by Katie Daniel | November 23, 2015 2:24 pm
by Harry J. Lubitz, CSI, CDT
The use of snow retention devices originated several hundred years ago in areas of Scandinavia and the Alps. In these cold regions, builders and homeowners placed stones and logs on rooftops to increase their friction with the snow. The purpose of a snow guard is to facilitate the evacuation of rooftop snow in a predictable and controlled fashion—evaporation (sublimation) and thaw—rather than by a sudden and dangerous rooftop avalanche.
Keeping snow in place was important to the structure owner as this construction predated the use of modern insulation. A good layer of snow on the roof in these Alpine climates helped reduce heat loss and keep inhabitants warm. Retaining snow on the roof provided the additional benefit of protecting anyone or anything (such as farm implements and animals) around the structure from accidental release of accumulated snow pack, and subsequent potential injury from its release.
Stones placed on the roof acted as individual ‘cleats’ to engage the snow bank and keep it in place (Figure 1). Logs were akin to continuous fences that engaged the bank of snow. Both methods proved effective in retaining snow and preventing a rooftop avalanche.
In these regions, people have lived with a constant threat of avalanching snow for centuries. Snow depths can reach up to 10 m (32 ft) annually in these parts of the world (Figure 2). Consequently, they learned how to deal with and reduce the hazards of sliding snow.
There is a science to understanding rooftop avalanches. When snow blankets a roof surface, a frictional, temperature-sensitive, adhesive bond is created between the snow particles and the roof material. A weak cohesive bond is also created at the ridge of the roof connecting the snow packs on each side of the roof. The vertical weight of the snow translates to vector forces—‘drag’ or gravity loads.
At first, the adhesive and cohesive bonds together are sufficient to resist the drag load. However, when the weather clears and the sun’s ultraviolet (UV) rays pass through the translucent bank of snow to the roof surface below, the radiant heat is absorbed by the roof’s surface. The heat energy raises the surface temperature, altering the frictional co-efficient as the bond turns into meltwater. This can happen when ambient temperatures are well below freezing due to the insulating characteristic of the bank of snow. With the frictional (or adhesive) bond now jeopardized, the drag loads exceed the strength of the cohesive bond at the ridge and the snow bank quickly releases, causing a rooftop avalanche.
This science applies to all roof surfaces, the difference being the texture and porousness of the roof materials. For example, metal roofs are more prone to dangerous avalanche hazards because of their slick surfaces, particularly since their polyvinylidene fluoride (PVDF) coatings are chemically related to non-stick Teflon—whereas a granular roof surface would pose a lower immediate risk.
Using snow guards
A snow guard should be used to:
Several provincial and municipal jurisdictions, along with local school boards and national retailers, have added this design requirement to their building designs. Good practice for snowfall regions suggests roof orientation be diverted away from pedestrian traffic. However, this is often impossible and pedestrian traffic may have to pass below unprotected eaves. Clearly, the safety of individuals walking by a building outweighs the minimal cost of adding snow guards to a project design.
There have been several high-profile court cases where people were injured or killed by snow falling from roofs. For example, in 2011 at Super Bowl XLV in Dallas, Texas, at least six people suffered a range of injuries when warming weather increased the rooftop temperature and caused layers of ice and snow to avalanche from the stadium roof.
Adding a sign to the building warning pedestrians of falling ice and snow is not an acceptable alternative to snow management. It might actually create a greater issue for the building owner because the sign admits to a problem with falling snow and the owner is not taking proper remedial action to protect the public. If someone were to be hurt by falling snow it may be an invitation for a lawsuit. The addition of snow guards is a simple ‘insurance policy’ to protect the designer, owner, and public from falling snow and ice. The goal is a well-managed snow pack, maintained on the roof by snow guards, where it can be naturally melted and eliminated (Figure 3).
Often, it is not just people at risk from the ravages of falling ice and snow. Vehicles, mechanical equipment, and landscaping can also be in the path of rooftop avalanches (Figure 4). The volume of ice and snow quickly exiting a roof can be measured in tonnes and can cause significant damage.
