Water Dripping From the Ceiling? How to Diagnose a Commercial RTU Condensate Leak
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- 5 min read
It usually starts with a phone call from a tenant. A ceiling tile has gone brown and soft.
There's a bucket under it. Someone has already used the word "flood."
The first instinct for most property managers is to call a roofer. Half the time that's the wrong call — and the roofer's invoice arrives before anyone figures that out.
If the water is appearing during hot, humid weather and there's a rooftop unit anywhere near the affected area, you're most likely looking at a commercial RTU condensate leak, not a roof membrane failure. Here's how to tell the difference, what's actually causing it, and what needs to happen in the first hour.
Roof leak or condensate leak? Three questions that sort it out
1. Is it raining? A roof leak follows the weather. A condensate leak follows the cooling load. If water is appearing on a dry, 31°C afternoon with the sun out, the roof is not your problem. If it only shows up during a downpour, it probably is.
2. Where is the water relative to the unit? Condensate leaks track down the outside of the supply ductwork or run along the underside of the roof deck before dropping. The stain is often several metres from the RTU curb, which is exactly why these get misdiagnosed. Map the ductwork, not just the ceiling.
3. What does the water look like? Condensate is clear and clean. Roof water picks up dirt, rust and debris on the way through and usually stains darker. Water that's clear on day one and rust-coloured on day three often means a corroded drain pan — the pan is leaking and rusting through.
What Causes a Commercial RTU Condensate Leak: Five Common Failures
Clogged condensate drain line
The most common cause by a wide margin. The drain line carries away several litres of water an hour in GTA summer humidity, and that constant moisture grows biofilm. Add dust pulled through the filters, a bit of insulation debris, and the line closes off. The pan overflows, and the water goes wherever gravity takes it — usually straight through the curb opening into the building.
This is a maintenance failure, not a mechanical failure. It is also the cheapest problem on this list to fix and the cheapest to prevent.
A trap that was never built right
This one gets missed constantly, and it's worth understanding because it explains repeat leaks on the same unit.
On a draw-through RTU, the drain pan sits under negative pressure. If the trap isn't deep enough to overcome that pressure, the unit pulls air backwards up the drain line and holds water in the pan instead of letting it drain. The pan overflows even though the line is perfectly clear.
On a blow-through unit, the pan is under positive pressure and the trap has to be configured differently, or the unit blows condensate out of the line entirely.
If a unit has leaked repeatedly, been "cleared" repeatedly, and leaked again, stop paying to snake the line. The trap is wrong. That's a one-time correction.
Rusted or cracked drain pan
Standard on older units. Galvanized pans sitting in standing water for fifteen Toronto summers corrode from the bottom up. Sometimes a pan can be repaired or a secondary pan installed; often, on a unit that age, the pan is the visible symptom and the compressor and coils aren't far behind. This is the point where a leak turns into a capital planning conversation.
Frozen evaporator coil
A coil that ices up will dump far more water than the pan is designed to handle the moment it thaws — and it thaws when the unit cycles off, often overnight when nobody sees it. Causes are usually restricted airflow (dirty filters, collapsed flex duct, failed belt) or low refrigerant charge from a leak.
Important: a frozen coil means something else is already wrong. Fixing the water without finding the cause guarantees a repeat.
The unit isn't level
Roof work, curb settlement, or a poor installation leaves the pan pitched away from the drain. Water pools at the wrong end and spills over the edge. Common after re-roofing projects — worth checking specifically if your leak started within a few months of roof work.
What to do in the first hour
Shut the unit down. No cooling means no condensate. This stops the water immediately and buys you time to respond properly instead of scrambling. Yes, the space will warm up. That's a better problem.
Protect the tenant's property. Poly sheeting, buckets, move equipment and inventory clear. A soaked server rack or a ruined inventory pallet turns a maintenance ticket into a claim.
Photograph everything before cleanup. Time-stamped photos of the stain, the standing water, any damaged property, and the rooftop unit. If this becomes an insurance or lease-recovery matter, contemporaneous documentation is the difference between a paid claim and an argument.
Pull the affected ceiling tiles. Wet mineral fibre tile holds water, sags, and eventually falls. Removing it also lets you see where the water is actually tracking from, which speeds up the diagnosis considerably.
Note the time and conditions. When did it start, what was the outdoor temperature, was the unit running. This is exactly what a technician needs and it narrows the cause before anyone gets on the roof.
Why this becomes a liability problem faster than people expect
Water inside a building has a clock on it. Mould growth in wet drywall and ceiling tile can begin within roughly 24 to 48 hours in warm conditions, and once it's established in the wall cavity you're no longer dealing with an HVAC repair — you're dealing with remediation, tenant health complaints, and potentially a habitability dispute.
There's also the question of who pays. In most GTA commercial leases, rooftop HVAC equipment and the building envelope sit with the landlord, while damage to tenant improvements and contents sits with the tenant's own insurance — but the wording varies enormously, and a leak caused by deferred maintenance can shift responsibility in ways a leak caused by sudden equipment failure does not. This is worth knowing before the leak, not during it.
Repair, or is the unit telling you something?
A single clogged drain line on an eight-year-old RTU is a maintenance item. Fix it, add the drain line to the PM scope, move on.
But start treating it as an end-of-life signal when you see:
A corroded-through pan on a unit past roughly 15 years
Repeat freeze-ups traced to a refrigerant leak the technician can't isolate
Multiple leaks across multiple units in the same year — that's a fleet-age problem, not five separate incidents
An R-22 or aging R-410A unit where a significant repair means putting money into equipment you'll be replacing under refrigerant regulations anyway
At that point the water damage is the cheap part. The question becomes when to schedule replacement on your terms rather than in August with a tenant threatening a rent abatement.
Related reading: [Commercial RTU Replacement Cost: What GTA Property Managers Need to Know]
The prevention is genuinely boring
Condensate leaks are one of the few HVAC failures that are almost entirely preventable, and the prevention is unglamorous: clear and flush the drain lines each spring, check pan condition and pitch, verify the trap is correct for the unit type, keep filters on schedule so the coil doesn't freeze, and confirm drainage after any roof work.
That's a scheduled maintenance visit. The alternative is an emergency call, a ceiling repair, a tenant claim, and a conversation you'd rather not have.
Dealing with water coming through a ceiling right now? Shut the unit down, protect the space, and call us. Burban Air Systems Ltd. provides commercial HVAC service across Toronto and the GTA, including emergency response for rooftop unit failures.
