How Are Detroit Flat Roofs Designed for Snow Loads?

The path from ground snow to structural demand runs through ASCE 7. The mapped ground snow load for the region is modified by exposure, which accounts for wind scouring on open sites, by the thermal factor, which accounts for heat loss melting snow from below, and by the importance factor for the building's risk category. The result is the balanced roof snow load, the uniform case. The unbalanced case then redistributes that snow: windward areas scour nearly clean while leeward zones and valleys accumulate deep drifts. Drift is where designs fail when it is skipped. ASCE 7 prescribes drift surcharge geometry at parapets, roof projections, and equipment based on the upwind snow source distance, and these triangular surcharges routinely control the design of the bays beside a tall parapet or penthouse. Rooftop units deserve the same attention: a new unit installed on an existing roof creates a new drift condition the original design never considered, which is why equipment additions on snow-country roofs need structural review, not just a curb detail.

Ponding is the instability that turns a survivable snow into a collapse. As snow loads deflect the roof downward, the deflection creates a low spot where meltwater and rain collect, and that water adds weight that increases deflection further. The engineer checks ponding stability explicitly: the roof must be stiff enough that this feedback loop converges rather than running away. Minimum slopes, tapered insulation systems that guarantee drainage even as the structure deflects, and camber in long-span members all serve this check. Overflow drainage is the last line of defense. Primary drains handle the design rainfall, but overflow scuppers or drains set a few inches higher ensure that a clogged primary drain cannot impound water to the full parapet height. The overflow elevation is coordinated with the structural design so the impounded depth at the overflow level is within the roof's capacity. On existing roofs with parapets added later or raised during reroofing, this coordination is frequently missing and gets corrected during the structural evaluation.

Every reroofing project in snow country is a structural question, because tear-offs are often replaced with heavier assemblies: multiple insulation layers, tapered systems, and new membranes add dead load the original joists or deck were not designed for. The engineer surveys the existing deck for deterioration, verifies member sizes and spans, and checks the structure under current code snow loads plus the new dead load. Where capacity is short, options range from sistering joists to adding new support lines. The evaluation also catches drift conditions created after original construction. Parapets raised for fall protection, new penthouses, and adjacent additions that changed the wind exposure all alter the snow loading, and the reroofing review is the natural moment to address them. Snow removal plans with trigger depths give owners of marginal roofs an operational tool, but they are a supplement to adequate structure, not a substitute for it.