When a commercial roof leaks, property owners naturally look at the big open areas of the membrane. But roofing professionals know the more likely culprit: the flashing. The open field of a well-installed flat roof is usually the most durable part of the system. The weak points are where the roof meets something else, like a wall, a drain, a pipe, an HVAC unit, or the roof edge.
Flashing is the material that seals those transitions. Understanding how it works, and how it fails, can help you catch problems early and avoid interior water damage.
What Is Roof Flashing?
Flashing is any material installed to waterproof the joints and penetrations of a roof. On flat commercial roofs, flashing is usually made from the same membrane as the roof (TPO, EPDM, PVC, or modified bitumen), sheet metal, or a combination of both. Its job is to direct water away from vulnerable seams and keep it from getting behind the roofing system.
Because flashing sits exactly where different materials meet and move differently, it takes more stress than any other part of the roof.
The Main Types of Flashing on a Commercial Flat Roof
1. Base flashing
Base flashing runs up vertical surfaces like parapet walls and curbs and seals to the roof membrane. The National Roofing Contractors Association (NRCA) generally recommends base flashing extend at least 8 inches above the roof surface, so that water ponding or drifting snow can’t overtop it.
2. Counterflashing
Counterflashing is a metal piece set into or fastened to the wall that overlaps the top of the base flashing. It protects the upper edge of the base flashing from water running down the wall.
3. Coping caps
Coping caps are metal caps that cover the top of parapet walls. When coping joints open up, water can enter the wall itself and then appear inside the building far from the actual entry point.
4. Edge metal (drip edge / gravel stop)
This metal trim is installed at the roof perimeter. It secures the membrane edge and directs water into gutters. Edges face the highest wind-uplift forces on any roof, and the industry standard ANSI/SPRI ES-1 exists specifically to address edge metal wind resistance.
5. Penetration flashing
Every pipe, vent, conduit, and support that passes through the roof needs its own seal. This includes pipe boots, pre-molded flashings, and pitch pans (also called pitch pockets), which are metal pans filled with sealant around irregular penetrations.
6. Curb flashing
Curb flashing seals raised curbs around HVAC units, skylights, exhaust fans, and roof hatches. These are high-traffic areas where technicians frequently walk and set down tools.
7. Drain and scupper flashing
This is the seal around interior drains and through-wall scuppers. Since all the water on the roof flows here, any weakness gets tested every time it rains. Our guide on draining water from a flat roof explains why these details are so important.
Why Flashing Fails
Thermal movement. Metal, membrane, masonry, and wood all expand and contract at different rates. Over years of hot summers and freezing winters, that constant movement stresses flashing seams and fasteners.
Sealant breakdown. Many flashing details depend on sealant, like termination bar caulking, pitch pan fillers, and counterflashing joints. Sealants dry out, shrink, and crack with UV exposure, so they need periodic inspection and replacement.
Shrinking pitch pans. The filler in pitch pans often shrinks away from the penetration, leaving a gap that channels water directly into the roof.
Bridging. When base flashing isn’t properly adhered into the corner where the roof meets the wall, it “bridges” the gap. Building movement then tears or pulls it loose.
Wind uplift at edges and corners. Loose edge metal lets wind get under the membrane, which can peel it back during storms.
Rooftop equipment work. HVAC service, satellite installs, and new penetrations are frequently added by other trades without proper roofing details. This is one of the most common causes of new leaks.
Ice and snow. In the Chicago area, snow drifts against parapet walls and meltwater refreezing at drains put extra pressure on flashing all winter.
For more on this, see our articles on common causes of commercial roof leaks and the most common causes of roof damage.
Warning Signs of Flashing Problems
From the roof:
- Base flashing pulling away from walls or curbs
- Cracked, missing, or dried-out sealant
- Open joints in coping caps or edge metal
- Rust on metal flashing or counterflashing
- Pitch pans with shrunken or cracked filler
- Wrinkles, tears, or loose membrane at corners
From inside the building:
- Stains along exterior walls, especially near parapets
- Leaks that appear only in wind-driven rain
- Water near HVAC units or skylights
- Efflorescence (white mineral deposits) on interior masonry
Leaks that appear far from the obvious source are often flashing-related, since water travels inside walls and along the deck before it shows up.
Repair or Replace the Flashing?
Many flashing problems can be handled with targeted roof repair: resealing, replacing pitch pans, re-adhering base flashing, or re-securing edge metal. However, if flashing is failing across the entire roof, it usually signals the system is reaching the end of its life. Pairing new flashing with an old membrane can create a mismatch in how the two age. In those cases, a flat roof replacement or restoration may make more sense.
When a leak appears during bad weather, a temporary flat roof repair can protect the interior until a permanent fix can be done correctly. As we explain in why flat roof repair in wet weather goes wrong, rushing permanent flashing work in the rain rarely ends well.
How to Keep Flashing Watertight
- Inspect twice a year, ideally spring and fall, plus after major storms. Our roof inspection service focuses on every flashing detail.
- Replace sealant proactively, not after it fails.
- Control rooftop access. Require that anyone installing or servicing equipment coordinates with your roofing contractor.
- Keep drains and scuppers clear so water doesn’t back up against flashing.
- Set up a maintenance plan. See commercial roof maintenance plans and our roof maintenance program.
FAQ
Is flashing part of the roof warranty?
Usually, if it was installed as part of the original system by the roofing contractor. Flashing added later by other trades often is not covered.
Can I just caulk a leaking flashing?
Caulk can be a short-term measure, but most flashing leaks need proper membrane or metal repair to hold long term.
How do I know if my leak is from flashing or the membrane?
A professional inspection, sometimes with water testing or moisture scanning, is the most reliable way to find the true source.
Protect the Weakest Points of Your Roof
All Roofs Inc. has been inspecting and repairing commercial flat roof flashing across Chicagoland since 1994. If you’re seeing stains near walls, equipment, or drains, contact us for an inspection. For urgent leaks, we offer 24/7 emergency roofing service.
