Commercial Roof Snow Load & Ice Dam Risks
When it comes to the weight of snow, the type of snow is as important as the depth of snow. Fresh “powder” type snow is typically lighter than wet packed snow. Ice is heavier than snow. During the winter months, a roof system can be exposed to all three combinations over a several month period.
General guidelines to help estimate the weight of snow:
- Fresh snow: 10-12 inches of new snow is equal to one inch of water, or about 5 lbs per square foot of roof space.
- Packed snow generally is heavier than new snow: 3-5 inches of old snow is equal to one inch of water, again about 5 lbs per square foot of roof space.
- Ice is also heavier than snow. One inch equals about a foot of fresh snow.
- The total amount of accumulated snow and ice is what matters in evaluating snow load risk. For example, the accumulated weight of two feet of old snow and two feet of new snow could be as high as 60 lbs per square foot of roof space, which may stress the limits of even the best designed roof.
If you are in the “danger zone” according to chart above or if the loads you estimate based on the thickness of the various types of snow and ice exceed 20-25 psf, you should consider having the snow removed from your roof.
Preventing Roof Collapse
Factors that could dictate how your particular facility will perform under the weight of ice and snow. These factors are listed below, which includes engineering considerations that could help you avoid roof collapses this winter.
- Live and dead load design;
- Age of the building and the roof;
- Condition of the roof;
- Maintenance during or after a major snow storm
Addressing Roof Strength
If it is determined that the roof system is not adequately designed to withstand the snow falls being encountered, a building owner should consider strengthening the roof as soon as possible or before the next winter. 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.
Safe snow removal may reduce some of the snow load on your roof. Consider contracting with a professional for snow removal. If your workers will be removing snow keep the guidelines below in mind. To avoid roof collapse, snow removal should begin prior to reaching the snow load limit of the roof.
Always follow Occupational Safety and Health Association (OSHA) Regulations and Standards, particularly fall protections for roof work. Avoid using shovels or snow blowers. Instead, use a heavy duty push broom with stiff bristles or roof rake to brush off the snow down the slope of the roof. For most single-story buildings with steep sloped roofs, a roof rake may be used for while remaining on the ground to pull snow down the roof slope. Do not pull snow back against the slope or sideways since the snow may get underneath the cover and can break shingles.
Ice Dam Risks
Ice dams are ridges of ice that form at the edge of a roof and prevent melting snow (water) from draining off your roof. The water that backs up behind this “dam” can leak into your business and cause damage to walls, ceilings, insulation, and other areas. Additionally, when the roof doesn’t drain properly, snow, ice, and water remains trapped on the roof, adding loads that put your roof at greater risk.
Preventing Ice Dams
IBHS offers the following guidance to help prevent damage from ice dams:
- Increase insulation above ceilings.
- Create a roof preventative maintenance, including periodic roof drainage inspections.
- Install self-regulating heating cables on gutters, downspouts, and around roof drains.
- Keep all drains, scuppers, gutters, and downspouts free of debris and vegetation.
- Prune trees that may hang over the roof to prevent an accumulation of tree leaves and branches that may clog or slow roof drainage.
- Improve ventilation. Consider installing electric power vents with thermostats.
Removing Ice Dams
We do not recommend chipping or breaking ice dams because this can damage the roof. The following guidance is for the most common types of commercial roof systems.
Steep Sloped Roof Systems:
- If the building has a history of ice dams, remove the snow to reduce the risk.
- If the building is too tall to reach with a roof rake from the ground, hire a roofing professional. For more information, please see Selecting a Roofing Professional.
- Remove or relocate heat sources that are installed in open areas directly under the roof.
- Increase ventilation in attic spaces:
- New gable roofs: Soffit/ridge vents provide good ventilation.
- Gable end vents: place an electric fan over vents to increase the flow of air.
- Hip roofs: Box or static vents are practical improvements.
- Insulate recessed light fixtures in the ceiling to reduce heat entering the attic. Look for visible light inside the attic. If present, insulate or seal.
- Insulate or seal all attic penetrations: partition walls, vents, plumbing stacks, electric and mechanical chases and access doors.
- New roof installation: Seal the roof deck using at least two layers of underlayment cemented together or a self-adhering polymer modified bitumen sheet. Extend the moisture barrier at least 24 inches from the edge of the eaves to beyond the inside of the exterior wall.
Flat, Monoslope and Low Sloped Roof Systems:
- Flat roofs are particularly vulnerable to water leaks if ice dams keep water from flowing into roof drains. Removing the snow will remove the source of a potential ice dam.
- If ice dams form around drains, place heating cables on the roof and connect the cables to the drains to create a path for the melting ice to follow.
- Consider installing heating cables in a zigzag manner inside gutters.
- If there is extensive ice build-up around the drains, consult a roofing professional.
- When the roof is dry, inspect the roof cover. Look for mold, mildew and vegetation, all of which are signs of a problem with the slope of the roof cover system and drainage. A roofing professional can advise you about re-pitching the roof cover.
Contact us at 973-383-2024 if you have a service need or click here to visit our website to learn more about SERVPRO of Northern Sussex County's System Services.