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Finding Nemo: Avoiding Snow Load Collapse


A major winter storm is expected to impact the Northeast and New England tomorrow into Saturday. As much as one to two feet of snow is forecast from the New York City metro area to Maine, with localized heavier amounts possible. This, in addition to wind gusts as high as 60-75 mph, will create significant impacts to transportation, power systems and property. Coastal flooding is also possible from Boston northward.

With storms of this magnitude, we often see structural collapse due to excessive snow load. According to the National Council of Structural Engineers Associations, “Although there are exceptions, it is generally the case that snow related roof collapses are due to larger than average loads on a small portion of roof, as opposed to nominally uniform loads over the whole roof. The most common of these ‘larger than average’ snow loads are drift loads of one kind or another, and to a much lesser extent, sliding snow loads or ice dams at the eave of a roof.”

snow load collapseFEMA’s Snow Load Safety Guidance document summarizes the warning signs of overstress conditions during a snow event, as well as the safety issues and risks a snow event can pose to buildings. Here are some of the key points:

Warning Signs of Overstress Conditions During a Snow Event
Overstressed roofs typically display some warning signs. Wood and steel structures may show noticeable signs of excessive ceiling or roof sagging before failure. The following warning signs are common in wood, metal, and steel constructed buildings:
• Sagging ceiling tiles or boards, ceiling boards falling out of the ceiling grid, and/or sagging sprinkler lines and sprinkler heads
• Sprinkler heads deflecting below suspended ceilings
• Popping, cracking, and creaking noises
• Sagging roof members, including metal decking or plywood sheathing
• Bowing truss bottom chords or web members
• Doors and/or windows that can no longer be opened or closed
• Cracked or split wood members
• Cracks in walls or masonry
• Severe roof leaks
• Excessive accumulation of water at nondrainage locations on low slope roofs
If any of these warning signs are observed, the building should be promptly evacuated and a local building authority and/or a qualified design professional should be contacted to perform a detailed structural inspection.

Unbalanced Snow Load from Drifting and Sliding Snow on Structures — Key Safety Issues and Risks
Snow accumulation in excess of building design conditions can result in structural failure and possible collapse. Structural failure due to roof snow loads may be linked to several possible causes, including but not limited to the following:
• Unbalanced snow load from drifting and sliding snow. When snow accumulates at different depths in different locations on a roof, it results in high and concentrated snow loads that can potentially overload the roof structure.
• Rain-on-snow load. Heavy rainfall on top of snow may cause snow to melt and become further saturated, significantly increasing the load on the roof structure.
• Snow melt between snow events. If the roof drainage system is blocked, improperly designed or maintained, ice dams may form, which creates a concentrated load at the eaves and reduces the ability of sloped roofs to shed snow. On flat or low slope roof systems, snow melt may accumulate in low areas on roofs, creating a concentrated load.
• Roof geometry. Simple roofs with steep slopes shed snow most easily. Roofs with geometric irregularities and obstructions collect snow drifts in an unbalanced pattern. These roof geometries include flat roofs with parapets, stepped roofs, saw-tooth roofs, and roofs with obstructions such as equipment or chimneys.

What to Do After a Snow Event
After a snow event, snow removal may be in order. To determine whether snow removal is necessary, one may
enlist valuable resources such as a local building authority and/or a qualified design professional, who will be familiar with the snow conditions of the region and the design capacities of local buildings per the building code. If it is determined that the snow should be removed, snow removal should only be performed by qualified individuals.

The qualified individual should follow necessary protocols for safe snow removal to minimize risk of personal injury and lower the potential for damaging the roof covering during the snow removal process.

If subsequent snow events are anticipated, removing snow from the roof will minimize the risk of accumulating snow causing structural damage. One benefit of immediate snow removal is that the effort required to remove the snow from the rooftop is reduced.

Methods of Snow Removal
Here are some recommended methods of snow removal that allow the qualified individual to remove snow safely and minimize risk of personal injury and property damage.
• Removing snow completely from a roof surface can result in serious damage to the roof covering and
possibly lead to leaks and additional damage. At least a couple of inches of snow should be left on the roof.
• Do not use mechanical snow removal equipment. The risk of damaging the roof membrane or other rooftop items outweighs the advantage of speed.
• Do not use sharp tools, such as picks, to remove snow. Use plastic rather than metal shovels.
• Remove drifted snow first at building elevation changes, parapets and around equipment.
• Once drifted snow has been removed, start remaining snow removal from the center portion of the roof.
• Remove snow in the direction of primary structural members. This will prevent unbalanced snow loading.
• Do not stockpile snow on the roof.
• Dispose of removed snow in designated areas on the ground.
• Keep snow away from building exits, fire escapes, drain downspouts, ventilation openings, and equipment.
• If possible, remove snow starting at the ridge and moving toward the eave for gable and sloped roofs.
• In some cases a long-handled non-metallic snow rake can be used from the ground, thereby reducing the risk. Metal snow rakes can damage roofing material and pose an electrocution risk and should be avoided.
• Upon completion of snow removal, the roofing material should be inspected for any signs of damage. Additionally, a quick inspection of the structural system may be prudent after particularly large snow events.
Warning! Snow removal is a dangerous activity that should only be done by qualified individuals following safety protocols to minimize risks. If at any time there is concern that snow loads may cause a collapse of the roof structure, cease all removal activity and evacuate the building.

Safety Measures for Snow Removal
Below are some safety measures to take during snow removal to minimize risk of personal injury.
• Any roof snow removal should be conducted following proper OSHA protocol for work on rooftops. Use roof fall arrest harnesses where applicable.
• Always have someone below the roof to keep foot traffic away from locations where falling snow or ice could cause injuries.
• Ensure someone confirms that the area below removal site is free of equipment that could be damaged by falling snow or ice.
• Whenever snow is being removed from a roof, be careful of dislodged icicles. An icicle falling from a short height can still cause damage or injury.
• When using a non-metallic snow rake, be aware that roof snow can slide at any moment. Keep a safe distance away from the eave to remain outside of the sliding range.
• Buried skylights pose a high risk to workers on a roof removing snow. Properly mark this hazard as well as other rooftop hazards.

In the event of a collapse, make all efforts to safely evacuate the building and call 911. You will also need to notify your insurance company. You will need to file an insurance claim and document all property losses and their values. Consider calling a public adjuster, like Professional Loss Adjusters, to assist in the preparation and negotiation of your claim. We perform a thorough review and analysis of your policy to determine what is required to maximize the recovery. We make a on-site analysis of the damage, and prepare a detailed estimate of the cost to reconstruct the property and replace the lost contents. In doing so, we secure the best possible settlement for you. Our service fee represents a small percentage of the insurance company’s settlement. This is easily offset by a better settlement than you would be able to obtain on your own.

It is devastating to suffer a loss to your home or business. We can help reclaim what is rightfully yours and help you begin rebuilding as quickly as possible. Hopefully, the information outlined about will help prevent such a loss. Questions? Contact one of the claims adjusters at Professional Loss Adjusters at 1-888-747-8260.

If you have any additional questions on this topic or other mitigation topics, contact the FEMA Building Science Helpline at or 866-927-2104.
You may also subscribe to the FEMA Building Science e-mail list serve, which is updated with publication releases and FEMA Building Science activities. Subscribe at

—Leonard Theran


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