How Is Structural Support Designed for Rooftop HVAC Units?

Rooftop unit structural support design verifies that roof framing can carry the unit's dead load plus maintenance live load, that curbs or dunnage bear on structural members, and that anchorage resists wind uplift and seismic forces per the building code. The work starts with the equipment cut sheet: operating weight, footprint, and the location of the unit's support points. Then I map those against the roof framing plan. If the unit lands between joists, the design adds headers, dunnage steel, or a reinforced curb to get the load into the members. The anchorage design follows from the site's wind speed and seismic design category — in high-wind and high-seismic regions, the hold-downs and curb attachment are doing serious work.

A complete RTU support package has four parts. First, gravity analysis: the unit's operating weight distributed to its support points, checked against the capacity of the members below, including the code-required maintenance live load around the unit. Second, the curb or dunnage design itself — member sizes, connections, and bearing details. Third, lateral and uplift design: wind and seismic forces on the unit, the anchorage into the structure, and the overturning check. Fourth, coordination with the roofing: penetrations, flashing, and making sure the structural work doesn't void the roof warranty. On existing buildings, I also check the as-built framing against the original drawings, because roofs get re-decked, members get notched for old work, and what the drawings show isn't always what's up there.

The trouble jobs are almost always replacements. The old unit weighed 800 pounds; the new high-efficiency unit weighs 1,400. The curb is the wrong size. The electrical and gas were run for the old footprint. And the building owner wants it done before cooling season. My advice: get the structural review done before you buy the unit. Moving a curb location six inches on paper costs nothing; reframing a roof around an already-purchased unit costs real money.

• Weight increase: new units are often heavier than the 20-year-old units they replace
• Curb mismatch: different footprint means new curb, new flashing, new structural bearing
• Clustered units: multiple RTUs in one bay can overload members that handle one unit fine
• Screen walls: architectural screens add wind sail area the original design never saw
• Vibration: spring isolators change the load path and need their own support design