Why Is My Roof AC Leaking Water Inside

Air conditioning units mounted on roofs can leak water into interior spaces for several reasons, creating stains, odors, and structural damage if left unaddressed. This article explains common causes, step-by-step inspection guidance, DIY fixes, when to call a professional, and preventive measures to protect the building and HVAC system. Understanding the likely sources of leaks helps prioritize repairs and reduces downtime.

Symptom	Probable Cause	Urgency
Water around drain pan	Clogged condensate drain	High
Water from duct penetrations	Failed roof flashing or curb seal	High
Water during heavy rain	Damaged roof membrane or improper unit curb	High
Intermittent dripping	Coil freeze/thaw or overflow	Medium

How Rooftop AC Systems Leak Water Inside

Rooftop HVAC units produce condensate as warm air crosses cold evaporator coils. Condensate is normally collected in a drain pan and removed via a condensate line. Leaks occur when that system fails or when the roof penetration and curb sealing around the unit are compromised.

Rainwater can also enter if the roof membrane or flashing around the unit curb is damaged, if the curb is deteriorated, or if pooling occurs from poor roof drainage. Both internal condensate failures and external roof breaches can cause interior water intrusion.

Common Causes Of Water Leaks From Roof AC

Clogged Condensate Drain Lines—Debris, algae, or sludge can block drain lines or trap primers, causing condensate to back up and overflow the drain pan.

Rust Or Damaged Drain Pan—Metal pans corrode or crack over time, allowing water to bypass the drain system and leak into the building.

Improper Unit Curb Or Poor Sealing—If the AC unit is not properly mounted or the flashing seal around the curb is degraded, water can track under the unit and penetrate the interior.

Clogged Condensate Pump—When rooftop units rely on condensate pumps to move water to roof drains, pump failure leads to overflow and interior leakage.

Coil Freeze And Thaw—Restricted airflow or low refrigerant can cause evaporator coils to freeze; when they thaw, excessive meltwater may overwhelm the drain system.

Roof Membrane Damage Or Ponding—Rains and ponding near the unit can exploit weaknesses in the roofing membrane or flashing, directing water into the building envelope.

How To Inspect A Leaking Rooftop AC

Start by documenting where water appears inside the building, then trace vertically to identify the rooftop unit directly above. Take photos of stains, water paths, and the HVAC unit from the roof to assist technicians if needed.

On the roof, visually inspect the unit curb and surrounding membrane for cracks, splits, or loose flashing. Check for ponding water, blisters, or previous repair patches that may have failed.

Open the access panel carefully with power off. Inspect the drain pan for standing water, rust, or holes. Verify that the condensate line is clear by running water into the pan and observing flow at the roof drain or discharge point.

DIY Fixes And Temporary Measures

Clear Condensate Lines—Use a wet/dry vacuum at the condensate line cleanout to remove clogs. Flushing with a bleach solution (1 part bleach to 16 parts water) can control algae but must be used carefully to avoid damage and chemical exposure.

Replace Or Patch Drain Pans Temporarily with waterproof tape or an EPDM patch to stop immediate leaks, but plan for full replacement if corrosion is significant.

Reset Or Replace Condensate Pump if the pump has failed; cleaning the float switch and testing the pump may restore operation temporarily while scheduling a full repair.

Improve Roof Drainage by clearing debris and ensuring scuppers and roof drains are open. As a short-term step, install temporary sandbags or reroute water away from the unit to reduce ponding.

Professional Repairs And When To Call A Technician

Call an HVAC professional when the leak source is unclear, when electrical components are involved, or when the condensate system requires replacement. Qualified techs can safely access coils, perform refrigerant checks, and replace corroded pans or pumps.

Engage a roofing contractor when flashing, curb integrity, or membrane failure is suspected. Roof work requires specialized materials and warranties that HVAC technicians typically do not provide.

For complex problems involving both HVAC and roofing, coordinate a joint inspection so both trades can identify interactions between curb design and condensate management to ensure a lasting repair.

Preventive Maintenance To Avoid Future Leaks

Schedule regular HVAC maintenance at least twice a year to clean coils, inspect drain pans, and flush drain lines. Seasonal checks before cooling and after winter reduce the risk of coil freeze and blockages.

Install Access Ports And Secondary Drainage so condensate has a backup path if the primary line clogs. Secondary overflow pans with float switches tied to alarms reduce the chance of unnoticed leaks.

Keep Roofs Clear And Maintain Flashing by inspecting roof membranes and curb seals annually, especially after storms. Apply durable sealants and replace aging curb boots to prevent water intrusion.

Costs And Timeframes For Common Repairs

Minor condensate line cleaning typically costs $75–$250 and can be completed within a few hours. Replacing a corroded drain pan or condensate pump generally ranges from $300–$1,200 depending on access and part quality.

Roof curb repairs or flashing replacement performed by roofers usually cost $500–$3,000 based on the extent of membrane work and materials. Major curb replacement combined with roof membrane restoration can be higher and may require coordinating work with building managers.

Safety And Liability Considerations

Water from HVAC systems may contact electrical wiring and ceiling finishes; document damage and shut down affected circuits if safe to do so. Addressing roof leaks quickly reduces mold growth risk and potential structural damage that can complicate insurance claims.

Building owners should keep maintenance records, service invoices, and photos to support warranty claims or insurance submissions. For leased spaces, confirm responsibilities outlined in the lease for HVAC and roof maintenance.

Frequently Asked Questions

Can Rainwater And Condensate Be Distinguished?

Yes. Condensate typically drains in a steady stream when the AC runs, while rain-driven leaks correlate with storms and may leave debris tracks or roof membrane breaches. Timing and visual inspection usually reveal the source.

Is Mold Likely After A Rooftop AC Leak?

Mold can develop within 24–48 hours in porous materials that remain wet. Rapid drying, removal of saturated ceiling tiles, and disinfection minimize mold growth. Consider a mold assessment for significant or prolonged water exposure.

Are There Upgrades To Reduce Future Risk?

Yes. Installing redundant drainage, engineered curb flashings, stainless steel or polyethylene pans, and remote condensate alarms helps prevent recurrence. Energy-efficient units with better airflow management also reduce coil freeze risk.

Checklist For Immediate Action When Water Is Found Inside

• Document The Leak—Photograph stains and active drips and note the time and weather.
• Turn Off Power To The Unit If Water Contacts Electrical Components.
• Contain And Remove Standing Water To Prevent Damage And Mold.
• Inspect The Roof For Obvious Breaches Or Ponding.
• Contact HVAC And Roofing Professionals If The Source Is Unclear Or Repair Exceeds Simple DIY.

Addressing rooftop AC leaks requires a methodical approach combining HVAC diagnosis and roof integrity checks. Prompt action, routine maintenance, and coordinated repairs between HVAC and roofing trades minimize damage and costs.