Ask a commercial roofer where they expect to find a leak and they will look at the mechanical equipment before they look at the membrane. Rooftop HVAC units sit at the center of more commercial roof leaks than any other feature on a low-slope roof.
It is not that the units themselves are bad. It is that every unit gathers a crowd of details in one place — a flashed curb, penetrations for refrigerant lines and power, a condensate discharge, and a service technician on the roof several times a year.
Cannon Roofing has traced leaks on light-commercial buildings across Wise County since 1994, and the path very often leads back to a rooftop unit.
Curb Flashing: The Detail That Fails Quietly
The unit sits on a curb, a raised frame built up from the deck. The membrane runs up the sides of that curb and terminates near the top, protected by the unit's own skirt or by a counterflashing.
That termination is the weak point. Sealant at the top shrinks and splits, termination bar fasteners work loose, and the membrane pulls back slightly with years of thermal movement. Once water gets behind the flashing at the top of a curb, it is inside the assembly and out of sight.
Curb height matters too. A curb that sits too low leaves no room for the membrane to turn up above the water line, so heavy rain puts standing water above the flashing. And when an old unit is swapped for a different footprint, curb adapters are sometimes fitted badly, leaving gaps at the corners.
Condensate Lines and Constant Wetting
An air conditioner pulls moisture out of the air, and that water has to go somewhere. Too often it goes straight onto the roof membrane.
A line discharging onto the surface keeps one area damp through an entire Texas cooling season. That patch grows algae, holds grit against the membrane, stains, and on a slow-draining roof it turns into a small permanent pond. Condensate should discharge to a drain, a gutter, or at minimum a splash pad positioned where water runs off promptly.
Blocked condensate pans are the other half of the problem — when the pan overflows, the water usually ends up inside the building, and the roof gets blamed for a leak it did not cause.
Foot Traffic: The Damage Nobody Reports
HVAC technicians are not roofers, and they are not trying to hurt the roof. But service visits leave marks:
- Dropped tools and screws, which get stepped on and driven into the membrane.
- Dragged equipment — compressors, tanks, and ladders — scuffing the surface.
- Panel access work done kneeling on the membrane with sheet metal edges underneath.
- Abandoned parts and packaging left to blow into drains.
None of it looks dramatic. A punctured membrane the width of a screw shank will still soak a lot of insulation over a rainy season, which is why leak tracing so often begins around equipment — see our guide to finding leaks on a commercial flat roof.
Walkway Pads Solve Most of This
Walkway pads are the cheapest insurance on a commercial roof. Run them from the roof hatch or ladder to every unit, and around the service side of each unit where technicians stand.
Pads spread load, protect the membrane from tools and knees, and tell everyone on the roof where they are supposed to walk. On a roof with several units, they pay for themselves the first time they stop a puncture.
Coordination Between Roofers and HVAC Contractors
Most equipment-related leaks trace back to a communication gap. A unit gets replaced, a new penetration gets cut for a line set, an old curb gets abandoned in place, and nobody tells the roofing contractor.
A few habits prevent almost all of it:
- Notify your roofer before mechanical work begins, and have the roof checked afterward.
- Let the roofer detail any new penetration, or at minimum inspect it.
- Remove or properly cap abandoned curbs and penetrations instead of laying a panel over them.
- Support line sets and gas piping on proper blocks or sleepers, never directly on the membrane.
- Confirm that warranty terms are not compromised by another trade cutting into the roof.
Making that a standing rule is far easier than tracing the leak afterward. It works best alongside a commercial roof maintenance program, where equipment areas get checked twice a year, and a documented roof inspection with prompt roof repair when something is found.
Frequently Asked Questions
Why do rooftop HVAC units cause so many roof leaks? Each unit brings a cluster of vulnerable details together: a flashed curb, refrigerant and electrical penetrations, a condensate discharge, and regular foot traffic from service technicians. Every one of those is a place water can get in.
What is a curb, and why does its flashing matter? A curb is the raised frame the unit sits on. The roof membrane turns up the curb and terminates near the top, under the unit's skirt or a counterflashing. If that termination loosens or the curb is too low, water gets behind the membrane.
Can condensate water damage a commercial roof? Yes. A line that discharges onto the membrane keeps one area wet through the whole cooling season, which encourages growth, holds grit against the surface, and can create a standing pool where the roof drains slowly.
Should my HVAC contractor tell the roofer before working up there? It helps enormously. New penetrations, unit change-outs, and abandoned curbs all need roofing details done correctly, and a post-work roof check catches dropped screws and punctures before the next rain finds them.
Leak showing up near a rooftop unit? Call Cannon Roofing at (940) 627-1045 or request a free quote. We detail equipment curbs and trace commercial leaks across Decatur, Wise County & Greater North Texas.