Why Rooftop Equipment and Structural Engineering Go Together
Rooftop mechanical units are heavy — commercial RTUs commonly weigh 800 to 4,000 pounds. When that weight is concentrated on a roof curb, it imposes point loads on the roof framing that may or may not have been anticipated in the original structural design. Adding, replacing, or relocating RTUs without a structural review means assuming the roof can handle loads it may never have been designed for.
When Structural Review Is Required
New Equipment Larger or Heavier Than What It Replaces
If you're replacing an RTU with a newer unit of similar size and weight at the same curb location, the structural impact is likely unchanged. But if the new unit is heavier, larger, or being placed at a new curb location, the structural engineer needs to evaluate whether the existing framing can support it.
Permit Requirements
Many jurisdictions require PE-stamped structural calculations and details for rooftop mechanical permits, regardless of whether the equipment is new or a replacement. The requirement is often triggered by the mechanical permit itself. Check with the permitting authority or ask your mechanical contractor — structural documentation is commonly part of the mechanical permit submittal package.
New Curb Locations
Relocating an RTU to a new position on the roof almost always requires structural engineering. The structural framing below the new location may be entirely different from the original curb location — and needs to be evaluated for the new loads.
Older Buildings With Unknown Structural Capacity
Older commercial buildings — pre-1990 construction especially — were often designed to code standards with lower equipment load allowances than modern units require. A structural review verifies that the existing framing is adequate or identifies the reinforcement needed.
What the Structural Review Includes
The structural engineer reviews the existing roof framing (beams, joists, connections) in the vicinity of the RTU location, calculates the imposed loads from the new unit, checks the capacity of the existing members and their connections, and either clears the installation or designs reinforcement. The deliverable is typically a one- to three-page structural calculation package and a detail drawing showing the curb framing and any required reinforcement.
Coordinating With the Mechanical Engineer
The mechanical engineer selects the equipment and determines the curb location and size. The structural engineer uses that information to evaluate the roof framing. Both sets of documents should be coordinated before the permit submittal — a mismatch between the mechanical and structural drawings creates comment responses that delay the permit.