While it is clearly important to protect people and equipment, it is also important to defend the building itself from avalanching snow. Extensive rooftop damage can be caused in an instant. Gutters, fascia, trim, signage, light fixtures, antennae—anything attached to a roof—can fall prey to an avalanche of sliding ice and snow (Figure 5). Ice and snow sliding uncontrollably down a roof valley can open up seams and destroy valley flashing (Figure 6). A small investment in snow guards can protect a building owner from extensive repairs and long-term maintenance headaches.
Engineering snow guards on metal roofs
The forces of snow on a rooftop should be mathematically calculated to ensure product success. Simply stated, the calculation takes the vertical weight of the snow and reduces it by the sine of the roof angle to arrive at the angular force exerted on the snow guard.
In the case of a metal roof, the force snow guards can withstand can be calculated simply by following these steps (Figure 7):
1. Obtain the roof snow load.
2. Multiply the roof snow load by the sine of the roof angle.
3. Multiply the sum of Step 2 by the eave-to-ridge measurement of the roof.
4. Multiply the sum of Step 3 by the seam-to-seam width measurement of the roof.
The final answer is the angular force. (There are several commercially available calculators on the Internet that can walk through this process and complete the calculation.)
Once the angular force is calculated, snow guards can be chosen by consulting the published test data from the manufacturers. To determine the number of snow guard rows needed, the tested allowable load of the snow guard being considered should be divided into the angular force sum. For example:
1. If angular force equals 461 kg (1016 lb), and a cleat snow guard device with the tested allowable load of 125 kg
(470 lb) has been chosen, four rows of these parts per seam will be needed to withstand the angular force (125/461=3.68, or ‘4’).
2. If angular force equals 461 kg (1016 lb), and a fence-type snow guard device with a tested allowable load of 476 kg (1050 lb) has been chosen, only one row of this type of device would be needed to withstand the angular force (476/461=.968, or ‘1’).
It is very important to understand snow guard systems must be load-to-failure-tested on specific roof manufactures and product profiles by a third-party-certified independent laboratory for the guards to properly perform in different environments and roof systems. This data must be part of the submittal package provided by the roofing contractor planning the snow guard system, and must be specified as such.
If a snow guard submitted by a roofing contractor has not been tested, or the data is not included, it should not be used. An untested system is merely a wild guess by the roofing contractor and will probably fail.
If a snow guard system is planned with multiple rows, their placement is critical for success. It is generally accepted all rows of snow guards should be placed on the lower half of the roof surface. The first row of snow guards is most often placed approximately 300 mm (12 in.) from the eave edge of the roof. This placement accomplishes two objectives. First, it minimizes the uncontrolled snow shed below the snow guard to 304 mm. Second, it engages the roof snow pack down low in the eave to ridge dimension where it is most dense, maximizing the guard’s holding capacity.
With fence-type snow guards, when a second row is required (see calculation process above), it is most often placed approximately 20 per cent of the eave to ridge distance and a minimum of 1.9 m (6 ft) from the first row of snow guards. (This distance will vary based on the actual eave to ridge distance of the roof.) If a third row of snow guards is required, it is most often placed approximately 40 per cent of the eave to ridge distance and spaced similarly as the first two rows. In any event, the objective is to place the rows evenly and esthetically, while keeping them in the lower half of the roof surface to engage the snow pack in its densest concentration [See Metal Roof Designs for Cold Climates.]
Snow guards on metal roofs
Metal roofs provide a unique and slippery surface for the adherence of snow. The following factors should be considered when determining what form of roof snow retention should be used.
Ground vs. roof snow
A common mistake when designing snow guard devices is using ground snow load in calculations rather than roof snow load.
Ground snow load is defined as the weight of snow on the ground surface, while roof snow load is defined as the weight of the snow on the roof surface used in the design of the building structure. Ground snow load values in Canada are established using data collected by Environment Canada.
If buildings were flat without any obstructions, ground snow loads would be similar to roof snow loads. However, this is rarely the case. Snow will collect on a roof in different depths based on various conditions and levels of roof design. Wind and shading are the deciding factors on how snow accumulates on a roof. Generally, the wind will create snow buildup against higher surfaces (e.g. walls and parapets), drifts on lower roof sections, snow accumulation deposits in roof valleys, and drifts on the leeward (downwind) side of objects, obstructions, and ridges, known as ‘aerodynamic shading’ (Figure 8).
