Why does my commercial roof leak around the rooftop AC unit?

Because the area around a rooftop unit packs more roof penetrations and stress into one spot than anywhere else on the building. The curb the unit sits on, the gas line and electrical whip feeding it, the condensate it discharges, and the foot traffic from every service call are all potential leak sources. When a leak shows up near a unit, that equipment zone is the first place to inspect, not the open field of the roof.

Who is responsible for a leak at a rooftop HVAC unit, the roofer or the HVAC company?

As a rule of thumb, the HVAC contractor owns the equipment, its internal drain pan, and its condensate line, while the roofer owns the membrane and every place it is sealed to the building, including the curb flashing, pipe boots, and pitch pans. The tricky part is that leaks often happen where the two overlap, such as a curb disturbed during a unit swap. That is why rooftop work should be coordinated between both trades rather than left to one.

Can rooftop HVAC work void my roof warranty?

It can. Most manufacturer warranties disclaim damage caused by other trades working on the roof, so if an HVAC crew cuts a new penetration or reworks a curb and seals it their own way, the coverage around that area may be affected. The protection is to have your roofer make or inspect any roof seals, and to keep photos and invoices of the work. Every warranty is different, so read yours and confirm the details with your roofing contractor.

Rooftop Units and Roof Leaks: Curbs, Pitch Pans and Pipes

A flat commercial roof almost never leaks out in the open field, in the middle of an empty membrane where nothing is happening. It leaks where something interrupts the roof, and on most commercial buildings the single busiest interruption is the rooftop HVAC equipment: the packaged units and their curbs, the gas lines and electrical whips feeding them, the condensate they pour out all summer, and the trades who climb up to service them. Every one of those is either a hole cut in a watertight surface or a repeated stress placed on it, and every one is a place a leak can begin. This guide walks the rooftop unit as a leak source, the specific details that fail, the slow damage condensate does to a membrane, and the question that quietly decides who writes the check when it does leak: whether the failure belongs to the HVAC contractor or to the roofer. It ends with a checklist for getting rooftop work done without voiding the roof warranty you paid for.

01 / THE PROBLEMWhy equipment is where roofs leak

A commercial roof is not really a big sheet of membrane. It is a big sheet of membrane plus every place that sheet gets interrupted, and the interruptions are where the roof does its hardest work. Out in the field, a modern single-ply or modified-bitumen membrane just lies there and sheds water, which it is very good at. The trouble starts wherever the membrane has to stop being flat and continuous: it has to turn up a wall, wrap a pipe, seal to a curb, or terminate at a drain. Those transitions are built by hand in the field, they carry the movement and the weather, and they are where the overwhelming share of real leaks live. Our guide to finding a flat roof leak makes the same point from the diagnosis side: when you go looking for the entry point, you go straight to the details, not the field.

Rooftop HVAC concentrates more of those details in one spot than anything else on the building. A single packaged unit sits on a curb, a raised frame the membrane has to seal to on all four sides. Around it are the gas line, the electrical whip, sometimes a low-voltage line, and the condensate discharge. Add a service disconnect, a couple of supports, and the path the technician walks to reach it, and one rooftop unit can easily account for a dozen separate penetrations and stress points in a small area. Multiply that by every unit on the roof and it is clear why, when a commercial building leaks, the smart first move is to look under and around the equipment before anywhere else.

THE ONE-SENTENCE VERSION

The field of a flat roof is the easy part; the leaks come from the interruptions, and rooftop HVAC packs more interruptions, curbs, pipes, gas lines, whips, and foot traffic, into a smaller area than anything else on the building.

02 / THE SUSPECTSThe rooftop details that actually fail

Rooftop leaks are not random. They come from a short list of details that fail in predictable ways, and knowing the list tells you where to look and roughly what the fix involves before anyone is on the roof.

Equipment curbs

The curb is the raised frame a rooftop unit sits on, and the membrane is flashed up and over it so water sheds away from the opening below. Curbs fail in a few familiar ways: the base flashing at the curb pulls loose or was never fully adhered, the sealant at the top edge dries and opens, or the counterflashing under the unit's own sheet metal skirt was left short. Because the unit sits directly on top, a curb leak is one of the hardest to see and one of the most damaging, since the water drops straight through the deck opening into the building rather than running to a visible spot. When a unit is replaced, the curb flashing is disturbed and often has to be rebuilt, which is exactly the moment a leak gets designed in if the roofer is not part of the job.

