What Causes Metal Roof Leaks? Common Causes, Detection & Repairs

Short answer: Most metal and Colorbond roof leaks come from failed seals at roof penetrations (vents, skylights, air conditioners), loose or worn roofing screws, poorly lapped or short flashings, and water drawn into sheet overlaps. Rust, blocked valleys and box gutters, and storm damage are the next most common causes. Condensation under the roof can also look like a leak.

A leaky roof is a showstopper when you consider the damage it can cause to your home. Whether it’s during or after rainfall, a dripping ceiling (or stained plaster) has a way of stopping you in your tracks.

According to an NRMA study, close to 40% of Australians have had a water leak in their home, while 21% have experienced it more than once. If you’re among those who have been troubled with a roof leak, you’re probably wondering where the leak came from, whether it could have been prevented, and how to fix it.

For those with metal roofs, including Colorbond and older corrugated iron roofs, leaks often stem from very specific causes. But don’t worry, we’re here to help. In this guide we’ll answer all your questions about metal roof leaks and how to fix them.

How to Detect Roof Leaks

When was the last time you had your roof checked out? If it’s been over a year, you’re not alone. In fact, 59% of Australians have not inspected their roofs in more than a year. But with more than 40% of all building problems linked to water infiltration, regular roof inspections are vital. Catching small issues early can help prevent them from turning into major repairs.

Metal roof leak detection: 4 easy steps

A leaky roof isn’t always obvious, but if left undetected it can lead to extensive (and often costly) damage down the road. To catch a leak early, you’ll need to play detective. Take these four steps to figure out if your metal roof is leaking.

1. Check the interior of your house

Look for signs of a leak inside your home.

• Check your walls and ceiling for water stains that are typically brown or yellowish in colour.
• Take note of any mould or peeling paint, as it could indicate water seepage or trapped moisture.

2. Inspect your roof from the exterior

Examine the roof for visible damage.

• Look for damaged panels, loose flashing, or missing fasteners.
• Watch out for any excessive movement of the metal panels when inspecting the roof (for safety, consider hiring a professional to conduct the inspection).

3. Examine gutters and downpipes

Assess the condition of your drainage system.

• Look for rust, holes, or sagging gutters that can cause water to overflow.
• Ensure downpipes are channelling water away from your home’s foundation, to avoid water gathering at the base.

4. Conduct a water test

If you’ve completed all the other steps and are still unsure of the leak’s source, try a water test. This involves spraying water on different parts of the roof to help you identify where the water is getting in. If you’d rather not get on the roof, a professional roof leak detection service can find the source safely

What Causes Metal Roof Leaks?

Metal roofs are incredibly durable, strong, and visually appealing. In fact, many roofing professionals claim that metal roofs are less prone to leaks compared to tile roofs. 

However, improper installation, subpar repairs, and extreme weather conditions can make metal roofs susceptible to leaks. The good news is that by understanding the cause of the leak you can address the issue immediately. Here are the top reasons why metal roofs might leak.

1. Roof penetrations (Vents, Skylights, AC Units)

Roof penetrations, such as vents, skylights, and AC units, can be a common source of leaks in metal roofs if the flashings or seals deteriorate or aren’t installed properly.

A couple of years ago, our team revisited a commercial property where we had installed hopper flashings to address previous leaks. We had received reports of new leaks in the same area. Upon site inspection, we identified one of the leading causes of metal roof leaks: penetrations added after the roof was installed.

In this case, the air conditioning units had been poorly installed, causing significant issues with flashings. The flashings were too short to adequately cover the ribs of the metal roof sheets, and they weren’t properly sealed. In addition, the trays of the metal roof sheets hadn’t been bent up to prevent water seeping in at the front of the penetration.

At the rear of the penetration, the flashings intended to redirect the water away from the air conditioning unit had been lapped the wrong way. It was a poor job that didn’t meet Australian Plumbing Standards and resulted in consistent leaks.

Our solution: We installed wider, longer flashings and ensured proper sealing at the joins. We also advised changing the direction of the laps over the roof sheets so that the upper lap overlaps the under lap, preventing water from seeping in.

2. Misaligned or Worn Roofing Screws

Roofing screws that aren’t aligned properly cause the majority of roof leaks. Their job is to hold the metal panel in place and create a tight seal with a rubber washer to prevent water from entering. When screws are overdriven (too tight), underdriven (too loose), or misaligned, they compromise the seal, allowing water to enter and cause leaks.

