Monsoon and Rooftop Units: Where the Water Actually Gets In
The leak that appears over the sales floor during the first big storm is usually not the roof. It is the unit sitting on it.
Water intrusion around commercial rooftop units usually originates at the curb seal, the flashing, degraded gaskets on access panels, or a blocked condensate drain backing up into the unit. Pre-monsoon inspection of curbs, flashing, panel gaskets, and drains prevents most storm-season interior leaks.
Why the unit is the usual suspect
When water shows up on a commercial ceiling after the first monsoon storm, the reflexive assumption is a roof leak. Often the roof membrane is fine and the water came in through or around the HVAC unit.
That makes sense structurally. A rooftop package unit sits on a curb — a raised frame penetrating the roof membrane — and it is the single largest intentional hole in the roof. Everything about keeping water out depends on the seal between that curb and the unit, and on the flashing where the curb meets the membrane.
Those assemblies are exposed to Arizona UV year-round, thermal cycling every day, and mechanical vibration from the equipment itself. They degrade, and nobody looks at them.
The entry points
- Curb seal. The gasket between the unit and the curb compresses and hardens over years. When it fails, wind-driven rain enters directly into the duct opening.
- Curb flashing and counterflashing. Where the curb meets the roof membrane. Degraded sealant here is a classic monsoon leak.
- Access panel gaskets. Panels get removed at every service visit and gaskets tear or get left misaligned.
- Blocked condensate drain. Water backs up in the pan and overflows inside the unit, which routes it straight into the ductwork.
- Damaged or missing panel screws. Panels that do not seat properly under wind loading.
- Economizer hood. A damaged or improperly installed outside air hood admits wind-driven rain directly.
- Conduit and wiring penetrations that were never properly sealed.
Why wind-driven rain is the specific problem
A monsoon storm does not deliver rain vertically. It delivers it horizontally at high velocity, often preceded by a dust event that packs debris into every opening.
An assembly that sheds normal rainfall perfectly well can fail entirely under horizontal water at 50 miles per hour. That is why leaks appear during storms and not during ordinary rain, and why a unit can pass a hose test and still leak in August.
It is also why the pre-monsoon inspection matters more than a reactive one. By the time water is on the ceiling, the damage is done and the storm is over.
The pre-monsoon inspection
- Curb seal condition and compression at every unit
- Flashing and counterflashing sealant condition
- All access panel gaskets, screws, and seating
- Condensate pan clear, drain flowing, secondary protection present
- Economizer hood and dampers intact and properly positioned
- Conduit, wiring, and any other penetration sealed
- Debris cleared from the unit and the surrounding roof area
- Drains and scuppers on the roof itself clear, so water does not pond against the curb
That last item is worth emphasizing. Ponding water against a curb will eventually find a way in regardless of how good the seal is. Roof drainage and HVAC curb integrity are the same problem.
After a storm
Walk the roof after significant weather. Look for displaced panels, damaged hoods, debris lodged in coils, standing water, and any sign of movement at the curbs.
Check coils for dust packing from the haboob that preceded the rain — that combination is common and it reduces capacity immediately at the hottest part of the season.
And check interior ceilings before the next storm. A small stain that appears after one storm becomes a collapsed ceiling tile after the third.
When the unit itself is the problem
Sometimes the honest answer is that the unit is at end of life and the cabinet, panels, and seals are no longer serviceable. Sealing a degraded cabinet is a temporary measure.
If a unit is past its useful life and leaking, the replacement conversation should include the curb. A new unit on an old curb with a poor fit is a leak waiting to happen, and curb adapters exist precisely to address that — done properly, they are part of the installation rather than an afterthought.
Frequently asked
Why does my commercial building leak around the rooftop HVAC unit?
Usually the curb seal, the flashing where the curb meets the roof membrane, degraded access panel gaskets, or a blocked condensate drain overflowing inside the unit. Wind-driven monsoon rain defeats assemblies that shed ordinary rainfall fine.
When should rooftop units be inspected for monsoon?
Before storm season, typically in May or early June. Check curb seals, flashing, panel gaskets and screws, condensate drains, economizer hoods, and all penetrations — plus roof drainage, since ponding water against a curb will eventually find a way in.
Can a clogged condensate drain cause a ceiling leak in a commercial building?
Yes, and it is a common cause. When the pan overflows inside the unit, water routes directly into the ductwork and from there into the ceiling. It looks exactly like a roof leak and has nothing to do with the roof.