Skylight Leaks: Why They Happen and How to Fix Them

10 min read

Skylights bring natural light into rooms that would otherwise be dark - hallways, bathrooms, internal living areas. They’re popular in Perth homes, and when they work properly, they’re fantastic. When they don’t, they’re one of the most frustrating roof problems you can have.

Of all roof penetrations - pipes, vents, aerials, whirlybirds - skylights leak more often than any other. It’s not because they’re poorly made. It’s because of where they sit and how much water they handle.

Understanding why skylights are prone to leaking helps you identify problems early and choose the right fix.

Why Skylights Leak More Than Other Penetrations

A plumbing vent pipe is a small, circular penetration sealed with a rubber boot. Simple geometry, one seal point, water flows around it easily. A skylight is fundamentally different.

They interrupt the roof plane. A skylight is a large rectangular opening cut into the roof surface. Water flowing down the roof hits the skylight frame and has to be redirected around it. That redirection requires flashing on all four sides - head flashing at the top, sill flashing at the bottom, and step flashing along both sides.

They have more flashing joints. Every corner where two pieces of flashing meet is a potential failure point. A skylight has at least four corner joints, plus the junctions between the flashing and the frame. Compare that to a pipe boot with zero joints.

They sit in the direct path of water flow. Unlike a whirlybird sitting on a ridge, a skylight sits mid-slope where the water volume is highest. During Perth’s heavy winter rains, the volume of water flowing over and around a skylight is substantial.

They move. Skylights expand and contract with temperature changes. Perth’s temperature range - from winter mornings near zero in the hills to summer afternoons above 40 - means constant thermal cycling. This movement stresses sealant joints, and over time, they crack and separate.

All of this means that even a perfectly installed skylight will eventually need maintenance. And an imperfectly installed one will leak much sooner.

Common Causes of Skylight Leaks

Degraded Flashing Sealant

This is the most common cause. The sealant used at flashing joints, around the frame, and between the flashing and the roof surface has a finite lifespan. In Perth’s UV-intense climate, silicone and polyurethane sealants typically last 8-15 years before they begin to crack, shrink, and pull away from surfaces.

Once the sealant fails, water has a direct path into the roof cavity. The leak may be small at first - just enough to leave a damp patch after heavy rain - but it will get worse as the sealant continues to deteriorate.

Signs of sealant failure include visible cracking or separation around the skylight frame, and leaks that only appear during heavy rain or when rain comes from a specific direction (indicating which side’s sealant has failed).

Corroded or Displaced Flashing

Over time, the metal flashing around a skylight can corrode, especially in Perth’s coastal suburbs where salt spray accelerates metal deterioration. Corrosion thins the metal and creates pinholes that let water through.

Flashing can also be displaced by wind, thermal movement, or trades who’ve walked on the roof without proper care. Even a slightly lifted flashing edge creates a capillary gap that draws water underneath.

Cracked Acrylic Dome

Older skylights use acrylic domes that become brittle with UV exposure. Perth’s intense sunlight is particularly hard on acrylic - after 15-20 years, domes develop crazing (fine surface cracks), yellowing, and eventually structural cracks.

A cracked dome doesn’t always leak immediately. Fine crazing lets in moisture slowly, and you might notice condensation inside the dome before you see an actual drip. But once a crack opens up, water comes through freely.

Condensation (Not Actually a Leak)

This is the one that catches people out. You see water dripping from your skylight, you assume it’s leaking, but the roof is completely watertight. The water is condensation forming inside the skylight shaft.

Condensation occurs when warm, moist air from your living space rises into the skylight shaft and meets the cold underside of the dome or glass. The moisture condenses on the cold surface and drips back down.

This is most common in:

• Bathrooms - high humidity from showers
• Kitchens - steam from cooking
• Laundries - moisture from dryers
• Winter - when the temperature difference between inside and outside is greatest

The telltale signs that it’s condensation rather than a leak:

• It happens on cold mornings, not during rain
• Water appears on the inside surface of the glass or dome
• The shaft or frame shows no water staining from above
• It stops when you open a window or run an exhaust fan

Blocked Weep Holes

Many skylight designs include weep holes - small drainage channels in the frame that allow any water that gets past the primary seal to drain back out onto the roof. If these weep holes become blocked with dirt, paint, or debris, water accumulates inside the frame and eventually overflows into the shaft.

This is a surprisingly common issue on skylights that have been painted around during roof restorations. If the painter isn’t careful, paint can block the weep holes. The skylight works fine for months, then one heavy downpour overwhelms the frame’s capacity and water pours in.

Incorrect Installation

Some skylights leak from day one because they were installed incorrectly. Common installation errors include:

• Flashing not integrated with tiles or sheets properly - the flashing should tuck under the course above and over the course below, just like any other roof flashing. If it’s just siliconed to the surface, it will eventually fail.
• No upstand at the head - the top edge of the skylight needs a raised flashing (upstand) that directs water around the frame rather than letting it flow straight into the junction.
• Shaft not sealed - the shaft between the roof surface and the ceiling should be vapour-sealed to prevent condensation. Many installations skip this step.

How to Distinguish a Real Leak from Condensation

Before spending money on repairs, make sure you’re actually dealing with a leak. Here’s how to tell:

It’s a leak if:

• Water appears during or shortly after rain
• The amount of water correlates with rainfall intensity
• Water staining appears on the shaft walls (indicating water entering from above and running down)
• The leak persists regardless of indoor humidity levels

It’s condensation if:

• Water appears on cold mornings regardless of rain
• Droplets form on the underside of the glass or dome
• It’s worse in high-humidity rooms (bathroom, kitchen)
• It stops when you improve ventilation
• The shaft walls are dry - water only appears on the glazing surface

If you’re still unsure, try this test: on a dry day, run a hose over the skylight for 10 minutes. If water appears inside, it’s a leak. If it stays dry, you’re dealing with condensation.