Pitch pans and pitch pockets

A pitch pan, also called a pitch pocket, is a small open metal box built around an odd-shaped penetration, a cluster of pipes, a strut, an angle iron, that cannot be flashed cleanly any other way. The pan is packed and topped with a sealant that sheds water off the penetration. It is also, honestly, one of the weakest details on any roof, because that sealant is exposed to full sun and freeze-thaw and shrinks, cracks, and dishes over time. A pitch pan is not a permanent detail, it is a maintenance item, and one that has quietly dried out and cracked is a classic slow leak. On a Michigan roof the freeze-thaw cycle works a neglected pitch pan hard every winter, which is one more reason the flat roof maintenance checklist puts eyes on every pan at least twice a year.

Gas lines, conduit, and small pipes

The gas line and electrical whip feeding a rooftop unit have to cross the membrane too, and how they are supported matters as much as how they are sealed. A gas line run flat on the membrane, or on undersized supports that let it sag, becomes a little dam that traps water and grit and a place a technician steps when the roof is crowded. Pipe penetrations that were sealed with nothing more than a bead of caulk at the base, instead of a proper flashed boot or a pitch pan, are on a countdown from the day they were installed. These are small details individually, but they are numerous, they are low to the membrane where water collects, and they get disturbed every time the equipment is serviced.

Foot traffic and dropped tools

The least glamorous leak source is simply people walking on the roof. Every service call brings a technician, a helper, tools, and sometimes a replacement compressor dragged across the membrane. Concentrated foot traffic on the same path abrades the surface, and a dropped tool or a set-down sheet-metal edge can puncture a membrane outright. This is why you see walkway pads: they are not decoration, they are a sacrificial path that takes the traffic so the membrane does not. A roof that is serviced regularly but has no protected walk path to the units is quietly being worn out by the very people keeping the air conditioning running, and the punctures they leave behind are among the most common membrane breaches we find on an inspection.

Nearly every rooftop leak traces back to a detail that was built by hand and then disturbed by someone servicing the equipment. The field of the roof is rarely the problem. The traffic around the machinery almost always is.

03 / THE SLOW KILLERCondensate: the drip that dissolves a roof

The leak nobody sees coming is not a hole at all, it is the water the equipment makes on purpose. All summer a rooftop cooling unit pulls moisture out of the building air and discharges it as condensate, gallons of it on a humid Michigan afternoon, and where that water lands determines whether it is harmless or slowly destructive. When condensate is simply dumped onto the membrane at the base of the unit, it does three things over time, none of them good.

First, it ponds. A steady discharge onto a low spot keeps that area wet far longer than rain ever would, and standing water is the enemy of every membrane and the thing manufacturer warranties most often exclude. Second, it grows things: condensate carries dust and organic matter, and a chronically wet patch becomes a home for algae and biological growth that holds even more moisture against the surface. Third, on some assemblies the discharge is slightly acidic or laden with residue that accelerates the aging of the membrane. The result is a soft, stained, prematurely aged patch of roof, sitting directly under the one piece of equipment most likely to need a curb rebuild later, so the weakest membrane and the busiest penetration end up in the same square yard.

WHERE THE DAMAGE HIDES

Condensate damage rarely shows up as a dramatic leak. It shows up as a chronically wet, stained, softening area right at the base of a rooftop unit, the kind of thing that reads as cosmetic until the membrane there gives way. Routing condensate to a drain or a splash block, instead of onto the roof, is one of the cheapest ways to add years to the membrane under your equipment.

The fix is not exotic. Condensate should be routed to a drain or led across a splash block so it is not pooling on bare membrane, and the discharge point should be checked as part of routine maintenance, because a line aimed at a drain in the spring is often aimed at a puddle by fall. Catching a wet, softening area early usually keeps it in the $350 to $3,200 repair range on our published price canon, where the same area, ignored until the deck rots and the unit has to be lifted, becomes a far larger job.

04 / THE OWNERSHIPWho owns which failure, the HVAC contractor or the roofer

When a leak shows up near a rooftop unit, the first phone call is often the most confused one, because two different trades touched that spot and each can plausibly point at the other. Sorting out who owns the failure is not about blame, it is about calling the right contractor the first time so the leak actually gets fixed instead of bounced back and forth while the water keeps coming in.

