On a new commercial construction project, roofing and rooftop mechanical and electrical work must be carefully sequenced to prevent leaks, roof damage, construction delays, and warranty issues. The basic rule is simple: roof penetrations, curbs, supports, and other items that must pass through or attach to the roof should be coordinated and installed at the appropriate stage before the final roofing system is completed.
However, rooftop equipment itself is not always installed before roofing. The exact sequence depends on the building design, roof system, equipment, structural requirements, and project schedule.
What Comes First: Roofing or Rooftop Equipment?
Typically, the roof deck and substrate are completed first, followed by installation of roof-related curbs, structural supports, drains, sleeves, and other permanent penetrations. The roofing contractor can then install the roof system around these components and properly flash and seal them.
NRCA guidance states that roof curbs, penetrations, roof drains, and plumbing pipes should be in place before the roof is installed. Curbs should also be securely fastened to the structure or roof deck before roofing materials are applied.
This sequencing is important because cutting new openings through a completed roof can compromise the membrane and create unnecessary flashing and waterproofing work.
A Typical New-Construction Sequence
A coordinated project may generally follow this order:
1. Structural roof deck is completed
The structural contractor completes the roof deck and verifies that it is ready to receive the roofing assembly. Required openings and structural framing should be identified before roofing begins.
2. Rooftop curbs and structural supports are installed
HVAC curbs, equipment supports, roof drains, pipe penetrations, sleeves, and other permanent roof components are positioned and secured according to the drawings.
3. Mechanical and electrical penetrations are coordinated
Mechanical contractors establish locations for ducts, vents, refrigerant lines, conduits, piping, and other services. Electrical contractors coordinate conduit, equipment connections, grounding requirements, and rooftop pathways.
4. Roofing system is installed
Once the roof substrate and required penetrations are ready, the roofing contractor installs insulation, cover boards, membrane or other specified roofing components. Flashings are integrated around curbs and penetrations as part of the roofing system.
Manufacturer guidance similarly emphasizes that roof openings, curbs, pipes, sleeves, ducts, vents, and other penetrations should be solidly set before roof-system installation begins.
5. Rooftop mechanical equipment is installed
After the roof can safely support construction activity, HVAC units and other rooftop equipment may be set onto their completed curbs or mounting systems. Heavy equipment should be handled carefully because moving large loads across a finished membrane can damage the roof.
6. Electrical and mechanical connections are completed
Electricians and mechanical contractors then complete final connections, controls, piping, ductwork, and equipment commissioning. Any work that penetrates the finished roof must be coordinated with the roofing contractor.
Why Trade Coordination Matters
The biggest risk is allowing one trade to complete work that another trade must later disturb. For example, if an HVAC contractor installs a curb after the roof membrane has been completed, the roofing contractor may have to return to modify the membrane and flashing. Similarly, electrical conduit installed through a completed roof without proper flashing can create a leak point.
Roof penetrations are particularly important because pipes, conduits, ducts, and other openings interrupt the continuity of the roof assembly. Proper flashing and detailing are necessary to maintain a watertight system.
For this reason, the general contractor should establish a roofing and rooftop equipment coordination plan before work begins. The plan should identify who installs each curb, penetration, support, flashing, and piece of equipment, as well as who is responsible for final weatherproofing.
The Bottom Line
Roofing should be sequenced so that permanent rooftop curbs, penetrations, drains, and supports are installed and coordinated before the roof membrane is completed, while heavy rooftop mechanical equipment is generally installed only when the roofing system can be protected from damage.
The best results come from coordination among the general contractor, roofing contractor, mechanical contractor, electrical contractor, structural engineer, and equipment suppliers. When these trades follow a coordinated sequence, the project can maintain roof integrity, reduce rework, protect manufacturer warranties, and deliver a watertight building envelope.
Related Questions
- Which should come first: the new roof or the new rooftop units?
- How do you sequence roofing around a building automation system upgrade?
- How are ductwork and equipment supports raised when insulation is added?
- What fire requirements apply to rooftop mechanical enclosures?
- How does the general contractor’s schedule pressure affect roof quality?
- Who pays for equipment disconnect and reconnect on a roofing project?
- What warranty issues arise when a roof is installed early in construction?
- Should rooftop equipment be secured or shut down before a hurricane?
- Who is responsible for dry-in during a new construction project?
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