Why do roofs collapse under snow when the snowfall was not a record?
Roofs collapse under snow without a record snowfall because the snow does not distribute evenly across the roof: in the great majority of snow-related roof collapses, the ground snow load for the event was within, sometimes well within, what the structure was designed for, and the roof failed anyway.
Wind moves snow from higher roof surfaces to lower ones and deposits it against obstructions, producing local accumulations several times the balanced depth, so what concentrates the snow load is the shape of the roof. Design provisions address snow drift explicitly, which means a drift-related roof collapse usually raises a question about the design or a later alteration rather than about the weather. Snow drift design provisions are also frequently misapplied.
What is a balanced roof snow load, and why is it not enough on its own?
A balanced roof snow load is the load for a uniform accumulation of snow across a simple roof, and it is only the starting point of roof snow load design. The design process begins with a ground snow load for the location, adjusted for exposure, thermal condition and importance to give the balanced roof load.
Load provisions then require several additional snow load cases to be checked: drift at steps and projections, unbalanced load on sloped and gabled roofs, sliding snow from a higher roof onto a lower one, and rain on snow. A roof design that checked only the balanced snow load case has skipped the conditions that most often govern.
Where does snow drift form on a roof, and why does it concentrate the load?
Snow drift forms wherever an obstruction interrupts the flow of wind carrying snow across a roof, and at that point the snow deposits. Steps between roof levels, parapets, mechanical units, penthouses, screen walls and adjacent taller buildings all create these conditions.
A snow drift forms as a triangular accumulation against the obstruction, and its peak intensity can be several times the balanced roof snow load. The upwind fetch — the length of roof feeding snow to the obstruction — largely determines how much snow accumulates in the drift, which is why a long low roof next to a roof step is a characteristic snow drift problem.
Can a new building or rooftop addition create snow drift on an existing roof?
Yes: a building constructed later next to an existing one can impose snow drift loading on the existing roof that was never part of the existing roof’s design, even on a roof that has no step of its own. So can a new rooftop unit, a screen wall or a solar array.
The original structure may be entirely adequate and become inadequate through a change outside it. Establishing when neighboring construction or rooftop additions occurred, relative to the existing roof’s design date, is therefore a standard line of inquiry in snow drift roof collapse matters.
How is the snow load actually present on a collapsed roof reconstructed?
The snow load actually present on a collapsed roof is reconstructed from water equivalent data where available, from photographs of the snow drift geometry, and from post-collapse measurement of snow depth and density on adjacent unfailed roofs. Weather station records alone are not enough: they give snow depth and precipitation, but depth alone does not give load, because snow density varies enormously between fresh, settled and rain-soaked snow.
Snow water equivalent data, where available, is far more useful than depth for reconstructing roof snow load. Photographs taken before and during a roof collapse frequently show the snow drift geometry directly, and post-collapse measurement of the remaining snow depth and density on adjacent unfailed roofs gives a direct measurement of what was sitting there.
Why is rain on snow so dangerous for a roof?
Rain on snow is the aggravating case for a roof because an existing snowpack absorbs rainfall and holds it, adding load without adding depth. A roof carrying a modest snow load can gain substantially in a few hours of rain, and roof drainage is generally blocked by the snow itself, so the water does not run off.
Where a roof collapse followed a rain event on an existing snowpack, the load reconstruction has to account for the absorbed water, and depth-based estimates of the snow load will understate the load considerably.
What does a roof’s collapse pattern reveal about snow drift loading?
A roof’s collapse pattern usually points to where the snow drift was, because drift loading is spatially concentrated: failures initiate where the drift was deepest — typically at the low side of a roof step or against a parapet — and propagate outward.
A roof collapse that began adjacent to an obstruction, in a structure where the remainder of the roof stood, is consistent with snow drift loading rather than with general overload. Debris mapping and the extent of the standing portion of the roof together establish this.
Can snow removal contribute to a roof collapse?
Yes: clearing snow from a roof can create the very condition that causes failure if it is done unevenly, leaving a large accumulated snow load on one portion of a structure whose adjacent bays have been relieved.
Snow removal equipment adds concentrated load to a roof, and workers cutting through the snow to the roof membrane can damage the membrane. Where snow removal was in progress or recently completed before a roof collapse, the removal sequence and the resulting snow distribution become part of the analysis.
Does showing that drift load exceeded the design balanced load settle why a roof collapsed?
No: establishing that snow drift load exceeded the design balanced load does not conclude a roof collapse analysis, because the structure’s actual capacity may differ from its design capacity.
A roof structure’s actual capacity can differ from its design capacity through deterioration, corrosion, prior damage, modifications, or members that were never built as specified. Both the load side and the capacity side of the comparison need independent work, and a conclusion about a roof collapse drawn from the load side alone is incomplete.
What evidence should be preserved after a snow-related roof collapse?
After a snow-related roof collapse, snow depth and density should be measured on the collapsed roof and on adjacent standing roofs before the snow melts, which is a matter of hours to days. Photographs of the snow drift geometry should be collected from every available source, including security cameras and bystanders.
The records to preserve after a snow-related roof collapse are the structural drawings and any records of rooftop additions, adjacent construction or alterations. The framing members themselves should also be preserved, because they establish whether the roof’s capacity matched the design.
This guidance on snow-related roof collapses is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular roof collapse requires hands-on examination by a credentialed expert.