For anyone planning to erect a steel building, a thorough knowledge of snow and rain loads is vital, particularly in the northern states. The maximum anticipated snow density on a structure’s roof at any one time is called the Design Snow Load. Live load is defined as the result of the occupancy of a pre-engineered building, while snow load computations are based on a precise location atop the building. The total snow load amount is calculated based on measurements taken at several locations on the roof. The estimated ground snow load total, the flat roof snow load are combined with the exposure and thermal amounts. Additionally, the roof pitch is input into the calculations. All of these factors work together in rather complex formula to determine the Design Snow Load ““ but, believe it or not, there is even more to consider.   

Wind energy works with evaporation to displace some building roof snow, causing the ground snow load figure to increase versus the roof snow load amount. Snow sliding and snow drift are additional common phenomena which must be accounted for. The design snow load on a lower building roof must be increased ““ by as much as four times ““ when another higher roof exists close enough to deposit sliding snow. Walls and parapets can experience considerable amounts of snow depth, so a greater snow load should be incorporated by considering the total square footage of the roof, parapet and wall elevations.

It is critical that the building engineer considers potential uneven amounts of snow atop either hip or gabled steel building roofs. Specific formulations for the given precise loading required for this design must include the total area involved, pitch of the roof, and the pitched and flat roof snow loading total.

Partial loading is another critical consideration involved in complete analysis of snow loading. Typically, the application of partial loading is specified in the design of structural supports like purlins or frames working with a multi-span configuration as opposed to clear span construction. Varying amounts of design snow load may be required among different areas of a building. Achieving correct snow load balancing requires careful planning.

Rain-on-snow and rain loads must also be factored into the calculations for proper roof loading. In certain areas of the country snow can abruptly turn to rain ““ hence, the requirement for rain-on-snow load. If the steepness of a roof is minimal the added water can’t drain away readily and it is absorbed by the existing snow. The solution to this heavier load includes adjusting the roof pitch in a combination with roof bracing. “Rain load”� or total weight of rainfall becomes a factor in situations where roof rain drainage is ineffectual. The dependability of just about any pre-engineered structure can be improved by verifying ideal rain drainage from the steel roof. Potential rooftop failure from rain load may be circumvented by using exterior rather than inner conduits.
As you can see, determining the correct roof load design is a very complicated matter, but one that can be successfully navigated with the help of resources available through your qualified steel building manufacturer.