Roof Flashing: What It Is, Why It Fails, and How to Fix It

10 min read

If you’ve ever had a roof leak and been told “the flashing’s gone,” you’re not alone. After failed pointing, deteriorated flashing is the second most common cause of roof leaks in Perth homes.

But flashing is one of those roofing components most people have never thought about until it causes a problem. Here’s a plain-language guide to what it is, where it is, why it fails, and what to do about it.

What Is Roof Flashing?

Flashing is thin metal sheeting - usually Colorbond steel, galvanised steel, zinc, or lead - installed wherever the roof surface meets another surface or changes direction. Its only job is to stop water getting in at these junctions.

Think about all the places on your roof where water could potentially enter if there wasn’t something to stop it:

• Where the roof meets a wall
• Where two roof planes meet in a valley
• Around chimneys
• Around vents and pipes
• Around skylights
• Where an extension meets the original roof
• At the edges of the roof (eaves and barge)

Every one of those locations has flashing - a shaped piece of metal that bridges the gap and directs water safely onto the roof surface rather than into the building.

Types of Roof Flashing

Different junctions need different flashing configurations. Here are the main types you’ll find on Perth homes:

Step Flashing

Used where a roof slopes down alongside a wall (common where a single-storey section meets a double-storey wall). Step flashing consists of individual L-shaped pieces, each overlapping the one below, stepping down the roof in line with the tiles or sheets.

It’s called “step” flashing because each piece is offset - stepping - down the slope. This allows water to flow over each piece and down to the next without pooling.

Apron Flashing

A single continuous piece of bent metal installed horizontally where a roof slope meets a vertical wall at the bottom of the slope. You’ll see apron flashing at the base of walls that rise above a lower roof section.

Apron flashing is simpler than step flashing because the junction is horizontal rather than running down a slope.

Valley Flashing

Valleys are the internal angles where two roof planes meet and channel water downward. Valley flashing is a wide strip of metal laid along this channel underneath the tiles or sheets.

Valley flashing handles more water than any other flashing on your roof - every drop of rain that falls on both adjacent roof planes flows into the valley. This is why valley failures cause some of the most significant leaks.

Perth homes typically have valley flashing made from:

• Colorbond steel - the most common on modern homes
• Galvanised steel - common on homes built pre-1990s
• Lead - found on some older homes, extremely durable but increasingly replaced due to health concerns

Counter Flashing

A second piece of flashing installed over step flashing or apron flashing, embedded or sealed into the wall above. Counter flashing covers the top edge of the primary flashing to prevent water running behind it.

This two-layer system is the gold standard for wall-to-roof junctions. Unfortunately, many Perth homes only have single-layer flashing without counter flashing, which is one reason wall junctions leak so often.

Pipe and Vent Flashing

A cone-shaped piece of rubber or metal (or a combination) that fits around plumbing vents, antenna mounts, and other pipe penetrations through the roof. The base sits flat on the roof surface while the cone seals around the pipe.

These are sometimes called “roof boots” or “dektite” (after a common brand name).

Chimney Flashing

Chimneys require a combination of step flashing on the sides, apron flashing at the base, and a “back pan” or “cricket” at the top to divert water around the chimney rather than letting it pool behind it.

Chimney flashing is the most complex flashing system on a roof and often the first to fail - there are simply more joints, more sealant points, and more opportunities for something to go wrong.

Why Flashing Fails in Perth

Perth’s climate is particularly hard on flashing. Here’s what goes wrong:

Thermal Movement

Metal flashing expands and contracts with temperature changes. In Perth, where roof surface temperatures can swing 50+ degrees between a summer afternoon and a winter night, that’s a lot of movement over the course of a year.

This constant movement causes:

• Sealant to crack and separate from the flashing or the surface it’s sealed to
• Nail or screw holes to elongate as the flashing shifts back and forth
• Overlapping joints to open as different pieces move at different rates

Thermal movement is the primary cause of flashing failure in Perth.

UV Degradation of Sealants

Every flashing installation relies on sealant at some point - usually where the flashing meets a wall, where two pieces overlap, or where the flashing meets the roof surface. These sealants (silicone, polyurethane, or bitumen-based) break down under Perth’s intense UV radiation.

A sealant joint that might last 15-20 years in a cooler climate may only last 8-12 years in Perth’s full sun. North and west-facing flashings fail fastest because they cop the most UV.

Corrosion

Different metals corrode at different rates, but all metal flashing eventually corrodes in Perth’s conditions:

• Galvanised steel - the zinc coating wears away over time, exposing bare steel to moisture and oxygen. Coastal homes see this happen significantly faster.
• Colorbond - the paint protects the steel, but scratches, cut edges, and damaged areas expose bare metal that rusts.
• Lead - extremely corrosion-resistant but softens in heat and can crack from thermal fatigue.
• Dissimilar metals - where two different metals touch (e.g., galvanised flashing against Colorbond sheeting), galvanic corrosion accelerates deterioration of the less noble metal.

Poor Original Installation

A surprising number of flashing failures in Perth aren’t caused by age or climate - they’re caused by poor installation in the first place.

Common installation problems include:

• Insufficient overlap - flashing pieces that don’t overlap enough, allowing wind-driven rain underneath
• No counter flashing - single-layer wall flashings that rely entirely on sealant to keep water out
• Wrong sealant - using silicone where polyurethane is needed, or using a sealant not rated for the temperature range
• No kick-out diverters - where step flashing meets a gutter, a kick-out piece should divert water into the gutter rather than behind it. Many installations skip this.
• Flashing too narrow - undersized flashing that doesn’t extend far enough under tiles or up a wall

These installation shortcuts often don’t cause problems immediately but fail within 5-10 years - well before properly installed flashing would show any issues.

