Snow guard design considerations

by sadia_badhon | February 5, 2021 10:34 am

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By David Kowch

With the growing trend of more environmentally conscious construction, metal roofs are becoming the choice of architects and builders. The lifespan of metal roofs is anywhere from 40 to 70 years. They are also environmentally friendly, as they can be recycled at end of their life cycle.

Snow guards

Every metal roof needs a properly designed and engineered snow guard system. A snow guard increases friction between roof and snow, retaining a snowpack on a roof, so it evacuates in a predictable and controlled fashion (evaporation and thaw) rather than by a sudden and dangerous rooftop avalanche.

Why use a snow guard?

Falling snow forms a temperature-sensitive bond to the surface of a metal roof. As that roof warms—either from the sun or building heat loss—the bond with the snow is broken and a thin film of melt water serves to lubricate the roof. This can have dramatic results as a several-ton blanket of snow suddenly slides off the roof and avalanches upon anything in its path—gutters, vehicles, landscapes, and even people. Once piled up below, that same snowbank can damage the exterior walls either directly or indirectly by funnelling melt water into, rather than away from, the wall. Snow guards allow snow to leave the roof slowly, either in small amounts of snow or as melted water, thereby avoiding avalanches.

The use of properly designed and engineered snow guards can:

• limit liability;
• protect building occupants and equipment such as air-conditioner units installed below upper roofs;
• safeguard structural and roof elements such as vent stacks, signage, lighting, and architectural elements; and
• reduce maintenance costs because snow can damage gutters and landscapes.

Sliding snow

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Sliding snow along the roof is impacted by various factors with each one affecting the snowpack in different ways.

Ambient thaw

When snow blankets the roof, the ambient air is typically at or below the freezing point. This creates a frictional bond, allowing the snowpack to remain on the roof. If the ambient air rises above freezing, the snowpack begins to melt from the top, and the heavier, wetter snow moves downward toward the roof panel. As this cycle continues, the heavier, wetter snow forms melt water along the roof panel, thus weakening the frictional bond between the snowpack and roof. The frictional bond would eventually be reduced to the point where the snowpack slides or avalanches down the roof.

Solar thaw

Solar thaw works in the opposite manner of ambient thaw; the snowpack is melted from the bottom along the roof. Solar thaw occurs when sunlight passes through the snowpack and heats up the roof surface. This, in turns, creates melt water along the roof and snowpack. It is interesting to note thawing is a common factor for sliding snow especially when the roof colour is darker, which has a higher absorption value than a lighter hue.

Heat loss thaw

Similar to solar thaw, “…heat loss thaw occurs between the snowpack on the surface of the roof. Heat loss thaw allows for heat from within the building escaping through roof construction warming the surface of the roof to temperatures above outside ambient air,” per the Metal Construction Association’s (MCA’s) Metal Roof Designs for Cold Climates.

Snow loads

The ability of snow guards to retain sliding snow on the roof is dependent on many factors. Snow loads are one of the most important, but commonly overlooked, considerations.

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Since snow loads on roofs vary according to geographical location (climate), site exposure, and shape and type of roof as well as from one winter to another, subsection 4.1.6 of the National Building Code (NBC) expresses the specified snow load, S, on a roof or other surface as the sum of two components—one being the product of a series of factors—multiplied by an importance factor:

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I_s = importance factor for snow load; S_s = ground snow load, in kPa, with a one-in-50 probability of exceedance per year; C_b = basic roof snow load factor; C_w = wind exposure factor; C_s = roof slope factor; C_a = accumulation factor; and S_r = associated rain load, in kPa. However, the rain load at any location on a roof need not be taken greater than the load due to snow (i.e. S_r ≤ S_s[C_bC_wC_sC_a])(consult “Structural Commentaries” in the National Building Code (NBC) User’s Guide 2015).

Snow loads should be calculated for every project requiring snow guards as each one will have varying factors affecting the loads. Calculating snow loads on a roof should never be dismissed or taken lightly.

Calculating snow loads

Figure 1 is an example of a snow load calculation based on a typical 24-gauge, 38-mm (1 ½-in.), single-fold standing-seam roof with 406-mm (16-in.) seam spacing. The roof in Figure 1 is 14 m (45 ft) eave to ridge, the roof-designed snow load is 2155 Pa (45 psf), the roof slope is 6:12 the seam spacing, or panel width is 406 mm.

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The example does not take into account factors such as unbalanced snow loads, snow pileup, or upper roof snow shedding down to a lower roof. These factors must be considered when calculating snow loads, and are discussed in NBC 2015, Division B 4.1.6.2, Volume 1.

For the roof in Figure 1, there will be 547 kg (1207 lb) of force in each 406-mm panel width pushing on the snow guards.

With 547 kg of snow pushing on the snow guards there is still more work to be done. There is a multitude of snow guards on the market today, some use adhesive clamps to attach to the standing seam itself while others screw through the panel. It is easy to assume all the available snow guards work the same way, but this thought can lead to snow guard failure.

