HVAC Curb Installation on Commercial Roofs: What You Need to Know

If you’re planning to add or replace HVAC units on a commercial building in Gainesville or the surrounding areas, the way those units are mounted matters just as much as the units themselves. Improper HVAC curb installation can lead to leaks, roofing system failure, and lost warranty coverage.

At L&L Roofing & Construction, we specialize in metal roofing systems—and that includes precise, code-compliant HVAC curb integration that protects your investment and keeps your building dry and energy-efficient.

What Is an HVAC Curb?

An HVAC curb is a framed support structure installed on a commercial roof to support a rooftop HVAC unit. It also serves to raise and isolate the HVAC unit above the roof surface and allow for secure, watertight ductwork transitions.

But here’s the deal: not all curbs are installed correctly. And that’s where problems start.

Key Requirements for Proper HVAC Curb Installation

Here’s what L&L Roofing ensures every time:

✅ Use of Pre-Manufactured or Custom “Floating” Curbs

• We use continuously-welded .080” aluminum or 18-gauge stainless steel curbs—never galvanized curbs, which corrode faster and void warranties.

• These “floating” curbs are designed to move with the metal roof system, preventing cracks and seal failures from thermal expansion.

✅ Proper Shingled Curb Flashing

• We install curbs in an “under/over” or shingled configuration: the upslope curb flange goes under the panels, and the downslope flange goes over them.

• This setup prevents backwater intrusion and is essential for low-slope or flat roofs common in commercial settings.

✅ Adequate Curb Height and Clearances

• Every curb stands at least 6 inches tall to keep water out, even in heavy rainfall.

• We maintain at least 12" clearance between the panel ends and the upslope diverter, and 6" clearance on the sides, ensuring proper drainage.

✅ Seam Clamp Systems for Non-Penetrating Installations

• When possible, we use non-penetrating seam clamps to secure frames—keeping your metal panels intact and avoiding unnecessary holes.

Avoid These Common Mistakes

Unfortunately, we see these errors too often on commercial roofs in our service area:

• Letting HVAC or sheet metal contractors handle the curb install (they often don’t understand waterproofing or thermal movement).

• Using curb materials that degrade fast (like galvanized steel).

• Ignoring panel layout, resulting in poor drainage and panel-rib interference.

• Pinning curbs to the structural deck, which restricts movement and causes leaks.

Our team ensures that doesn’t happen.

Double Curb Systems for Heavy HVAC Units

If your unit is large and heavy, we use a double curb system:

• Structural curb: supports the HVAC unit and ties into the building framing.

• Flashing curb: surrounds it, integrates into the roofing, and handles waterproofing.

This approach separates load-bearing from waterproofing—delivering superior durability and leak resistance.

Why This Matters for Your Roof Warranty

Most manufacturers require roof-mounted penetrations (like HVAC curbs) to be approved by them and installed per their specs. If not, your roof warranty might be voided.

When you work with L&L Roofing, we coordinate directly with the metal roofing supplier to:

• Get curb compatibility confirmed

• Use only approved installation methods

• Preserve your long-term system warranty

Serving Gainesville’s Commercial Community with Integrity

From flat TPO commercial roofs downtown to standing seam metal roofs in Cumming, we bring honesty, expertise, and faith-driven service to every job. When your HVAC system depends on the roof—and your roof depends on the curb—you want professionals who know how to do it right.

FAQs

How high should an HVAC curb be on a commercial roof?
At least 6 inches high. This keeps water out and protects the duct connections.

What’s the best material for a curb?
Welded aluminum or stainless steel. They resist corrosion, don’t warp with heat, and seal tight with the roof.

Can HVAC curbs be installed after the roof is done?
Yes—but it requires extra care. We recommend involving a qualified roofer (not just the HVAC tech) to make sure seals and flashing are done right.

Do HVAC curbs need to match the roof profile?
Absolutely. Pre-fab curbs must fit the specific panel profile of your metal roof. We work with manufacturers to get it right every time.

Want your HVAC curb installed right the first time?
Call L&L Roofing & Construction at (770) 874-0372.
We’ll help you protect your investment with craftsmanship you can trust.