It’s that time of year again when we will begin to see ice and snow accumulations. The article below not only lists factors involved in roof load capacity but also gives solutions to help minimize the risk of roof collapse.
Wendy Light, CIC, CWCA
Facilities are susceptible to risks from excessive snow and rain on the roof.
Evaluate roof load capacity
Knowing how much snow a roof can handle can help avoid collapse
A building’s ability to stand up to weight of ice and snow depends on:
- Live and dead load design
- Age of the building and the roof
- Condition of the roof
- Maintenance during or after a major snow storm
Factors involved in roof load capacity:
1. Live and dead load design:
A. Review available as-built structural plans to evaluate the live and dead load capacity of the roofs. You may find load information on the notes page or on a roof framing plan.
B. Roof dead loads are considered to be the weights of materials and equipment that remain constant, such as the weight of the roof deck, cover, and roof mounted equipment. C. Live loads are those which may exert a variable pressure due to the surrounding environmental conditions such as rain, snow, and wind loads. D. An analysis completed by a structural engineer can determine the loads your roof can withstand.
2. Age of the building:
A. Newer buildings may be better able to withstand heavy snow since more recent building codes provide much better guidance for snow loads, particularly the increased loads near changes in roof elevations where snow drifts and snow falling from the upper roof can build up on the lower roof near the step.
B. Older roofs can suffer from corrosion which can reduce their ability to resist high snow loads. Inspect the underside of the roof deck for signs of deterioration. A history of trapped moisture and leaks will accelerate the aging process of the deck.
3. Roof type:
A. Buildings with low slope and flat lightweight roofs, such as metal buildings, steel decks on steel bar joists, and wood, generally provide less protection from overload than heavy roofs, such as structural concrete.
B. The safety margins used by engineers are based on a combination of the weight of the roof (dead loads) and the snow loads (live loads). Consequently, there is usually a larger margin of safety against excess snow loads for heavy roofs than for light weight roofs.
C. For low slope and flat roofs, the step-down area between roof sections is particularly susceptible to roof overload because of the tendency for ice and snow collection, especially during periods of windy weather.
4. Varying Elevations:
A. Roof top equipment and roof projections, such as mechanical equipment that is over 2 feet tall, can cause snow accumulation due to drift, creating the need for higher snow load consideration in these areas.
B. An even more serious condition can be created when a taller building or a taller building addition is built adjacent to an existing building. Unless the existing building is strengthened in the area next to the new taller building or addition, snow accumulation on the lower roof near the step could produce much higher loads than those considered by the original designer for the existing building.
A. Conduct periodic roof inspections during and after a snow fall, during which fluctuating temperatures may lead to thawing and refreezing of ice, rain and snow.
B. If safely possible determine the weight a 1 ft. x 1 ft. area of snow by using a bucket and scale. A fish scale works well to weigh the bucket with snow.
C. Compare the weight of the snow with roof snow load capacity.
D. Keep an eye out for any signs of roof or ceiling deflection that might suggest water is beginning to “pond” on your roof.
Maintenance before and after a snowstorm can minimize risks
- Maintenance: Conduct periodic roof inspections during and after a snow fall, during which fluctuating temperatures may lead to thawing and refreezing of ice, rain and snow.
- If safely possible determine the weight a 1 ft. x 1 ft. area of snow by using a bucket and scale. A fish scale works well to weigh the bucket with snow.
- Compare the weight of the snow with roof snow load capacity.
- Snow removal: To avoid roof collapse, snow removal should begin prior to reaching the snow load limit of the roof.
- For safe removal that won’t endanger you or damage your roof, consult a roofing contractor. Occupational Safety and Health Association (OSHA) Regulations and Standards should always be followed when working on a roof.
- Fall protection should always be implemented when working on a roof.
- A heavy duty push broom with stiff bristles or roof rake may be used to brush off the snow down the slope of the roof.
- Do not pull snow back against the slope or sideways since the snow may get underneath the cover and can break shingles.
- A shovel or snow blower should not be used since they may tear up the roof cover system.
- If you see indications that the roof is deflecting under the weight of the snow in certain areas, be sure to keep people away from those areas and seek the help of a professional snow removal expert.
- Ice dams: The formation of ice dams can increase loads on the roof. See the project: Preventing Ice Dams on Businesses.
- When the weather clears: Consider strengthening the roof before the next winter.
- An analysis completed by a structural engineer can determine the maximum loads your roof can withstand as well as provide practical solutions to improve the strength of your roof.
- The structural analysis should also include available retrofit reinforcement methods such as: increasing roof framing, wall bracing, columns, etc.