The clean version of the line is this. The HVAC contractor owns the machine and everything that is part of the machine: the unit itself, its internal drain pan, the condensate line up to the point it leaves the equipment, and the gas and electrical connections as connections. The roofer owns the membrane and every place the building envelope is sealed: the curb flashing, the pipe boots and pitch pans, the base flashings, and the field of the roof. In the tidy case, a leak from a cracked internal drain pan is an HVAC problem, and a leak from a failed curb flashing is a roofing problem, and each trade fixes its own.

Real roofs are messier, because the two trades meet exactly where the leaks are. When an HVAC crew sets a new unit they disturb the curb flashing, when they run a new gas line they create a penetration the roofer has to seal, and when they discharge condensate onto the membrane they age a surface the roofer warrants. So the honest answer to who owns the failure is often both, and the failures multiply precisely when nobody coordinated the handoff. This is also where roof warranties get quietly voided, because most manufacturer warranties disclaim damage caused by other trades working on the roof, a point our guide on commercial roof warranties spells out. If an HVAC crew cuts a new penetration through a warranted membrane and seals it their own way, the roof around it may no longer be covered, and the owner finds out only when a claim is denied.

THE RULE OF THUMB

The HVAC contractor owns the equipment, its drain pan, and its condensate line. The roofer owns the membrane and every place it is sealed to the building, curbs, boots, pitch pans, and base flashings. The leaks happen where those two responsibilities overlap, which is why the two trades have to be coordinated, not just dispatched.

05 / THE CHECKLISTCoordinating rooftop work without voiding the roof

Most of the damage in this guide is preventable, and the prevention is coordination rather than money. When rooftop equipment is installed, replaced, or serviced, a few habits keep the roof watertight and the warranty intact. Treat the following as a checklist for any rooftop work, whether it is a full unit swap or a routine service call.

Tell your roofer before the HVAC crew goes up. Any time a unit is set, replaced, or has new lines run, a new penetration or a disturbed curb is coming. A roofer looping in before the work means the seals are made right and inspected, not discovered as a leak next spring.
Have the roofer, not the HVAC crew, make the roof seals. HVAC technicians are experts at the machine, not the membrane. Curb flashings, boots, and pitch pans should be built or restored by the roofing contractor so the detail matches the roof system and stays inside the warranty.
Insist on protected walkways to every unit. Walkway pads on the service paths take the foot traffic that would otherwise wear through the membrane. On a roof that is serviced regularly, this is one of the highest-return details there is.
Route condensate to a drain or splash block, never onto bare membrane. Get the discharge off the roof surface and confirm at every visit that it still lands where it should, because lines drift.
Keep the paperwork. Photos and invoices for any rooftop work, and any repair a roofer makes afterward, are what protect the warranty if another trade's damage is ever questioned. A documented inspection habit is the cheapest coverage you can buy.
Inspect the equipment zone on a schedule. Every curb, pan, boot, and condensate outlet should be walked at least twice a year and after major storms, which is exactly the routine the maintenance checklist is built around.

None of that requires a big budget, and all of it is cheaper than the leak it prevents. Where a rooftop detail has already failed, the repair usually lands in the $350 to $3,200 range, with larger work running higher, and a full low-slope reroof when a membrane reaches end of life sits in the canon $9,000 to $16,000 commercial range. You can run a first-pass number on the cost calculator, get a ballpark from the instant estimator, or browse the rest of the commercial library from our roofing services hub. For a building that finances the larger work, an illustration only: a $13,500 project over a 10 year term at 9.9% APR is about $178 per month, subject to approval, not an offer of credit. And if a rooftop leak is active right now, our 24/7 emergency tarping stops the water while the permanent repair gets scoped in daylight.

NO OBLIGATIONKEY TAKEAWAYS

A flat roof leaks at its interruptions, not in the field, and rooftop HVAC concentrates more penetrations and foot traffic into one area than anything else on the building.
The short list of failing details is curbs, pitch pans, small pipe and gas-line penetrations, and puncture from service foot traffic, which is why walkway pads exist.
Condensate dumped onto bare membrane is the slow killer: it ponds, grows biological matter, and ages the surface exactly under the busiest unit; route it to a drain or splash block.
The HVAC contractor owns the equipment and its drain line; the roofer owns the membrane and every seal to the building, and leaks happen where those overlap.
Coordination protects both the roof and the warranty: loop the roofer in before HVAC work, let the roofer make the seals, and protect the walk paths.