Roof Leak or HVAC Condensate? A Commercial Building Diagnostic Checklist

A ceiling drip below a rooftop air-conditioning unit doesn't automatically call for a roof patch. Start by comparing when the water appears with two things: rainfall and actual cooling operation. Then have the appropriate trades trace the equipment drainage and roof interfaces separately. Timing helps choose the investigation; it doesn't prove the source.

Our recommendation is simple: don't approve a patch solely because the stain sits below a rooftop unit. Ask what water source and route the proposed repair will address. For commercial buildings in the Lower Mainland and Fraser Valley, that question can keep a mechanical drainage problem from becoming a series of unrelated roofing work orders.

Two water sources, one ceiling stain

During cooling, moisture can collect at the cooling coil. The equipment's condensate pan and drain arrangement collect and remove that water. If drainage fails, water can escape the equipment rather than leave through its intended route.

Johnson Controls' rooftop-unit application guide explains a particularly useful distinction: condensate overflow can spill from a unit and travel down through its roof curb into the building. The curb is the raised support around the roof opening. Water showing up there is therefore not, by itself, proof of defective roof flashing.

Rainwater presents a different question: is water getting past a roof membrane, flashing or another weather-exposed interface? Both investigations may involve the same rooftop unit, but they follow different water paths. Clearing an equipment drain does not repair an open flashing lap; repairing that lap does not correct an overflowing condensate pan.

Compare the trigger before choosing the trade

Use observations already available from occupants, service reports and the building's controls. Record actual cooling status if available, rather than assuming that a running fan means the unit is cooling.

• Water appears during cooling in dry weather: ask the mechanical service provider to investigate the condensate path. Tell them whether the symptom starts during operation or is noticed afterward.
• Water appears during rain, including when cooling is inactive: prioritise investigation of rain entry at the roof and equipment interfaces. Include the roofing contractor and involve the equipment provider where the suspected route enters the cabinet.
• Water appears during rainy days when cooling also runs: keep both possibilities open. A visit that checks only one source may leave the central question unanswered.
• The timing is unclear: arrange coordinated investigation rather than treating the nearest visible defect as the answer.

These are routing clues, not diagnostic tests. Don't deliberately cycle equipment or apply water to reproduce a leak yourself. Ask the responsible service provider to plan any operational checks, with roof-side coordination where needed.

For example, imagine an office ceiling that drips on a dry afternoon while cooling is active. A mechanical inspection is a sensible first step. If the technician finds and corrects an overflow, the next question is whether that explains the original interior location. If the same location later becomes wet during rain with cooling inactive, reopen the investigation instead of assuming the mechanical repair failed.

What the mechanical inspection should answer

A useful mechanical report should say more than “drain cleared.” Ask the provider to identify the equipment, describe the fault found and explain how it could produce the reported water.

The review can address the condensate pan, outlet, drain line and trap, along with any pump or overflow protection fitted to that installation. Ask whether drainage was checked under relevant operating conditions and whether the discharge reaches its intended destination.

Trap design is an equipment-specific issue. The Johnson Controls application guide explains that a trap unsuited to the unit's negative static pressure can prevent proper drainage. A pipe that looks open isn't enough to establish correct operation. Leave trap dimensions, alterations and service methods to the mechanical provider using the installed unit's instructions.

If an overflow alarm occurred, include it in the work order. Johnson Controls' overflow-switch explanation describes how its Premier units can stop cooling while the supply fan continues running. That behaviour is model-specific, but it shows why “the fan was on” is a poor substitute for an equipment status report. Don't bypass an alarm to keep cooling available; have the service provider address its cause.

What the roofing inspection should answer

Ask the roofing contractor to examine the membrane and flashing around the affected equipment, nearby penetrations and the relevant drainage area. Findings should distinguish a suspected entry point from a confirmed route and identify which components need another trade's attention.

The boundary matters. RCABC's SBS curb-flashing provisions distinguish continuous roof-field-to-curb waterproofing from waterproofing equipment installed on the curb, which is assigned to others. Those are SBS RoofStar project provisions, not a universal repair detail for every existing roof. The practical lesson for an owner is to make sure the investigation covers the interface between the trades, rather than leaving each to inspect only its own side.

Keep rooftop access and equipment opening with the responsible trained service teams. You can provide valuable interior observations without climbing onto the roof or removing panels.

A handoff checklist that connects the two visits

Put the following in one shared incident record so the mechanical and roofing teams are investigating the same symptom:

1. Location: interior room and marked drip position, plus the relevant rooftop equipment ID and roof-plan reference.
2. Timing: when dripping started and stopped, rainfall observations, cooling status and any alarms or recent service.
3. Paired views: an interior photo and service-team roof/equipment photos labelled to show their relationship, with close-ups of findings.
4. Separate conclusions: what each trade observed, what it corrected, and what remains unconfirmed or outside its scope.
5. Follow-up: who will check the original interior location when the relevant cooling or rainfall conditions return.

RCABC's roof-maintenance guidance recommends retaining roof plans, inspection photographs, previous problems, corrective work and mechanical-equipment changes. Keeping this incident together makes those records useful at the next service call.

Close the work order with an explanation of the water route and the follow-up observations, not just two invoices marked complete. If roof-side entry remains suspected, request a commercial roof leak investigation and share the mechanical findings at booking. That gives the next visit a specific question to answer.