Snow Loads: Guide to the Snow Load Provisions of ASCE 7-22

This introduction provides an overview of the key concepts discussed in the subsequent chapters of this book. The book provides practicing structural engineers with a detailed description of the snow loading provisions in Section 7.0 of the ASCE Standard 7-22, Minimum Design Loads for Buildings and Other Structures. It presents the research and philosophy that underpins the provisions and illustrates the application of the provisions through numerous examples so that the user not only knows how to use the provisions but also knows the reasoning behind them. The book is devoted to partial loading, which is covered in Section 7.5 of ASCE 7-22. It covers provisions in ASCE 7-22 for open framed equipment support structures and addresses snow loads for solar paneled roofs. The book offers views, thoughts, and ideas on snow loading issues that are not addressed in ASCE 7-22.

The roof snow loads specified in ASCE 7-22 are based on or related to the ground snow load. This approach is used because of the relative abundance of ground snow measurement information in comparison to roof snow load measurements. For most of the central Midwest, bounded by Indiana on the east and Nebraska on the west, the ground snow loads are primarily a function of latitude. The equal ground snow load lines generally run East to West. In the eastern United States, ground snow load generally increases with latitude, but two additional variables also influence ground snow load: site elevation and the distance from the shoreline. The new ground snow load maps and Hazard Tool have eliminated the need for the seven state tables in ASCE 7-16. To avoid requiring structural engineers to perform snow load calculations where snow does not control the design, ASCE 7-22 has added low ground snow load clarifications and exceptions.

Section 7.3 in ASCE 7-22 provides for calculation of the flat roof snow load and prescribes a minimum roof snow load. Areas with infrequent snowfalls and small accumulations tend to have higher ground-to-roof conversion factors than colder areas with substantial ground snow accumulation. The exposure factors are a function of the terrain or surface roughness category and the location of the structure with respect to various obstructions within the given surface roughness category. The intent of the ASCE 7 thermal factors is to quantify the differences in ground-to-roof conversion factors specifically due to heat loss through the roof layer. The roof snow loads prescribed in ASCE 7-22 are based on the ground snow load, modified by various exposure and thermal factors. However, there are other surfaces such as near ground decks and roofs of subterranean spaces for which the usual building related exposure and thermal factors do not apply.

Section 7.4 of ASCE 7-22 provides information needed to convert flat roof snow loads into sloped roof snow loads using the roof slope factor. As skiers know, it is difficult to “keep” snow on steep trails. For roofs, the reduction is caused by snow sliding off after initially being deposited or snow not adhering to the roof surface in the first place. At lower roof slopes where the sliding mechanism is slip at the snow/roof surface interface, R. Sack presents values for North and South facing slopes, while Chiba values are for East-facing slopes. Section 7.4.5 provides sloped roof snow loads for air supported structures with slippery exterior fabric surfaces. Given the various factors in ASCE 7-22 on which the roof snow load is based, one may ask how well observations compare with prescribed loads. The ASCE provisions provide an acceptably small probability of serious overload, whereas the others do not.

The partial loading is required for continuous structural members, such as roof purlins in metal building systems, where a reduction in snow loading on one span results in an increase in stress or deflection in an adjacent span. Although partial loading for a roof can be compared to the checkerboard loading pattern used for floor design, an actual checkerboard pattern is not prescribed for roof snow. Instead, continuous-beam systems are investigated for the effects of three loading cases. The general patterns presented in Cases 1, 2, and 3 could also arise because of roof snow removal operations. There are other structural systems, besides purlins and frames in metal building systems, where the removal of the roof snow from one portion will result in an increase in stress or deflection at another portion.

Unlike the partial loads, the unbalanced loads in Section 7.6 of ASCE 7-22 in most cases are drift loads. Except for saw-tooth type roofs, the windward portion of the roof is the snow source, and the leeward portion accumulates a percentage of the drifted snow. This is termed an “unbalanced” condition, because the leeward portion has more snow than the windward portion. This chapter discusses the unbalanced snow loading on hip and gable roofs, curved roofs, saw-tooth roofs, and dome roofs. For hip and gable roofs, the balanced loading condition results from a drifting process in which wind removes snow from the windward side of the ridge line. Unbalanced loads on sawtooth roofs are fundamentally different from unbalanced loads for gable, curved, or even dome roofs. Balanced loads for domes are the same as the corresponding balanced loads for curved roofs.

Roof snow drifts account for roughly 75% of all snow related roof damage. Drifts accumulate differently when the higher-level roof is on the windward side from when the higher level roof is on the leeward side. Windward drifts start out as quadrilateral shapes owing to a downward curling vortex, which forms when wind impinges on the vertical wall directly beyond the drift. For most roof steps, a leeward drift or a windward drift or some combination of the two is possible. However, in design the windward and leeward drift heights are calculated separately, and the larger value is used to establish the design drift loading. Although the adjoining roof step case is more common, occasionally the lower level roof is simply adjacent to the upper level. The adjacent roof drift presumably would start with a rise-to-run of roughly 1:4 like the adjoining roof drift.

