Evaporative Air Conditioner Roof Leaks: Causes and Fixes

9 min read

Perth and evaporative cooling go together like summer and 40-degree days. The dry climate makes evap coolers effective and economical, which is why you’ll find one sitting on the roof of roughly half the homes in the metro area.

The downside? That big metal box on your roof is pumping water constantly while it runs, it’s exposed to Perth’s brutal sun year-round, and it sits on a hole cut through your roofing material. It’s one of the most common sources of roof leaks we see across Perth, and the cause isn’t always obvious.

How Evaporative Coolers Cause Roof Leaks

An evaporative cooler works by pumping water over pads and blowing air through them. There’s a constant flow of water during operation, with excess water draining back into a base tray and either recirculating or overflowing to a drain.

That means there’s always water present on your roof when the unit is running - and sometimes when it’s not. Here are the main ways that water ends up inside your house instead of staying where it should.

Blocked or Overflowing Drain Pan

The base tray of the cooler collects water that drips off the pads. This tray has a drain or overflow outlet that should direct excess water safely away. When the drain blocks - with mineral buildup, leaves, dust, or dead insects - the tray overflows.

Overflowing water runs across the base of the unit and down onto the roof surface. If the sealant around the roof penetration has any gaps, that water goes straight into your ceiling space.

This is the single most common cause of evap-related roof leaks in Perth. Mineral buildup from Perth’s hard water is a constant issue, especially if the unit doesn’t have a dump valve that regularly flushes the system.

Cracked or Corroded Base Tray

The base tray is typically made of galvanised steel or plastic. Over years of holding water in Perth’s heat, it deteriorates:

• Steel trays develop rust holes, especially at the corners and around weld seams where the galvanising wears thin.
• Plastic trays can crack from UV exposure and thermal cycling, particularly around fittings and drain connections.

A cracked tray leaks water directly onto the roof surface beneath the unit - a spot that’s impossible to see without physically lifting or moving the cooler.

Failed Sealant Around the Roof Penetration

When an evap cooler is installed, a hole is cut through the roof to allow the ducting to pass into the ceiling space. This penetration is sealed around the edges with flashing and sealant to prevent water entry.

Over time - and in Perth, “over time” means faster than in cooler climates - the sealant breaks down. UV radiation, thermal movement, and vibration from the unit all contribute to sealant failure. Once the seal cracks, any water on the roof surface around the unit has a direct path inside.

This is often the hardest leak to diagnose because the unit itself is sitting over the penetration, hiding the failed sealant from view.

Corroded or Disconnected Ducting

The ductwork connecting the cooler to your ceiling can corrode at joints, especially where condensation forms on the inside of the duct. Corroded joints allow water to drip into the ceiling space rather than being contained within the duct system.

Flexible ducting connections can also pull apart over time due to vibration, wind movement, and thermal expansion, creating gaps where water vapour condenses and drips.

Roof Damage Under the Unit

The area of roof directly beneath an evap cooler takes a beating. It’s constantly damp, carries the weight of the unit, and never gets maintenance attention because the unit is sitting on top of it.

Common issues include:

• Corroded metal roofing - standing water and constant moisture cause rust on Colorbond or Zincalume sheets, sometimes rusting right through.
• Cracked or deteriorated tiles - the mounting frame can crack tiles, and moisture sitting under the unit promotes moss growth and tile deterioration.
• Degraded sarking - the waterproof membrane under the roofing material can deteriorate from prolonged moisture exposure.

Seasonal Issues: The Start-of-Summer Surprise

A large percentage of evap-related roof leak calls come in October and November - right when Perth homeowners fire up their coolers for the first time since autumn.

Here’s why:

• Seals have dried and cracked over the cold months when the unit wasn’t running.
• Drain outlets have accumulated debris from months of dust and leaf fall.
• Water-soaked components have corroded over winter from sitting rain water that pooled in the tray.
• Wasps and spiders have built nests inside drain lines and overflow outlets during the off-season.

The unit gets switched on, water starts flowing, and within hours there’s a leak that wasn’t there six months ago. The problem was developing all winter - it just needed water flowing to reveal itself.

Pre-Season Maintenance Prevents Most Leaks

Before you switch your evap cooler on for summer, a basic service should include:

1. Clean or replace the cooling pads
2. Flush and clean the base tray
3. Clear the drain and overflow outlets
4. Check the dump valve operation
5. Inspect the tray for cracks or rust
6. Check ducting connections
7. Run the unit and watch for any water escaping where it shouldn’t

This is typically done by an air conditioning technician, not a roofer. But if the leak is coming from the roof penetration or the roof surface beneath the unit, that’s when you need a roofer.