It is important to keep in mind some snow guard manufacturers use their ultimate load or failure point to design snow guards, which is not the best practice. For example, a snow guard manufacturer has load tested their attachment point to the standing seam, and determined the ultimate or failure point is 455 kg (1000 lb). Hence, they claim their attachment point to the standing seam can withstand 455 kg. This can lead to failures, as the ultimate or failure point of the load test was used. When a safety factor of two is applied to the ultimate load test, the allowable load would be 227 kg (500 lb), which is what the snow guard should be designed around. By applying a safety factor of two, the attachment point to the standing seam has a buffer, in case snow loads are higher in a given year.

To resist the loads imposed on snow guards it is recommended to install them the full length of the building as opposed to just over doorways, garage doors, or entrances. Isolating snow guards over doorways, garage doors, or entrances may overload them, or the snow may simply curl over the guard. When the snow slides down the roof and impacts the snow guard, it forms a wedge shape and can increase the initial snow load calculations, resulting in guard failure (Figure 2).

Budgets

As mentioned, snow guards are an integral part of the overall project when dealing with metal roofing, as they keep the snow from falling off the roof.

Typically, snow guards are an afterthought; they are missed during the initial scope of work. Sometimes, inadequate snow guards are used and need to be replaced. At this point, chances are the budget for the project has been spent and additional funds would be required to fix the issue, which means someone has to come up with more money, and that is never an easy conversation.

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Snow guards typically come with a limited manufacturer warranty of between three and 25 years. However, design, engineering, and installation determine if the snow guard is going to fail or not. A snow guard can be designed properly and approved by an engineer, but it would not matter if installation is incorrect.

It is recommended to work with a company specializing in snow guards before the project goes out to tender or before final budgets are calculated. These companies can provide snow load calculations specific to the project’s location and roof’s geometry. They have an understanding of the factors affecting snow loads, such as snow pileup or drift due to adjacent roofs or parapet walls, upper roofs shedding snow down to lower roofs, and how dormers and valleys can affect snow accumulation, and will be able to recommend the proper number of snow guard row(s) required as well as snow guard layout. This initial work not only assists with proper budgeting, but also eliminates guess work for contractors or surprises that may arise when more rows of snow guards that initially planned are required.

Another factor to consider when budgeting for snow guards is that cheaper is rarely better. There is a tendency in the industry to supply snow guards made from bent sheet metal formed into a triangular shape that are either screwed to the standing seam or screwed through the rib of the panel down to the underlayment, which is, in most cases, plywood. These types of snow guards rely on the strength of sheet metal screws to resist hundreds of kilograms of snow and ice sliding down the roof. However, the screws and sheet metal ribs are not designed for that. The sheet metal screws are employed to secure a metal roof to wood or purlins. The initial cheaper solution will cost the client more money to repair. If the snow guard must be screwed through the metal roof, it is important to install wood blocking between rafters where possible and secure the snow guard to the wood blocking. Again, it is important to do load calculations to determine if one or multiple rows are required.

Cleats or pad style

These are individual or unitized parts that are secured to the standing seam, screwed through the roof or adhered to it. These types of snow guards are used in low snow load areas or for esthetic reasons. The drawback to these snow guards is their holding strength tends to be low and require multiple rows along the whole roof. Adhesive snow guards rely on an adhesive to secure the snow guard to the roof. The problem with these is they can only be installed during warmer months. The adhesive must have proper conditions for it to cure and, usually, require multiple call backs to replace them. Further, adhesives break down due to exposure and weaken over time, resulting in failure.

Bars or fences

Commonly referred to as pipe or rail-type snow guards, they are designed to provide a continuous wall in which the shedding snow will impact. These types of snow guards can be attached to standing-seam roofs via clamps or attached to exposed fastened roofs via screws. The bar or fence-styled snow guard is most commonly used across Canada and can be less expensive than the cleats or pad styles.

The taller the snow guard the better

A deeper snowpack does not equal a taller snow guard. A taller snow guard is not any more effective than a shorter one because the snow does not slide from the top. The sudden release of the snowpack happens along the roof panel at the base of the snowpack when the frictional bond between the snowpack and roof panel has been reduced. It is the base of the snowpack that must be restrained, and not the top.

Choosing a snow guard

When choosing a snow guard, ask the manufacturer about product testing. As mentioned earlier, clarify if the snow guard is designed around the ultimate load or failure point or around an allowable load. It is important to remember the snow guard is going to be the one piece of equipment that will stop the snow from falling off the roof with potentially disastrous outcomes to the building.

[7]David Kowch is the business development manager, overseeing all operations at Sky Products. Kowch has over 15 years of business, project, and client management experience. Kowch can be reached at davek@skyproducts.ca[8].

Source URL: https://www.constructioncanada.net/snow-guard-design-considerations/

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