When a commercial building needs both a new roof and new rooftop HVAC units, one of the most important questions is: Which should be installed first—the roof or the rooftop units?
In most commercial roof replacement projects, the best approach is to coordinate the HVAC replacement with the roofing work rather than completing either project independently. In many cases, the old rooftop units should be removed before the new roof is installed, allowing the roofing contractor to properly repair or replace the roof around the equipment curbs. The new rooftop units can then be installed after the roofing and flashing work is complete.
However, the exact sequence depends on the building, equipment, roof system, curb configuration, and project requirements.
Why the Sequence Matters
Rooftop HVAC units sit directly on or above the roofing system. Their curbs, electrical connections, ductwork, condensate lines, and other penetrations all require careful waterproofing.
If a new roof is installed first and new HVAC equipment is placed afterward without proper coordination, the installation may require cutting, patching, or modifying the new membrane. That can create unnecessary leak risks and potentially complicate the roof warranty.
Florida projects also require careful attention to rooftop equipment installation. Commercial rooftop mechanical equipment must be properly supported and designed for applicable wind loads, while roof-mounted units generally require appropriately raised curbs or supports.
A Common Best-Practice Sequence
For a commercial building receiving both a roof replacement and new rooftop units, a coordinated sequence often looks like this:
1. Inspect and plan the project.
The roofing and HVAC contractors should inspect the existing roof, curbs, structural conditions, equipment dimensions, duct connections, electrical requirements, and access points.
2. Select and confirm the new rooftop units.
The HVAC equipment should be selected before finalizing roofing details. The contractor needs to know the unit dimensions, weight, curb requirements, service clearances, and connection locations.
3. Remove the existing units.
The existing RTUs can be disconnected and removed, typically using appropriate rigging or crane equipment. Once the equipment is removed, the exposed curb and surrounding roof can be properly evaluated.
4. Repair or replace the curbs and prepare the roof.
If the new equipment has different dimensions, a curb adapter or replacement curb may be necessary. Equipment compatibility and structural capacity should be verified before installation.
5. Install the new roofing system and complete flashing.
With the equipment out of the way, roofing contractors can properly install the membrane, insulation, flashing, and waterproofing around the equipment locations.
6. Set and secure the new rooftop units.
After the roof-side work is ready, the new units can be lifted into position, properly anchored, sealed, and connected. Manufacturer guidance emphasizes that the curb must be properly positioned and sealed to create a watertight equipment connection.
What If the New Roof Is Installed Before the HVAC Units?
Sometimes the new roof must be installed before the new equipment is available. This can work, but it requires close coordination between the roofing and HVAC contractors.
The roofing contractor may need to install temporary or permanent flashing details around the curb while leaving specific areas accessible for the equipment installation. Any subsequent roof penetrations, curb modifications, or membrane repairs must then be completed according to the roofing manufacturer’s requirements.
This is particularly important for commercial buildings where maintaining a manufacturer’s warranty is a priority.
The Bottom Line
So, which comes first: the new roof or the new rooftop units?
For many commercial projects, the preferred sequence is remove the old rooftop units, prepare or modify the curbs, complete the roof replacement and waterproofing, and then install the new rooftop units. But there is no universal sequence for every building.
The most important factor is coordination. Roofing, HVAC, structural, and mechanical requirements should be reviewed together before work begins.
For Florida commercial property owners, this coordination is especially important because rooftop equipment must account for applicable building-code and wind-resistance requirements.
If your building needs both a commercial roof replacement and rooftop HVAC replacement, Shieldline Roofing can help coordinate the roofing scope so equipment curbs, penetrations, flashing, and waterproofing are addressed as part of the overall project—not as an afterthought.
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