The weight of one foot of fresh snow ranges from 3 pounds per square foot (psf) for light, dry snow to 21 psf for wet, heavy snow. When evaluating an existing roof for snow loads, an engineer will want to know the year it was built, the materials involved and the load the roof was designed to support, to start. But there are many other variables that need to be considered when evaluating existing roof loads under snow conditions. Outlined here are some of the many factors that impact the snow load carrying capacity a structure:
- Materials & Design:Engineers use the building code formulas to determine the appropriate snow load for their new design. A detailed study, prepared in 2002, set the ground snow loads throughout New Hampshire and is the basis for all new construction projects. Structural engineers use various design standards for steel, wood and concrete that include factors of safety and account for serviceability issues such as deflection. Older structures, governed by earlier building codes, may not meet current standards.
- Detailing and Construction:The type and condition of the bracing and roofing materials can contribute to (or undermine) its strength. Some older steel framed buildings used a cantilever beam layout to minimize the beam sizes by using the load of one beam to reduce the stress in the adjacent beam. Because snow buildup occurs in an unbalanced manner, roof failures have been attributed to this type of construction. Minor renovations to an existing structure can reduce the carrying capacity when, for example, bracing is removed to add new ducts. A recent study, published in STRUCTURE magazine, found that the bulk of New England roof failures were related to construction or detailing deficiencies and were not a result of excess snow loads.
- Pitch & Thermal Conditions:The slope and type of roof surface determines how much snow is retained on the roof. Flat and low-pitch roofs are more commonly prone to overloading because they hold onto snow more easily than steeper ones. Flat roofs without adequate drainage are at increased risk of failure due to ponding that occurs as a result of excessive deflection. Adding insulation to the structure reduces the heat loss causing less melting, and results in larger loads. Similarly, the absence of any heat will increase the snow retained even more.
- Roof Layout Geometry:The location of hips, valleys, high roofs and low roofs, and raised elements – like skylights and dormers – create snowdrifts and therefore factor into determining additional loading. Similarly, the addition of snow guards on a sloped roof change the dynamics of the snow behavior on the roof. Parapets and large roof top equipment can cause snow drifts on flat roofs. The addition of a new structure adjacent to existing structures often create drift conditions that are not accounted for adequately during renovation projects.
- Depth and Snow Density:Determining the weight of snow based on depth is not possible unless you know the density of the snow on a particular roof. The density is the weight of the water in a set volume of snow. Snow on a roof will compact over time as temperatures fluctuate and as new snow layers are added to the roof. Rain-on-snow increases the density of the snow thus increasing the weight. Measuring this density is not terribly scientific and is not as important as how the roof is actually performing.
- Sun and Wind Exposure:Natural elements such as sun and wind impact how much snowfall is retained on a roof. From a code perspective, 70% of a single snowfall event is expected to remain on a roof under normal wind conditions. Wind and sun can create unbalanced snow load conditions on a gable roof when more snow is retained on one side of the ridge than the other.
- Maintenance:Proper, or improper, maintenance plays a role in how well a building will perform under load. Some older roofs suffer from steel beam and connector corrosion, or rotting wood, which reduces the building’s ability to withstand heavy snow loads. Proper maintenance, including repairs to any damage or leaks, is important to ensure the structural integrity of the entire structure.