Signs Your Flashing Is Failing

Here’s what to look for:

From Inside

• Water stains on walls or ceilings near wall-to-roof junctions - this is the classic sign of failed step or apron flashing.
• Water stains in the centre of the ceiling - often indicates valley flashing failure, especially if the stain is along a line rather than a single point.
• Water appearing around a vent pipe or skylight - pipe flashing or skylight flashing failure.
• Musty smell in certain rooms after rain - moisture is getting in somewhere, and flashing junctions are a primary suspect.

From Outside (Ground Level)

• Rust staining running down from a flashing location - visible rust streaks on walls or roof surface below a flashing point.
• Visible gaps between flashing and wall - you can sometimes see where flashing has pulled away from a wall surface.
• Lifted or buckled flashing - thermal movement can cause flashing to bow or lift, particularly apron flashing.
• Missing sealant - if you can see the top edge of step flashing without any sealant or counter flashing covering it, water is getting behind it.

From the Roof (Professional Inspection)

• Cracked or missing sealant at all flashing edges
• Corrosion or rust on flashing surfaces
• Nail or screw heads that have rusted through
• Flashing that moves when touched - it should be firmly fixed
• Debris accumulated behind flashings - leaves and dirt trapped against flashing cause moisture retention and accelerate corrosion

Repair vs Replacement

Whether to repair or replace flashing depends on the extent of the damage and the type of failure.

When Repair Is Sufficient

• Sealant failure only - if the flashing metal is in good condition but the sealant has cracked, removing old sealant and applying new high-quality sealant can be effective. This is a common repair during routine roof maintenance.
• Single point failure - one piece of step flashing has lifted or one joint has opened. Resecuring and resealing the affected area is straightforward.
• Minor surface corrosion - light rust on galvanised flashing can be treated, primed, and painted to extend its life.

When Replacement Is Necessary

• Widespread corrosion - if the flashing has rusted through in multiple places, it needs replacing. Patching corroded flashing is a short-term fix at best.
• Structural deformation - flashing that has buckled, cracked, or split from thermal fatigue can’t be reliably repaired.
• Repeated failure - if the same flashing point has been resealed multiple times and keeps leaking, the flashing itself is likely the problem and needs replacing.
• Valley flashing failure - valley flashings carry too much water to risk a patch repair. If a valley is leaking, replace the full valley flashing.

Temporary Fixes

If you’ve got a flashing leak and can’t get a professional out immediately:

• Roof sealant from a hardware store - products like Sika roof sealant can be applied to visible cracks and gaps in accessible flashing as a temporary measure. Clean the surface first and apply in dry conditions.
• Flashing tape - self-adhesive bituminous tape can be applied over small cracks or gaps. It’s not a permanent fix but can stop a leak for weeks or months.
• Do not use silicone from a tube as a long-term fix - bathroom silicone is not formulated for roof temperatures and UV exposure. It will fail within months.

These are strictly temporary. Get a professional repair done as soon as practical.

How Flashing Is Addressed During Restoration

During a full roof restoration, every piece of flashing on the roof is assessed and addressed:

1. Inspection - all flashings are checked for corrosion, sealant failure, loose fixings, and proper overlap.
2. Removal of old sealant - failed sealant is stripped back to clean metal and clean wall surfaces.
3. Replacement where needed - any flashing that’s corroded, deformed, or undersized is replaced with new material.
4. Resealing - all flashing edges and junctions receive fresh, high-quality sealant rated for roof conditions.
5. Coating - exposed flashing surfaces are coated as part of the overall roof coating system, providing an additional layer of UV and corrosion protection.

This comprehensive flashing overhaul is one of the major benefits of a full restoration compared to piecemeal repairs. Every potential leak point is addressed in a single project, rather than chasing individual leaks as they appear over the years.

Common Perth Flashing Problems by Home Era

Different eras of Perth housing have characteristic flashing issues:

Pre-1970s Homes

• Lead flashing that’s cracked from decades of thermal cycling
• Minimal or no counter flashing
• Galvanised valleys that may be completely corroded

1970s-1980s Homes

• Galvanised flashing showing significant corrosion
• Poor original installation common in the building boom
• Valleys that are undersized for modern rainfall intensity expectations

1990s-2000s Homes

• Better installation standards but sealants reaching end of life
• Colorbond flashing in good condition but needing reseal
• Pipe flashings (rubber boots) degrading from UV

2010s Onwards

• Generally well-installed with modern materials
• Early sealant failure in some cases from poor product selection
• Minimal issues unless there were installation shortcuts

The Bottom Line

Flashing is the silent protector of your roof - you don’t notice it until it fails, but when it fails, the leaks can be significant. In Perth’s climate of extreme heat, intense UV, and coastal salt exposure, flashing has a harder life than in most Australian cities.

If you’re seeing water stains near wall junctions, around vents, or along ceiling lines that correspond with valleys, failed flashing is the most likely cause. A professional roof inspection can identify exactly where the failure is and whether it needs flashing repair or replacement.

If your roof is due for a restoration, that’s the best time to address all flashing in one go - it’s more thorough and more cost-effective than fixing individual points as they fail.

Got a leak you suspect is flashing-related? Call us on 08 6388 4492 and we’ll find the source.