How Are Roofs Designed for Nor'easter Snow Drifts?

Every roof obstruction creates an aerodynamic shadow where wind-blown snow drops out of the airstream. Parapets are the classic case: snow scours off the windward portion of the roof and piles against the leeward parapet in a drift whose height is a function of the upwind snow source distance and the parapet height. Roof steps are worse - the high roof acts as a giant snow source feeding a drift on the low roof below, and the drift surcharge there can exceed the uniform design snow load several times over. Rooftop units, penthouses, and adjacent taller buildings each create their own drift zones, and the engineer maps them all. Unbalanced loading adds a second demand on sloped roofs: the code requires checking one slope loaded while the other is stripped, because that is exactly what noreaster winds do. Hip and gable roofs, curved roofs, and roofs with valleys each have their own unbalanced cases. Sliding snow from an upper roof onto a lower one is checked where the geometry allows it. The point of all these cases is that the worst snow load on any part of the roof is rarely the uniform blanket - it is the drift, the unbalanced pile, or the slide, and the structure must handle each where it occurs.

New roof design integrates snow from the first framing layout. The engineer selects the ground snow load for the site from the code's maps, applies exposure, thermal, and importance factors, then computes drift surcharges at every obstruction and adds the unbalanced cases. Members are sized for strength and checked for deflection under the full drift load - a roof that sags under drift invites ponding as meltwater collects in the depression, a failure mode that has collapsed more than one New England roof. Connections, bearing, and the load path down through columns and walls to the foundation are all part of the same calculation. Existing roofs get a forensic version of the same analysis. The engineer documents the as-built framing - member sizes, spans, connections - and runs it against current snow demands including drifts the original designer likely never calculated. Common retrofit findings: joists adequate for uniform snow but not for the drift at the new rooftop unit, parapets added during a reroof that created a drift zone the framing cannot carry, and flat roofs with inadequate slope for drainage. Retrofits range from sistering and added beams to reconfigured rooftop layouts that eliminate the drift source. The evaluation report prioritizes by risk so the owner fixes the drift zones first.