Repair Options

Reseal: $300-$800

If the flashing is in good condition but the sealant has failed, resealing is the simplest and cheapest fix. This involves removing the old sealant, cleaning the surfaces, and applying new sealant to all joints.

A good reseal should last another 8-15 years depending on the sealant used and the UV exposure. This is the appropriate fix when the flashing, frame, and dome are all in good condition and only the sealant has reached end of life.

Reflash: $800-$2,000

If the flashing itself is corroded, damaged, or was incorrectly installed, it needs to be replaced. This is a bigger job because it involves lifting tiles or sheets around the skylight, removing the old flashing, installing new flashing properly integrated with the roof surface, and resealing everything.

Reflashing is warranted when you can see visible corrosion, holes, or displaced sections of flashing. It’s also the right fix if the skylight has been leaking since a previous repair that only addressed the sealant without fixing the underlying flashing repair issue.

Replace Dome: $500-$1,500

If the acrylic dome is cracked, crazed, or heavily yellowed, replacing just the dome can restore the skylight without the cost of a full replacement. This works well when the frame and flashing are still in good condition.

Dome replacement also improves energy efficiency and light transmission - a 20-year-old yellowed dome blocks a significant amount of light and has poor insulation properties compared to a new unit.

Full Skylight Replacement: $1,500-$4,000

When the frame is corroded, the flashing has failed, and the dome is deteriorating, it makes more sense to replace the entire unit rather than patchwork multiple components.

Modern skylights are significantly better than units installed 20-30 years ago. Current models feature:

• Double-glazed glass instead of single-skin acrylic - better insulation, no yellowing, longer lifespan
• Improved flashing systems - engineered to integrate with specific roof types rather than relying on sealant
• Better thermal break design - reduced condensation issues
• Options for ventilation - opening skylights that help manage moisture and heat

If your skylight is over 20 years old and leaking, full replacement is usually the better investment than repairing a unit that’s near end of life.

When Does Repair Make Sense vs. Full Replacement?

Repair makes sense when:

• The skylight is under 15 years old
• The frame and flashing are structurally sound
• The issue is limited to sealant failure or a single component
• The dome/glass is clear and uncracked
• The total repair cost is under half the replacement cost

Replacement makes sense when:

• The skylight is over 20 years old
• Multiple components have failed (sealant, flashing, and dome)
• The frame itself is corroded
• You’re having the roof restored anyway (combining the work saves labour costs)
• You want to upgrade from acrylic to glass
• Previous repairs haven’t resolved the leaking

Consider removing rather than replacing when:

• The skylight serves a room that no longer needs it (renovated layout)
• You’re tired of the ongoing maintenance
• The opening has caused structural issues with the roof frame

Removing a skylight and filling the opening with matching tiles or sheets is typically $800-$1,500 and eliminates a maintenance point permanently.

Condensation Solutions

If your skylight problem is condensation rather than a leak, the fix is about managing moisture and temperature:

• Improve ventilation - install or upgrade exhaust fans in bathrooms and kitchens. Make sure they vent to the outside, not into the roof cavity.
• Insulate the shaft - an uninsulated shaft creates a cold surface for condensation to form on. Insulating the shaft walls reduces the temperature differential.
• Use a diffuser - some skylight manufacturers offer condensation diffusers that catch drips before they fall. Not a fix for the root cause, but stops water from landing on your floor.
• Upgrade to double-glazed - double glazing keeps the inner surface warmer, dramatically reducing condensation. If your single-glazed skylight has persistent condensation issues, replacement with a double-glazed unit is the long-term solution.

Perth-Specific Considerations

Perth’s climate creates particular challenges for skylights:

UV intensity. Perth has some of the highest UV levels in Australia. Sealants and acrylic domes degrade faster here than in southern capitals. Budget for shorter maintenance intervals - reseal every 8-10 years rather than waiting for failure.

Thermal cycling. Summer days above 40 degrees followed by nights in the low 20s create significant expansion and contraction. Winter is less extreme but still cycles. This constant movement is the primary driver of sealant failure.

Heavy winter downpours. Perth’s rain comes in concentrated bursts rather than steady drizzle. The volume of water flowing over a skylight during a heavy downpour is substantial, and any weakness in the flashing or sealant will be exposed.

Coastal salt. For homes in Scarborough, Cottesloe, Fremantle, and other coastal suburbs, salt spray accelerates corrosion of metal flashing and frames. Coastal skylights need more frequent inspection and maintenance.

The Bottom Line

Skylights are the highest-maintenance penetration on your roof. That doesn’t mean they’re not worth having - the natural light they provide is genuinely valuable. But they need attention.

If your skylight is dripping, start by determining whether it’s a genuine leak or condensation. If it’s a leak, get the leak repair assessed sooner rather than later - water entering your roof cavity causes damage that escalates quickly.

For skylights under 15 years old with isolated sealant failure, a professional reseal is cost-effective and long-lasting. For skylights over 20 years old with multiple issues, replacement with a modern double-glazed unit is usually the smarter investment.

And if you’re getting a roof restoration done, that’s the ideal time to address skylight issues. The scaffolding is up, the roof is being worked on, and combining the work is more cost-effective than a separate job later.