Even correctly installed screws can work loose over time. Metal roof sheets expand in the heat and contract as they cool, and over years of hot Melbourne summers and cold nights, that movement slowly loosens screws and wears the washers. The rubber also breaks down in the sun, so on older roofs it’s common to find washers that have cracked or perished.

How we fix it: We replace failed screws with new roofing screws and washers, using a slightly larger gauge where the original hole has worn.

3. Seams and Overlaps

When two metal panels overlap small gaps can form, creating a risk of water leaking. This is called a capillary draw, where water travels between the tiny gap where the panels meet. During installation sealant is typically added between the two metal pieces, but this can wear down over time. If you’ve noticed your metal roof leaking at an overlap, this is usually the reason.

Low-pitched roofs are especially prone to this. On a roof with very little fall, heavy rain can’t drain fast enough, so water backs up the laps and gets pushed under the sheet above. This is common on older skillion and verandah roofs around Melbourne.

How we fix it: Depending on the roof, we reseal or replace the lapped sheets, or on very flat roofs, recommend replacing them with a profile designed for low pitches.

4. Missing or Failed Sealant

During roof installation, sealants are used under areas such as ridge caps, flashings, pitch pans, and reglets to keep water out. However, these sealants degrade over time and require regular maintenance. If sealants are missing or failing, water can seep in through the trims.

How we fix it: We remove the old, failed sealant before applying fresh, neutral-cure silicone that’s compatible with Colorbond. Putting new sealant over old rarely lasts.

5. Flashings at Walls, Chimneys and Ridges

Flashings are the folded metal trims that seal the joins where your roof meets something else, such as a wall, a chimney, the ridge, or a change in roof pitch. They’re one of the most common sources of leaks we find on Melbourne homes.

The usual problems are flashings that are too short, haven’t been turned up properly behind the wall cladding, or have pulled away from the wall as the building moves. On older homes, flashings are often lead or galvanised steel that have simply reached the end of their life. Leaks at the chimney are particularly common, because the brickwork and the roof move differently over time.

How we fix it: We replace short or failed flashings with new, correctly sized Colorbond flashings, turned up and sealed to Australian standards. 

6. Rust and corrosion

Colorbond and Zincalume roofs are built to last, but they can rust if the protective coating is damaged or exposed to the wrong materials. Once rust gets through the sheet, it creates small holes that let water straight in.

The most common causes we see are:

• Swarf: tiny metal filings left on the roof after cutting or drilling, which rust and stain the sheet. This is often left behind by other trades, such as solar or antenna installers.
• Incompatible metals: water running off copper pipes or lead flashings onto a Colorbond or Zincalume roof speeds up corrosion.
• Cut edges and scratches: exposed steel at cut ends or scratched areas.
• Debris and ponding water: leaves and dirt that sit on the roof hold moisture against the sheet.
• Age: older galvanised iron roofs, common on Melbourne period homes, eventually rust through.

How we fix it: Small areas of surface rust can sometimes be treated. Where rust has gone through the sheet, the affected sheets need to be replaced. If rust is widespread, a Colorbond roof replacement is usually more cost-effective than patching.

7. Blocked valleys, box gutters and rainheads

Valleys, box gutters and rainheads carry a lot of water off the roof. When they block with leaves, dirt or tennis balls, the water has nowhere to go. It backs up and overflows under the roof sheets and flashings, and into the ceiling.

Box gutters are a particular problem because they sit inside the roof line, so an overflow goes straight inside rather than over the edge. Undersized box gutters, missing overflows and rusted sumps are common causes of serious leaks in heavy rain.

How we fix it: We clear blockages, repair or replace rusted valleys and box gutters, and add overflows where they’re missing. 

8. Storm, Hail and Foot Traffic Damage

Melbourne’s storms can lift flashings, dent roof sheets and loosen screws. Hail can crack skylights and puncture old, thin roof sheets. Falling branches can also tear sheets or dislodge the ridge capping.

Foot traffic is another common cause that homeowners don’t think of. Solar installers, antenna and air conditioning technicians, and even roof cleaners can dent sheets, crush the ribs or leave swarf behind. If a leak appears soon after someone has been on your roof, that’s the first place to look. 

How we fix it: After a storm, we inspect the roof, secure any lifted flashings and replace damaged sheets. We can also provide a report for your insurance claim.

