Managing Snow Loads on Roofs and Decks

Snow can weigh a lot, as much as 21 pounds per square foot (FEMA P-957 2013). In areas that see snowfall in winter months, accumulation of snow on roofs and decks can cause deflection of structural members and can even cause collapse of under-designed structures. For areas that experience snow in winter months, structural designs for roofs and decks need to accommodate additional loads placed on these structures by snow accumulation (Figure 1). Building codes such as the International Residential Code (IRC) provide basic guidance for building designers.

Roof and Deck Snow Loads

Anticipated roof and deck snow load, or the weight of snow sitting at a particular place on a roof or deck, is highly variable and depends on a number of factors including:

• local ground snow load
• building topology (shape)
• snow type
• cumulative snow amounts due to successive weather events
• the presence of ice
• snow sliding onto a roof or deck from a roof above
• temperature of the space below the roof (insulation levels).

Ground Snow Load

This is the typical weight of accumulated snow on open ground at a particular geographic location. It does not necessarily represent the weight of snow on a roof or deck, because of the multiple factors at play listed above. In the United States, maps of ground snow load can be found in IRC 2021, R301.2. See Figure 2 and also see the Climate tab of this guide.

Specific design guidance like span tables and bracing requirements can be found for roofs in IRC R802, and for decks in R507. These tables cover common lumber species, construction practices, and spans. In cases where ground snow load is more than 70 pounds per square foot per R301.2, or where the planned construction system is not covered explicitly in the code, an engineer’s stamped structural drawing will be required. Engineered components, including trusses and manufactured buildings, should be designed to meet the local ground snow load requirements and should include documentation to verify compliance.

Generally, code requirements are intended to accommodate most foreseeable snow conditions and roof or deck characteristics. The following issues are worth considering however, especially at points during the snow season when excessive snow loading may become a concern.

Building Topology and Shape

Ground snow load indicates the weight of snow per square foot that can be expected in an open, unobstructed flat ground situation. Roofs and decks, with the exception of particularly large, flat examples, do not accumulate snow in such a uniform fashion. For instance, on a simple gable roof, snow depth will typically be less on the windward side (side where the wind is coming from) than on the leeward side. Obstructions and geometry transitions can provide areas that are “aerodynamically shaded,” and therefore accumulate more snow. This can cause uneven loading of the structure and can cause local areas of snow load that are significantly higher than the ground snow load. Figures 3 and 4 shows some examples of these conditions for roofs. Decks can experience the same uneven snow coverage due to adjacent buildings, guard rails, fencing, shrubs, etc., that prevent or encourage snow accumulation.

Sliding Snow

Generally, roofs under 30 degrees in slope (5:12 or 6:12) will not experience significant sliding. The angle that a roof needs to be sloped to prevent accumulation will vary according to such factors as snow consistency (dry versus wet), and roof surface texture (slick vs grippy), and the presence of “snow guards,” intended to prevent snow from sliding.

Sliding snow can present a danger to people, landscaping, and equipment on the ground. The dynamic force of sliding snow that falls from an upper roof onto a lower roof can produce an excessive dynamic load on the lower roof that could potentially overload the roof structure (FEMA P-957 2013). The potential for sliding snow should be considered when designing entryways for homes. Consideration should be given to whether or not commercially available snow guard or snow cleat products should be installed in certain locations on the roof to reduce the likelihood of snow slides; this will increase snow load in those areas.

Roof Insulation Quality

In a well-insulated building with good air sealing of the ceiling plane and good ventilation below the roof deck, roof surface temperatures will be near outside ambient air temperatures. This means that snow will not melt until the weather warms up. However, if attic insulation or air sealing is lacking, allowing heat from the house to transfer into the attic, and if attic ventilation is inadequate, allowing that heat to linger in the attic and warm the underside of the roof deck, the roof temperatures can rise above freezing. This can cause accumulated snow on the roof to begin to melt, even when the outside temperature is below freezing.

This snow melt can simply result in the loss of accumulated snow, reducing loads on roof structures. But often, this melting is the root cause of ice dam formation (BSC 2006). Ice dams form when the meltwater runs down the roof until it reaches the colder eave where it refreezes. Eventually this ice buildup forms a dam at the edge of the roof, which prevents drainage of additional melted snow. Liquid water builds up above the dam. Some of it spills over and drips off the edge of the roof forming icicles. Some of it backs up under the shingles and eventually finds holes in the roof waterproofing, to leak through into the attic where it can cause extensive damage. The weight of the water buildup can also cause roof collapse. The icicles (a frequent sign of ice dam formation) are themselves a hazard when they fall. Figure 5 shows the relationship between roof ventilation and ice dam formation. For more information about ice dam prevention, see the Solution Center guide “Roof Eaves Sealed with Self-Adhering Membrane in Cold Climates.”

