If you manage a commercial building long enough, you learn a frustrating rule: a lot of “roof leaks” are not really field-of-roof problems. They’re curb problems. The leak shows up near a hallway, a ceiling grid, or the edge of a conference room, and someone points up at the rooftop unit. Then you get the usual debate. Is it the HVAC? Is it the roof? Is it both?
Most of the time, the answer is the curb, which is the raised frame the rooftop unit sits on.
When the curb is properly sealed and flashed, it’s boring and reliable. When it isn’t, water gets in, travels, and shows up wherever gravity and the deck allow it.
Why RTU Curbs Leak in the First Place
A rooftop unit curb is a transition point. You have a big piece of equipment sitting on a raised frame, with a roof membrane tied to metal surfaces, fasteners, and corners. That’s a lot of “edges,” and edges are where low-slope roofs tend to fail first.
Here are the most common reasons curbs become chronic leak sources:
Movement and vibration
RTUs vibrate. Fans cycle on and off. The unit heats up and cools down. Over time, that movement can stress sealants, loosen fasteners, and open tiny gaps at corners.
Old or incorrect flashing details
Some curbs were never flashed correctly, especially on older buildings that have seen multiple HVAC swaps or roof repairs. Other curbs were flashed correctly once, but later work damaged the membrane tie-in, and nobody rebuilt it properly.
Water gets trapped behind the unit
A curb might be “fine,” but the roof around it holds water. Ponding water near a curb is a slow leak generator. Even a good detail gets beaten down when it sits in water after every storm.
Service work that unintentionally causes damage
HVAC techs are not roofers. They have to access panels, remove screws, cut through old sealant, or route conduit. It’s easy for a small puncture or lifted edge to happen during maintenance, and that damage often goes unnoticed until the next hard rain.
Condensation line confusion
Some drip issues are condensation or drain line problems that mimic a roof leak. The water appears in the same area and is blamed on the roof. A good inspection separates “rainwater intrusion” from “mechanical water.”
What Building Owners And Facility Teams Can Look For
You don’t need to climb onto the roof with a caulk gun, but you can spot patterns that tell you whether the curb is the likely source.
On the roof side, common warning signs include:
● Membrane pulled tight at curb corners, with small splits starting
● Wrinkling or fishmouths where the membrane turns up the curb
● Metal counterflashing that looks loose or bent
● Heavy, glossy sealant beads that look like “someone tried to stop something fast.”
● Rust stains around fasteners or curb corners
● Standing water behind or along one side of the unit
Inside the building, the pattern matters:
● Stains that appear after wind-driven rain, not just any rain
● Leaks that show up only during snow melt or freeze-thaw swings
● Water that appears in a different ceiling tile each time, especially near mechanical runs
The big clue is repetition. If the same zone leaks every season, it’s usually a detail that’s failing, not a one-time puncture.
Why quick curb “patches” often don’t last
A lot of curb leaks get “fixed” with surface caulk and hope. It may slow the leak for a month, then it’s back. The reason is simple: caulk is not flashing, and it can’t replace proper transitions.
If the underlying issue is membrane separation, corner failure, or water trapped in wet insulation, adding more sealant at the surface just delays the next leak. You can also create new problems by trapping moisture where it can’t dry.
A real commercial roof repair is about rebuilding the curb detail so water has no path in, even when the unit moves and the roof gets hit by a sideways storm.
What A Proper Curb Repair Usually Involves
The exact scope depends on the roof type and curb construction, but lasting repairs tend to include a few consistent steps:
Confirm where the water is coming from
A good roofer will inspect the curb, surrounding membrane, and nearby penetrations. They’ll also look at ponding and drainage, because you can’t repair a curb into a swimming pool and expect it to hold.
Remove failing materials instead of burying them
If there’s loose flashing, brittle membrane, or layers of old patch, those need to be removed back to sound material. Stacking patch on patch is how you get “repairs” that look thick and still leak.
Rebuild the flashing transitions
This means restoring proper membrane tie-ins, corner detailing, and termination. The goal is a curb that stays watertight even when the unit vibrates and the roof moves through seasons.
Address wet insulation if it’s present
If the roof around the curb is saturated, the repair should include replacing wet sections. Otherwise you’re sealing a problem that keeps degrading the system.
Fix the conditions that caused repeat failure
Sometimes that means adjusting drainage, adding a cricket behind the unit, or correcting a chronic ponding zone. Sometimes it means coordinating with HVAC so future service doesn’t damage the roof detail.
Protect Your Building with Professional Commercial Roofing Services
If you’ve had more than one leak in the same area, or if you’re seeing repeated staining around an RTU, it’s worth treating the curb as a priority. Curb leaks tend to get worse, not better, and the hidden costs add up quickly: ceiling tile replacement, interior damage, downtime, and emergency calls during the worst weather.
MacDermott Roofing provides trusted commercial roofing services throughout Metro Detroit, for more than 50 years. If you want an inspection that focuses on curb details, drainage conditions, and the real source of repeat leaks, contact MacDermott Roofing today to schedule a professional commercial roof inspection and stop chasing the same stain every season.