9. Condensation

Not every drip is a leak. On metal roofs, warm moist air from inside the house can condense on the underside of the cold roof sheets overnight, then drip onto the ceiling. It often looks just like a leak, but it happens on cold, clear mornings rather than when it rains.

If water appears when it hasn’t rained, condensation is the likely cause. It’s usually fixed with the right insulation blanket and ventilation, rather than roof repairs.

Can You Prevent Roof Leaks?

Yes. In fact, 93% of water leaks can be minimised or prevented altogether. So, what can you do to help prevent your metal roof from leaking? Focus on ensuring proper flashing, using waterproof products, and maintaining your roof regularly. Responding quickly at the first sign of a leak or after a big storm is also crucial.

How to Repair Your Metal Roof Leak

So, how do you fix a leaking metal roof? It depends on what was causing the leak in the first place, and prior to conducting any repairs at home, it’s best to consult with a metal roof leak repair expert first. Let’s dive into what the repairs might look like.

Roof Sealant

Roof sealant can work as a quick, temporary solution for small leaks. They’re a great solution for addressing minor problems immediately before they can cause mould or more serious damage.

Remove or Replace Screws

As explored earlier, roofing screws that aren’t properly aligned can cause water entry points. This can be fixed by removing and replacing the screws. This repair method can often be incredibly labour intensive and time consuming, making it a less-practical solution for larger repairs or severely damaged roofs.

Roof Replacement

If your roof is at the end of its lifespan, or requires extensive repairs, replacing individual metal panels or even the entire roof may be a more efficient and cost-effective solution in the long run. This option, while the most costly, provides a fresh start with a new roof that can offer durability and peace of mind for years to come.

Does Your Metal Roof Need Leak Repair?

If your house is filled with buckets and DIY patch jobs every time it rains, it’s time to consider if your metal roof needs to be repaired.

Call a licensed roof plumber if you notice any of these:

• Water coming through downlights, exhaust fans or cornices when it rains
• Ceiling stains that grow after each storm
• Rust spots or holes you can see from the ground
• A leak that comes back after it’s been sealed
• Lifted flashings or ridge capping after strong winds
• A metal roof that’s more than 25 to 30 years old and has never been inspected

Our team handles metal roof leak repair in Melbourne, from finding the source to fixing it properly the first time.

Metal Roof Leak FAQs

Why is my Colorbond roof leaking?

The most common causes are failed seals around penetrations like skylights and air conditioners, loose or worn roofing screws, short or poorly lapped flashings, and water backing up at sheet overlaps. Blocked valleys and box gutters, rust, and storm damage are the next most likely culprits.

How long can you leave a leaking roof?

Not long. Even a small leak can soak insulation, stain plaster and cause mould within days or weeks, and wet ceilings can sag or collapse. If you can’t get a repair straight away, put down a bucket, move valuables, and book a roof plumber as soon as possible.

When should I worry about a roof leak?

Worry straight away if water is coming through light fittings, the ceiling is sagging, or the leak is getting worse with each storm. Turn off power to any wet light fittings and call a professional. Even a small drip should be looked at before winter.

Can I fix a leaking metal roof myself?

You can put down a bucket or apply a temporary patch from inside, but repairs on the roof itself, like replacing screws, sheets or flashings, are plumbing work in Victoria and need a licensed roof plumber. Working on a roof is also dangerous without the right safety equipment.

Does silicone fix a metal roof leak?

Only as a short-term fix. Silicone can seal a small gap for a while, but it breaks down in the sun and won’t fix the cause, such as a short flashing or a worn screw. If you use it, choose a neutral-cure silicone suited to Colorbond, as some sealants can damage the coating.

How often should roofing screws be replaced?

There’s no set schedule. Screws and their rubber washers wear out at different rates depending on sun exposure and roof movement. Have your roof inspected every year or two, and replace any screws that are loose, rusted or have cracked washers.

How long does a Colorbond roof last?

A well-installed and maintained Colorbond roof can last for decades. The exact lifespan depends on your location and conditions, such as how close you are to the coast. BlueScope’s warranty also varies by location, so check the details for your area. 

Will home insurance cover a leaking metal roof?

It depends on your policy and the cause. Many policies cover sudden storm damage, but not leaks caused by wear and tear, rust or poor maintenance. Check your product disclosure statement, and take photos of any damage before repairs.