Single-Ply Roofing Flashings at Penetrations

When it comes to single-ply roofing systems—like TPO, PVC, or EPDM—the biggest risks for water intrusion aren’t always in the field of the membrane. They're at the penetrations.

Pipe boots. Vents. HVAC units. Skylights.
Every one of these rooftop features creates a hole in the roof system that must be carefully sealed with flashing. If the flashing fails, water gets in. And when water gets in, repairs get expensive fast.

At L&L Roofing & Construction, we’ve seen how improper flashing installs—especially around penetrations—lead to premature roof failure. This post breaks down what Gainesville-area commercial property owners and facility managers need to know.

What Is Flashing Around Roof Penetrations?

Flashing is the waterproof layer that seals any joint or interruption in the roof system. In single-ply roofing, flashing is used around:

• Pipe penetrations

• Vent stacks

• HVAC curbs and ducting

• Skylights and drains

• Conduits and cables

These are typically sealed with:

• Pre-molded boots or pipe seals

• Field-fabricated flashing using membrane

• Pourable sealers with pitch pans (used sparingly)

Each of these methods has to be installed correctly and in line with both the membrane manufacturer’s and code requirements.

Common Mistakes We See in the Field

We’ve repaired a lot of poorly flashed penetrations in Hall and Forsyth County. The most common errors?

• Incompatible materials: Using sealants or accessories not approved for TPO, PVC, or EPDM membranes.

• Lack of membrane reinforcement: Some installers skip reinforcing with cover strips—especially around pipes.

• No slope away from penetrations: Flat areas around a penetration lead to ponding water and premature deterioration.

• Unsealed penetration collars or boots: This is the most common source of slow, hidden leaks.

If your roofer isn’t trained or certified by the membrane manufacturer, these mistakes are more likely.

Code and Manufacturer Requirements

The 2021 International Building Code (IBC) requires that all roof penetrations be “flashed in a manner that prevents water entry into the building” and comply with the manufacturer’s tested system. That includes:

• Flashing height: 8 inches above the roof surface is typical for vertical penetrations.

• Welded seams: On TPO and PVC, seams must be hot-air welded for durability.

• Termination details: Flashing must be terminated with clamps, sealants, or edge metal depending on type.

Don’t assume “close enough” is good enough. The only right way is the code-approved way.

Our Best Practices at L&L Roofing

Here’s how we protect your building when flashing single-ply penetrations:

• Manufacturer-trained crews install each boot, wrap, and curb according to technical specs.

• Reinforced membrane flashing adds strength and longevity.

• Pourable sealer in pans only when necessary—never as a shortcut.

• Every penetration is checked during our warranty inspections.

Want to avoid leaks for 20+ years? It all starts with precision and integrity—especially at these high-risk areas.

FAQs

How do you flash a pipe in a TPO roof?

We typically use a pre-molded TPO pipe boot that’s welded to the membrane, sealed with a clamp at the top, and reinforced with a membrane patch at the base.

What causes most roof leaks in commercial single-ply systems?

The majority are due to poorly flashed penetrations—either bad welds, missing reinforcement, or incompatible sealants.

Can flashing be repaired or does it require full replacement?

Minor damage can be patched if the membrane is still under warranty and installed correctly. Otherwise, we recommend full re-flashing of the penetration.

At L&L Roofing & Construction, we believe the right way is the only way. That’s why we take the extra time to get the details right—especially where it counts most.

Have a flat roof with multiple penetrations? Wondering if your system is sealed properly?
Give us a call at (770) 874-0372 and we’ll walk it with you.