https://www.whirlwindsteel.com/blog/bid/264596/designing-a-building-to-withstand-heavy-snow
DESIGNING A BUILDING TO WITHSTAND HEAVY SNOW
Published July 11, 2018 by Whirlwind Team
If you build in snow country, or simply have the occasional wet snow or ice, your building design must take into consideration the pitch and sturdiness of your roof and building envelope. The issue is compounded if your building is in a populated area close to other buildings, pedestrians and streets, where safety may be compromised. Also, accumulation beyond intended design could lead to structural failure or collapse.
Snow and ice can appear at unexpected times. If you remember Superbowl XLV, hosted by Dallas, TX in 2011, you probably remember the disastrous events caused by falling snow and ice from the new stadium. A photographer scheduled to cover the game went to the hospital with his shoulder broken in four places. North Texas is not known for winter weather, but it still happens.
DO YOU NEED TO WORRY ABOUT SNOW?
Performing an ice and snow assessment brings together data to help you determine the need for additional roof strength or snow-shedding ability. Unlike a snow load study, which is meant to assess how much the roof should carry without failure, an ice and snow assessment focuses on events that could interrupt building operations or create a safety hazard.
In your assessment, you must consider several factors.
· The falling snow and ice prediction for the location is the starting point for your assessment.
· Historical records of falling ice or freezing rain accumulations provide a foundation for planning for the future.
· Prevention of sliding snow and ice is a prime consideration in populated areas.
· Water melt and icicle formation create another potentially dangerous condition.
· Drifting snow, windblown snow infiltration, freeze-thaw conditions and obstruction of windows and skylights round out possible issues for contemplation.
Most building codes use a 50-year return criterion for snow loading on a roof surface to determine risk.
PROBLEMS AND WARNING SIGNS OF SNOW DAMAGE
The weight of snow cannot be calculated in inches. Different types of snow have varying weights per inch. An inch of wet, compacted snow is much heavier than an inch of powder. Repeated snows could disguise multiple layers of ice and snow while adding unacceptable weight on the roof.
· 12 to 13 inches of snow equals about one inch of water.
· An inch of water or ice weighs about five pounds per square foot.
· Sagging of the roof or horizontal deflection of the walls may indicate damage due to heavy snow.
· Popping and creaking noises, bowing trusses and severe roof leaks are a further indication of damage from snow load.
· Doors and windows may become difficult or impossible to open or close.
The roof slope ratio indicates the ease of snow and ice shedding from the roof of your metal building, especially on the part of the roof where the sun hits.
LOCATION AND ROOF DESIGN TO MITIGATE SNOW DAMAGE
Where you place your structure on the site matters as much as its design.
· Consider prevailing winds before orienting your building on the site. Wind direction determines where snow will blow and drift against the building and on the roof.
· Align the main ridgeline to face the prevailing winds to eliminate the unequal distribution of snow across the roof and deep piles of snow on the leeward side of the building.
· Plant a windbreak of tall evergreen trees to block prevailing winds and blowing snow from reaching the structure.
When designing the roof, a straight, single-sided gable style is the most effective shape for snow management. A simple gable roof can shed water, snow and ice quicker than a flat roof or a building with a complex roofline. Avoid roofs with multiple dormers, valleys and other obstructions which create multiple places for snow accumulation and uneven snow loads.
The roof slope is another factor in preventing snow damage and safety issues on the ground. A shallow slope allows snow to linger on the roof and increase the strain on the building envelope. An extremely high slope could allow large amounts of snow and ice to fall at once. If the material lands on a person or vehicle, it could easily crush them. Snow can weigh 30 pounds per square foot or more. Three feet of accumulation yields 90 pounds per square foot of snow landing on someone’s head.
The recommendations for the best roof pitches on a metal building are 3:12, 4:12, 5:12 or 6:12. Higher pitches create large snow slides. Anything less than 1:12 allows the snow to accumulate to the point of collapse. The best roof type to prevent snow accumulation is a standing seam metal roof with a pitch of 2:12 to 3:12.
While metal buildings do not always have overhangs, some builders or owners believe allowing an overhang will keep sliding snow from accumulating against the walls of the building. Unfortunately, the overhang is unheated and such a design allows the formation of ice dams, which also damage roofs and panels, leading to leaks and other problems.
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ENTRYWAY POSITIONS
Considering the danger of falling snow and ice from a roof, the position of the entryway to the building should eliminate the issue. Placing the entry under a gable instead of underneath the slant of the roof should keep the issue at bay. Placing the entry on the opposite side from the prevailing winds will not only decrease the safety hazard but improve energy efficiency as well.
SHOULD YOU INSTALL GUTTERS IN SNOWY REGIONS?
Gutters and downspouts channel water to the ground and away from the building’s foundation, including snow and ice melt. But the gutter system only works if it is maintained. Blocked gutters and downspouts keep water from draining and, in cold weather, the water will refreeze and form ice dams and allow further accumulation that can damage the roof and the gutter system. Heating cables within the gutters may eliminate this issue.
Sliding snow and ice from the roof can pull gutters off the edge of the roof on the way down. Snow gutters can be installed to minimize the increase in energy efficiency brought about by insulation, snow doesn’t melt as readily because the heat never reaches the roof. The choice of whether to install gutters should be discussed with the building designer and engineer.
Much of the U.S. receives accumulations of snow, some more frequently than others. Therefore, the chance of heavy snow that could damage your roof or create havoc as it slides to the ground should always be a consideration in roof design and building placement.
In regions where heavy snow is a regular occurrence, such as in the Sierra Nevada Mountains or New England, decisions about roof style and pitch and the installation of gutter systems should be made with all safety factors in mind.
Designing a simple, gabled roof perpendicular to the prevailing winds and sufficient roof pitch will go a long way toward mitigating the hazards posed by snow.
