Rooftop Equipment Wind Uplift and Anchorage

Direct uplift is the plan area times the net uplift pressure for the roof zone, and the unit's own weight resists it -- often entirely. THE DIRECTION IS REPORTED IN WORDS RATHER THAN AS A SIGNED NUMBER, because a negative net uplift means the WEIGHT wins and it is easy to read the sign the other way. THE OVERTURNING COUPLE IS THE PART THAT GETS OMITTED ENTIRELY, and it is frequently the larger demand. Lateral wind on the unit's side profile acts at its centroid, well above the curb, and produces a moment that resolves across the unit's width as a couple, adding tension to the windward fasteners over and above the direct share. A tall narrow unit is harder to anchor than a low wide one of the same weight, because the lever arm grows and the resolving width shrinks at the same time -- and a unit whose weight fully resists direct uplift can still need real anchorage for the couple alone. THE LOAD PATH HAS TWO PARTS AND THE SECOND IS INVISIBLE. Fastening the unit to the curb is one connection; fastening the curb to the roof structure is another, and once the roofing is done nobody can see it. A unit adequately screwed to a curb that is only nailed to a wood deck has a load path that fails at the deck, and roof inspections after wind events find exactly that. This checks the first connection only. AND THE ROOF ZONE MATTERS MORE THAN THE PRESSURE LOOKS: uplift near an edge or a corner is much higher than in the field of the roof, and rooftop equipment carries its own provisions distinct from the roof covering's. This resolves entered pressures into fastener demands. It does not determine the design wind pressures, the roof zone, or the applicable provisions, check the curb-to-deck attachment or the deck itself, evaluate the curb, the rails, or the unit's own attachment points, address seismic demand, which may govern in high seismic areas, account for fastener group effects, edge distance or substrate capacity, or determine what any standard requires. ASCE 7 as adopted, the equipment and curb manufacturers, and the engineer of record govern.

uplift = plan area x the zone's net uplift pressure, resisted by the unit weight; lateral wind on the side profile acts at mid-height, and the moment resolves across the unit width as a couple adding tension to the windward fasteners.

The design pressures are ENTERED for the roof zone and height. Rooftop equipment carries its own provisions, distinct from the roof covering's, and edge and corner zones are much worse than the field.

Areas, one moment, and one division.

Estimate. AHJ and licensed professional govern.

Field names used by the API: unit_length_ft, unit_width_ft, unit_height_ft, unit_weight_lb, uplift_psf, lateral_psf, fastener_count, windward_fastener_count, fastener_capacity_lb, plan_area_ft2, uplift_lb, net_uplift_lb, weight_reserve_lb, lateral_lb, overturning_ftlb, couple_tension_lb, per_windward_lb, demand_ratio

• Design pressures entered for the zone and height the unit actually sits atASCE 7 as adopted
• Lateral force at mid-height the unit's centroid is assumed at mid-height of the side profilethe engineer of record
• Unit-to-curb only the curb-to-deck connection is a separate and invisible checkthe curb manufacturer and the roofer