For ice dam prevention, as well as energy conservation, plan to build a well-insulated attic with good airtightness between conditioned and unconditioned areas (BSC 2006). Positive ventilation under the roof deck is not strictly necessary in most regions that do not receive high levels of snowfall but does provide significant insurance against ice dam formation. Positive ventilation under roof decks should be considered wherever the ground snow load exceeds 50 lbs/sq ft for ice dam control. For more information about vented vs unvented roof assemblies, see the Solution Center guide Vented vs Unvented Attics.

Snow Types and Conditions

Snow characteristics can vary significantly. The specific type of snow, the manner in which it accumulates, and weather that occurs between snowfalls, all contribute to the ultimate snow load condition.

The weight of snow varies greatly – one foot of light dry snow weighs 3 pounds/ft2 while one foot of heavy wet snows weighs 21 pounds/ft2 (FEMA P-957 2013). Ice weighs significantly more than snow at 57 pounds/ft2 for a foot of ice, or about 5 pounds/ft2 for 1 inch deep of ice. Snow in the east tends to be heavier than the drier snow in the west except in some locations of wet snow in the Northwest and coastal Alaska (FEMA P-957 2013).

Snow can accumulate through multiple snow events, unless it either slides off or melts in the time between snows. Snow can also partially melt and then refreeze. This will reduce the depth of the snow while making it more dense due to water occupying formerly open space in the accumulated snow. It is also less likely to blow off the roof or deck once refrozen. Additional snow that falls on top will simply add to the cumulative weight of the snow supported by the structure. In temperature conditions that occur near the freezing point, “mixed” precipitation often falls consisting of both wet snow and rain. The mixed wet snow will be heavier than dry snow falling during colder temperatures. Similarly, rain fall on snow will tend to fill small voids in the snowpack, saturating it and making it quite heavy. Both situations will increase the load. It is important to consider these mechanisms that increase snow loads when designing roof and deck structures and when monitoring structures for signs of distress over the course of the winter.

In very general terms, unless the roof structure is damaged or decayed, most residential roofs regardless of the location of the house should be able to support 20 pounds per square foot of snow before they become stressed. Roofs in areas known for higher snowfall may be designed to handle higher snow loads. (IBHS 2021). (If you are designing a roof, follow the load requirements provided in code or by your local building department.)

To get a rough estimate of how much snowfall is likely to weigh, see these tips for homeowners from the Insurance Institute for Building and Home Safety (IBHS 2021):

Fresh snow: 10 to 12 inches of new snow is equal to about 1 inch of water, or about 5 pounds per square foot of roof space, so the roof could likely handle 4 feet of new snow before the roof it will become stressed.

Packed snow: 3 to 5 inches of old snow is equal to about 1 inch of water, or about 5 pounds per square foot of roof space, so anything more than 2 feet of old snow may be too much for a roof to handle.

Total accumulated weight: 2 feet of old snow and 2 feet of new snow could weigh as much as 60 pounds per square foot of roof space, which is beyond the typical snow load capacity of most roofs.

Ice: one inch of ice equals one foot of fresh snow (IBHS 2021).

How to Design for Snow Loads

Roofs

• Identify ground snow load at the proposed building site using IRC 2021 Section R301.2.
• For covered structural systems and ground snow loads up to 70 psf, use span tables in the IRC Section R802 or approved span tables from engineered lumber products.
• For systems not covered in IRC Section R802 or where ground snow loads are above 70 psf, a structural engineer will need to verify compliance with ASCE 7-16 - Minimum Design Loads and Associated Criteria for Buildings and Other Structures. Note that many engineered systems like roof truss packages will be engineered to these standards and should include documentation for review by local code authorities.

Decks

• Identify ground snow load at the proposed building site using IRC 2021 Section R301.2.
• For covered structural systems and ground snow loads up to 70 psf, use sizing and span tables in IRC Section R507 or approved span tables from engineered lumber products or composite decking products.
• For systems not covered in IRC Section 507 or where ground snow loads are above 70 psf, a structural engineer will be needed to verify compliance with ASCE 7-16 - Minimum Design Loads and Associated Criteria for Buildings and Other Structures.

Existing Buildings

See the Retrofit tab.