These accumulations and drifts create an unbalanced load on roof surfaces that may not be accounted for in the ground snow measurement. Unbalanced loads could produce a roof design load of three or even four times that of ground snow. Hence, if a snow guard is engineered to resist only the ground snow load, it would most certainly fail under the added weight of the roof snow. Consequently, it is important to obtain the roof snow load calculation from the engineer and use it in the guard’s calculation.
Miscalculation of tributary snow loads
The miscalculation of localized tributary loads is another frequent error. For instance, if a designer wants to protect people using an entryway, rather than protecting an entire eave, he or she designs a small section of snow guard just over the doorway. The tributary area of snow buildup behind that snow guard is not a simple rectangular area as many would suppose. The snow retained will represent a wedge shape that far exceeds the vector forces of a narrow strip of snow guard. The side angle of the wedge shaped accumulation will grow more obtuse as the slope increases (Figure 9). For example, a roof with a 12:12 pitch will create a 65-degree angle wedge behind the strip of snow guard, whereas a roof with a 4:12 pitch will create a 45-degree angle wedge behind the strip of snow guard.
This ‘penny-wise/pound-foolish’ approach of only using a snow guard over doorways will invariably lead to overloading the small snow guard strip and tearing it from the roof surface. Snow guards are an inexpensive roof accessory designed to reduce the designer’s and owner’s liability and protect people and property. Extending the snow guard device across the length of the eave will more evenly distribute the snow load on the device, and when properly engineered, provide protection for the life of the roof. Trying to save a few dollars on this safety device may lead to bigger long-term problems.
Designers are often disappointed with the look of bar, rail, pipe, or fence snow guard devices and opt for a clear plastic option to fit their snow control needs. Polycarbonate snow guard devices are perceived as an economic option and have the benefit of allowing the roof colour show through the device and blend with the roof. While adding plastic to the roof may seem a logical option as these devices look ‘clear’ in photos, the prismatic effect of the light striking the plastic surface makes them highly visible when placed on the roof. Additionally, these plastic devices are petroleum derivatives and do not stay ‘clear’ for long. The sun’s UV rays can turn them yellow after just a few years.
Another concern of designers is the additional cost they associate with painting a bar, rail, pipe, or fence snow guard to match the roof colour. Innovations in bar and rail snow guards have incorporated a strip on the matching metal roof into a track on the front of the device (Figure 10). This colour strip allows for the snow guard to accurately match the metal roof and blend it into the roof surface. The inserted metal strip is the exact same colour as the metal roof surface and will weather and look the same as the roof surface for the life of the roof. This metal strip also hides the balance of the snow guard components from view and eliminates the need and cost of painting the snow guard.
An important consideration when designing and specifying snow guard devices is how they will be fastened to the roof surface. It is a primary concern to specify fasteners that work with the roof surface and do not violate the metal roof manufacturers’ warranty. Using mechanically attached devices versus adhesive attached devices will maintain the integrity of the roof manufacturers’ finish warranty. Additionally, employing the correct mechanically attached device will maintain the integrity of the roof manufacturers’ watertightness warranty.
There is more than one fastener used with attachment devices for metal roofs and they need to be closely scrutinized to ensure they do not harm the roof and attach without voiding the metal roof manufacturers’ warranty. Many snow guard devices employ a mechanically attached fastener with a round, polished head that creates a ‘dimple’ in a standing seam. This acts as the active holding point of the fastener. The dimple is created by the round head of the fastener integrated with a ‘well’ in the snow guard clamp to accommodate the metal seam protrusion on the other side of the seam (Figure 11). The round point of the fastener is designed specifically to dimple the seam without penetrating it, and subsequently stay within the non-penetrating clause of the metal roof manufacturers’ warranty. The polish given to the round fastener head ensures the roof’s protective coating is intact, eliminating a place for metal corrosion to begin.
Other fasteners employ a cup-tip and are designed to ‘dig’ into the metal seam to create holding strength. This intrusion into the seam of the metal roof could potentially penetrate the seam of the metal roof and void the manufacturers’ warranty. The action of digging into the metal seam by the fastener also removes the roof’s protective coatings and exposes bare metal to the corrosive elements of the weather. This could potentially reduce the roof’s service-life. Cup-tip fasteners are not recommended in publications by Metal Building Manufacturer’s Association’s (MBMA’s) Metal Roofing Systems Design Manual section on fasteners, and the Metal Construction Association’s (MCA’s) Metal Construction Association Technical Bulletin.
