Rooftop unit (RTU) curb installations are one of the most common — and most commonly under-engineered — mechanical projects in Florida commercial construction. When an HVAC unit gets replaced or a new RTU goes on an existing building, the curb adapter and its attachment to the roof structure need to be engineered for Florida's extreme wind loads. Our team provides PE-sealed engineering with site visits to ensure everything is done right.
What Is an RTU Curb and Why Does It Matter?
An RTU curb is the raised metal frame that sits on the roof deck and supports the rooftop HVAC unit. It serves multiple functions: it elevates the unit above the roof surface to prevent water infiltration, provides a mounting platform for the equipment, creates the duct transition between the unit and the building's supply/return system, and transfers all gravity and wind loads from the equipment into the roof structure.
When an RTU is replaced, the new unit often requires a different curb size or configuration than the original. Curb adapters bridge the gap between the existing roof opening and the new unit's footprint. This transition piece must be structurally adequate to carry the equipment loads — including wind loads — through to the building structure. A poorly designed or improperly attached curb can fail during a storm, allowing the unit to shift or separate from the roof, which leads to water infiltration, structural damage, and potential injury.
Our RTU Curb Engineering Process
RTU curb projects benefit from a hands-on approach. For most of our curb installation jobs, a Professional Engineer from our team conducts a site visit to evaluate the existing conditions before any calculations begin. This allows us to verify the existing roof framing and structural support, measure the actual curb opening and confirm compatibility with the new unit, identify any deterioration, damage, or previous modifications to the roof structure, assess the attachment method and recommend improvements if needed, and document the conditions with photos for the permit package.
Engineering Analysis
After the site visit, we perform the full engineering analysis. The wind load calculations follow ASCE 7-22, determining the velocity pressure at the roof height and the forces acting on the equipment. The curb and its connections must resist lateral wind forces from any direction (the equipment acts as a bluff body exposed to wind), net uplift forces where wind suction on the unit exceeds its dead weight, combined overturning moments from eccentric wind loading, and gravity loads from the equipment's operating weight including refrigerant and any accumulated snow or rain.
The connection design is particularly important. We specify the fastener type, size, spacing, and pattern to ensure the curb is securely attached to the roof structure. For metal deck roofs, this typically means through-fastening into structural steel below. For wood-framed roofs, we detail connections into the rafters or added blocking.
Understand Your Roof Pressures
The wind pressures on your roof directly affect RTU curb design. Use our Roof C&C Wind Pressure Calculator to see how pressures vary across different roof zones — equipment near edges or corners faces much higher demands than interior installations.
What Makes Florida RTU Curb Projects Different
Compared to other states, Florida's RTU curb installations face several unique challenges. The design wind speeds are among the highest in the nation, with many coastal locations requiring design for 170 mph or more. FBC 2023 has specific requirements for mechanical equipment anchorage that go beyond what many other state codes require. Building departments, particularly in South Florida, expect detailed engineering documentation with clear load paths from the equipment to the primary structure. The combination of high winds and heavy rain means the weather-tightness of the curb-to-roof interface is as important as the structural integrity.
Our Deliverable
A typical RTU curb engineering package from our office includes: a site visit report with photographic documentation, wind load calculations per ASCE 7-22 and FBC 2023, curb and equipment anchorage design with fastener schedules, detail drawings showing the curb-to-structure connection, existing framing adequacy verification or reinforcement details if needed, and a PE-sealed package ready for permit.
Working With Contractors
We work closely with HVAC contractors and general contractors throughout the process. Our site visits are coordinated around your schedule, and we're available by phone to answer field questions during installation. The goal is to deliver a buildable, permit-ready package that gets your project inspected and closed out without delays.
Need RTU Curb Engineering?
If you have an RTU replacement or new installation that needs PE-sealed engineering and a site visit, get in touch with our team. We serve projects across Florida and can typically schedule site visits within a few business days.