Snow drifts frequently form at parapet walls and adjacent to rooftop units (RTUs). The drift on the upwind side of the RTU is a windward drift for the same reasons as for parapet walls. There are significant differences between leeward drifts at roof steps and windward drifts at a parapet wall, roof projection or roof step. Leeward drifts have a triangular shape, whereas windward drifts begin with a quadrilateral shape and given sufficient upwind snow source and wind, may morph into a triangular shape. A taller addition to an existing building is a common problem for structural engineers. The new roof geometry results in a roof step. One approach to minimize the leeward drift atop the existing roof is to construct a snow capture wall at the common column line.

From a structural standpoint, snow sliding off a roof is beneficial as long as the sliding snow does not collect in an undesirable location. The roof geometry and the site plan for the immediate adjacent areas should be such that the snow sliding off a roof does not pose a hazard to people, parked cars, or other adjacent objects. ASCE 7-22 does have design load provisions for snow that slides onto an adjacent roof, which are discussed in this chapter. Sliding loads can also occur on lower-level roofs that are separated from but close to a higher-level roof. Per ASCE 7-22 procedures, sliding loads are required for a separated roof that is close enough (within 15 ft horizontally) and low enough (ratio of vertical separation to horizontal separation greater than 1.0).

A rain-on-snow surcharge is occasionally one of the contributing factors in a roof collapse, although the author is unaware of case histories wherein rain-on-snow was the sole source of an overload that led to a collapse. The rain-on-snow surcharge associated with rain percolating through a roof snowpack is composed of two portions. The first, typically small portion is the load caused by vertical percolation through the upper unsaturated layer. The second, larger portion is the load caused by rain flowing downslope in the saturated layer immediately above the roof. As expected, the surcharge caused by flow in the saturated layer increases with increases in rainfall intensity, rainstorm duration, and eave-to-ridge distance, and it decreases as roof slope increases. All other things being equal, a low sloped roof with a large eave-to-ridge distance will have the greatest rain-on-snow surcharge.

Section 7.11 of ASCE 7-22 requires a ponding analysis for susceptible low-sloped roofs, and roofs where water can be impounded. The designer must use the full roof snow load in the ponding instability analysis. Section 7.12 requires the designer of a new facility that will be adjacent to an existing lower-roof facility to either analyze and possibly reinforce the existing roof for the now-expected drift or sliding loads (if both facilities are part of the designer’s scope of work) or make the owner of the existing facility aware of the “new” loads (if the existing facility owner is not the client or if the existing facility is not in the scope of work). The roof-step problem for these situations is particularly difficult because of the cost and the complexity of reinforcing an existing roof.

Section 7.13 provides design snow loads for so-called open framed equipment structures (OFES), of which petrochemical facilities are a prime example. The primary differences between OFES and more typical buildings are that various levels within OFES may or may not have flooring, OFES typically do not have exterior (perimeter) walls, and exposed cable trays and piping are common. If the piping or trays are at the same nominal elevation and the gap between them is small, falling snow could bridge the gap, increasing the snow load on both. This behavior is like a snow cornice one can observe on the leeward side of a mountain ridge and is likely because of the cohesive properties of snow.

This chapter presents O’Rourke and Isyumov’s recommendations for Snow Loads for Solar Paneled Roofs. The O’Rourke and Isyumov guide provide loads for the roof beams, roof girders and columns. They consider four types of solar panels: flush panels, tilted-closed panels, tilted-open panels, and elevated panels. The first five specific recommendations address the balanced roof snow load, which are a function of the ground snow load, the thermal factor, the exposure factor, and the slope factor. Like the sliding load recommendations, the recommendations for drift loads cover drift adjacent to the solar panels themselves owing to the panels behaving as roof top obstructions.

This chapter presents three complete snow load design examples. The first is a gabled roofed metal building with a roof step, located in a moderately heavy ground snow load area. It illustrates the determination of various types of drift loads. The second is a pole barn with a hip roof located in a low ground snow load area. It illustrates, among other things, the determination of unbalanced loads on hip roofs and checks for minimum roof snow loads and rain-on-snow loads. Finally, the third is a solar paneled roof in a moderately heavy ground snow load area. The design illustrates, among other things, the balanced load and drift load provisions.

This chapter presents answers to snow loading questions not specifically addressed by the ASCE 7-22 provisions. As such, it is not an official or unofficial interpretation of the ASCE 7-22 provisions. Rather, although the answers are generally consistent with the ASCE 7-22 provisions, they are specifically the author’s opinions.