To protect a metal roof so it may achieve its full useful life and maintain the manufacturer’s warranty, it is recommended to specify non-penetrating round-point fasteners in specification documents.
To maintain the metal roof manufacturers’ finish warranties, no chemical adhesive or sealant can be applied to the face of the panel. These chemical-based materials will not properly adhere to the metal roof surface and will damage the paint finish—subsequently invalidating the finish warranty.
Chemical sealants will weaken as they age because they are subjected to environmental factors such as hot and cold cycling, UV light exposure, and moisture—meaning they will not maintain a strong and measurable bond for snow guards. Ultimately, the vector force placed on a chemically adhered (i.e. adhesive applied) snow guard will cause the device to fail and break loose from the roof. This can be observed after a few years on most roofs employing adhesives as the attachments (Figure 12). The subsequent ‘scar’ created by the broken bond is both unsightly and could be a potential corrosion point in the future.
The science behind this inability to chemically adhere items to metal roof surfaces is simple. The polyvinylidene fluoride (PVDF) resins used in modern metal roof coatings are designed to be non-stick surfaces and resist substance accumulation. Consequently, an adhesive-applied snow guard is destined to have a high failure rate and will not provide long-term protection for people or property. Mechanically attached snow guards are the only solution for long-term peace of mind and preservation of roof manufacturers’ warranties.
It is imperative to understand the tested holding strength of the snow guards to be able to accurately plan a successful snow guard installation.
A mistake made when using snow guards involves cost issues—spending either too little or too much. At first glance, low unit prices make a small adhesively adhered parts seem like a bargain. However, the required holding strength of some parts are significantly lower than others, and they may be applied redundantly. Consequently, the cost of some ‘bargain’ parts is much higher because they may require more units per panel due to their low holding strength. The difference on a job could add up to thousands of dollars.
This is why it is important to compare not only costs, but also holding strength. In the case of snow guards, the greater the holding strength of the device, the fewer rows required in the system to hold the volume of the snow. Fewer parts impact both the initial product cost and the installation labour.
The penetration and fixity of the roof panels (roof warranty violations)
When applying anything to a metal roof, there are two cardinal rules that should never be broken—do not penetrate the roof panel, and do not fix or pin the panel in place to violate its ability to move by expansion and contraction.
This is clearly appropriate when planning snow retention devices. As a delegated design in specifications, it is imperative to prescribe these two cardinal rules to the subcontractor developing the ultimate design of the snow guards.
Snow guards with standing seam roof clamps and non-penetrating round point fasteners are specifically designed to protect the roof by attaching without penetration and allowing the roof to continue its thermal cycling. Installers need to take reasonable care during installation by following snow guard manufacturer installation instructions.
Proper snow guard specification techniques
Snow guards can appear in several places of the finished specification, and it is usually the specifier’s choice and preference where they wish to list them. The options for MasterFormat include:
When developing a specification for snow guards, there three key elements to request the subcontractor provide in the submittal package:
It is important to detail the specific attachment fastener type in the product description to ensure the snow guard is installed with non-penetrating round point fasteners to protect the integrity of the metal roof warranty. For example,
Set screws: 300 Series stainless steel, 18-8 alloy, 3/8 inch diameter, with non-penetrating round head point.
The creator of the specifications should make sure the building owner is provided with the best options to protect the building occupants and pedestrians, the property around the building and the building itself from the harm of falling ice and snow. The inclusion of properly planned, calculated, and tested snow guards will provide the building owner many years of trouble-free satisfaction while limiting both the owner’s and designer’s liability for injury and harm.
|A. Action Submittal:1. Shop Drawings: Include roof plans showing locations of snow guards on roof and attachment details and spacing.
2. Product Data:
B. Individual component dimensions:
Harry J. Lubitz, CSI, CDT, is the architectural and national accounts manager for S-5! Metal Roof Innovations, of Colorado Springs, Colorado and S-5! Canada in Ajax, Ont. Lubitz works with the design community to develop and improve architectural specifications for metal roofs and attachment systems. He has more than 25 years of experience in the building materials industry and is active in numerous architectural and professional organizations. He can be reached via e-mail at hlubitz@S-5.com.
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