How Water Travels: Why the Stain Is Never Where You Expect

One of the frustrating things about evap cooler leaks is that the water stain on your ceiling is almost never directly below the leak source.

Water entering the roof space around an evap cooler follows the path of least resistance:

• It runs along the top of ceiling joists
• It follows the underside of battens
• It pools on the sarking (if present) and runs to the lowest point
• It wicks along timber until it finds a gap or a nail hole to drip through

A leak at the cooler penetration on the north side of the roof can easily show up as a water stain in a bedroom on the south side. This is why you can’t diagnose the leak just by looking at the ceiling stain - someone needs to inspect the roof space and the cooler installation to trace the water back to its source.

Temporary Fixes

If you’ve got water coming in and can’t get a professional out immediately, here are some short-term measures:

• Turn off the cooler. No water flowing means no new water entering through the leak. Use a split system or fans until the leak is fixed.
• Place a bucket or towel in the ceiling space under the drip point if you can safely access it.
• Don’t try to seal the roof penetration from outside yourself. You’d need to move the cooler to access it, and doing that on a roof without safety equipment is dangerous.
• Check the base tray drain from the outside - if you can see the overflow outlet from a window or the ground, check whether water is actually flowing out of it when the unit runs. If it’s not, the drain is likely blocked.

These are temporary measures only. Water in a ceiling space causes mould, rots timber, and damages plaster. The longer it sits, the more expensive the fix.

Permanent Solutions

Reseal the Roof Penetration

This is the most common fix. It involves:

1. Moving or lifting the cooler unit to access the penetration
2. Removing old, failed sealant
3. Inspecting the flashing for damage or corrosion
4. Replacing flashing if needed
5. Applying new high-quality sealant rated for UV and thermal movement
6. Repositioning the unit

This is a job that requires both roofing knowledge (for the flashing and sealant) and sometimes aircon knowledge (to safely disconnect and reconnect the unit).

Replace the Base Tray

If the tray is rusted through or cracked, it needs replacing. Your aircon technician can handle this as part of a cooler service.

Repair or Replace Damaged Roofing

If the roof material under the unit has corroded or cracked, the affected sheets or tiles need replacing. This often gets done at the same time as resealing the penetration.

For metal roofs where the sheets under the cooler have rusted significantly, a roofer may recommend replacing the affected sheet entirely and installing a new flashing system around the penetration.

Consider a Roof-Mounted Cradle or Stand

Older installations sometimes have the cooler sitting directly on the roof surface. Modern installations use a raised cradle or stand that lifts the unit off the roof, allowing air to circulate underneath and preventing moisture from being trapped against the roofing material.

If your cooler sits flat on the roof and you’re having moisture problems, upgrading to a raised mounting system during your next service can prevent future issues.

When to Call a Roofer vs an Aircon Technician

This is a common point of confusion. Here’s a simple guide:

Call an aircon technician when:

• The cooler itself is leaking (tray, pads, pump, plumbing)
• The drain or overflow is blocked
• The unit needs servicing or replacement
• Ducting inside the unit needs repair

Call a roofer when:

• The leak is coming from around the roof penetration
• The flashing or sealant around the cooler cutout has failed
• The roof material under or around the unit is damaged
• You can see rust, cracked tiles, or gaps in the roof surface near the unit

Sometimes you need both. The aircon tech services the unit and the roofer fixes the roof - it’s two different skill sets and two different problems, even though they’re happening in the same spot.

The Bigger Picture: Evap Coolers and Roof Restoration

If you’re planning a roof restoration, the area around your evaporative cooler is one of the most important zones to address. During a restoration, we:

• Remove or lift the cooler (coordinating with an aircon tech if needed)
• Strip and replace all sealant and flashing around the penetration
• Repair any damaged roofing material
• Clean and treat the area that’s been hidden under the unit
• Apply new coatings to match the rest of the restored roof
• Reseal everything to current standards

It’s one of the most satisfying parts of a restoration because you’re fixing an area that’s been slowly deteriorating unseen for years.

The Bottom Line

Evaporative coolers are a practical cooling solution for Perth’s climate, but they need regular maintenance to prevent roof leaks. Most evap-related leaks come down to blocked drains, failed sealant, or corroded components - all preventable with pre-season servicing.

If you’ve got a water stain on your ceiling near your evap cooler, don’t ignore it. The longer water sits in your ceiling space, the more damage it does. Get the cooler serviced by an aircon tech, and if the leak is coming from the roof penetration or the roof surface itself, give us a call on 08 6388 4492 or get a free quote online.