it. As temperatures slowly start to rise above freezing, water begins to flow. Over the course of time, the snow mass melts, the water trickles out, Spring blossoms, and all is well. Now picture the same situation with a frozen accumulation of snow and ice. However, in this scenario the temperatures skyrocket to 50 degrees bringing along 2 inches of rain fall; all of a sudden water levels are on the rise. In the case of the beaver dam, water, snow, and some ice flows over the dam. Quite possibly the dam will be damaged in the process (does height matter? Yes, it does). With luck, the beaver has its place of hibernation built high enough that the water level doesn’t reach him. In the case of the snow retention device the wet slushy mass is going to slump against the snow retention device. If the device is tall enough to manage the volume of the snowy mass, all is well. If it is not the mass may crest, like a wave washing up against the shore, cascading over the top of the guards. As a generic rule of thumb, strictly for visual purposes, it has been observed that the guards will manage snow depths up to two times the height of the guard. For example, an Alpine FusionGuard is three inches tall, therefore, it should handle snow depths up to six inches.
For the second variable, we need to look at the frequency of devices. Using the beaver analogy, if Mr. Beaver builds a dam at the low end of a valley formed by two 5000’ mountain chains, it doesn’t take a genius to understand that in a 2’ deluge of water atop a snow mass, Mr. Beaver is likely to wake up somewhere down steam; if he’s lucky. However, if he starts at the top of the valley, building his dams to manage the water that is likely to accumulate; he is able to manage the water in smaller masses. Mr. Beaver is now more likely to survive extreme weather conditions. Then he or his offspring could migrate down the valley building a series of dams at intervals designed to handle the flow of water at its highest levels.
The same principle is true with snow retention devices. If the given snow mass or the potential volume dictated by local building codes cannot be managed by one tier of guards, it is possible to divide up the roof area into smaller sections by installing multiple tiers of guards. Simply stated, a snow retention device (this varies by product) will manage a finite amount of roof area dictated by the strength of the given device, the roof area to be managed, the pitch of the roof, and the area snow loads.
Moving on to the third variable, the type or style of guard/obstruction (Dams are typically continuous whereas obstructions are often scattered about). When I think of dams, I think of barricades from one end of an area to the other; a continuous obstruction. This is also how I think of “pipe style” snow guards; a continuous barricade. Some of us as kids had the opportunity to play in small streams, building dams of our own. As soon as these rocks began to be moved into place the flow of water was disrupted. Water still flowed around them, but generally slower and more managed. As more rocks were moved into place, the depth of the water would increase, and the flow of the water behind the rocks would slow. This is how our “pad style” snow guard’s work; they help to manage the flow of snow and ice as it exits a roof. The snow will still move around the pads (similar to the rocks in the stream) as it exits unobstructed areas. A good example of this approach can be seen as “Wing dams” (see image below) on rivers like the Mississippi and the Columbia.
For this reason, I describe “pad style” snow guards as snow management devices. The more obstructions (pads) used the more robust and effective the management system will be. That being said, pads are not a barricade. They are added obstructions that help manage a moving mass, but by physical design cannot stop a slumping snow or ice mass from eventually migrating around them. The rate at which this migration happens depends on a variety of things that might include the roof pitch, the amount and weight of snow mass, the density of the snow mass, the roof type, etc.
If you are considering using a “pad style” snow retention device, most common in residential construction due primarily to aesthetic concerns, the size matters. As compared to rocks in a stream, a bigger rock will disrupt the volume of water greater than a smaller rock will.
Lastly, the final variable we commonly use is the type of roof. This may also affect the choice of snow retention devices. There are snow guard devices made available for all roof types, from the glass on solar panels, to commercial flat roofs that slope to exterior drains. From slate, to tile, to synthetic slate, all the way to composite shingles, there is a snow retention device for any dwelling or municipality. A snow retention device used on a low slope commercial membrane might be different than the device needed for a steep slope membrane roof. This is a particular concern with newer synthetic shingle profiles that are greater than 1/4” thick. The thicker the shingles are at the exposed end, the taller the snow retention device needs to be.
An example of this might be to picture water flowing over a spill way that is 6” tall. Any device installed with the intention of obstructing the flow of the water coming through the spill way would need to be purposefully taller than the spill way it self. The taller and wider the device, the more effective it will be.
So how does all of this relate to the stated question?
Using the river flow management as an analogy; I have tried to provide a common visual image to help the reader understand why it is not practical to guarantee how a snow guard device will function given multiple variables including “Acts of God (Mother Nature)”. What I haven’t touched on is the expectation of the consumer.
When a decision is made to purposely manage the flow of water in a river, with the intention of stopping the flow and retaining water in place, the obvious solution is to erect a dam. When you apply the same logic to a roof top, the obvious solution is the same; use a “pipe style” snow retention solution.
With the exception of one project in the residential space and metal roofing (this project was owned by a roofing contractor who routinely uses pipe style snow devices), in the nearly 30 years I have been making and selling snow guards, no residential project that I can recall has opted to buy and install pipe style snow guards. Why? Building owners want the protection of a snow retention system but they don’t want to see it. It’s that simple. Building owners want guarantees (The Hoover Dam) but they also want to see aesthetically pleasing river rocks scattered about in a less obtrusive manner. Remember the description above about damming a river with rocks? The more rocks you use and the bigger they are, the better they will perform. Yet all the while water / snow is still going to slip past the obstruction.
If you are looking for a reliable snow retention solution for your project, choose a “pipe style” solution. If your intention is not to barricade the snow, but to gradually mange the amount of snow exiting the roof, the “pad style” guards are more of what you are searching for. Using only 2 rows off pads on most projects will sometimes cause more damage than they resolve.
There is hope! In the next article I will update you on the new Alpine SnowGuards Fusion and Snow Bird products. This hybrid approach combines the benefits of both snow retention systems, and the aesthetics of pads; all with the functionality of a pipe system. This allows for flexibility in the installation process.
When I think about engineering feats such as “The Hoover Dam”, it seems unfathomable. The time and energy that must have been required to plan, design, and implement the construction of such a monumental dam. But could a dam fail? Absolutely! No matter how much planning goes into a project, a variety of problems could evolve.
Stayed tuned for more exciting information to follow, Cheers to 2021!
Alpine Snow Guards