A skylight leak does not always originate where the water appears.

Water can enter through the roof-to-skylight interface, flashing, underlayment, curb, frame seals or nearby roof conditions and then travel before appearing indoors. NRCA guidance specifically notes that the point where water drips inside a building is not necessarily directly above the actual point of entry, and that condensation and other building components should also be considered during diagnosis.

There is another common mistake: treating every wet skylight as a roof leak.

Condensation can form on a cold interior glass or frame surface when indoor air reaches its dew point. A correct diagnosis therefore starts with:

Leak after rain? Condensation in humid/cold conditions? Or a combination of both?

Once the source is established, repair the water-entry or moisture-generation mechanism, not simply the visible stain.

Quick Summary

SymptomWhat to investigateTypical responseMoisture on interior glassCondensation conditionsCheck indoor humidity, surface temperature and ventilationWater appears after rainRoof/skylight water pathInspect flashing, curb and roof interfaceWater at frameFrame seal + flashing + drainageTrace the entry path before resealingWater appears below skylightEntire roof penetrationInspect above the visible stainLeak after wind-driven rainWeather-exposed interfacesInspect flashing, seals and drainageOld recurring leakProduct + roof interfaceDetermine whether repair or replacement is appropriate

The key point: diagnose the source before applying sealant. A visible water stain identifies where water arrived, not necessarily where it entered.

Is It Really a Skylight Leak?

Before repairing anything, separate two different moisture mechanisms.

Condensation

Condensation forms when an interior surface becomes cold enough for water vapour in the indoor air to condense.

Possible contributors include:

• high indoor relative humidity
• low outdoor temperature
• low interior glass-surface temperature
• cold frame surfaces
• thermal bridges
• inadequate insulation around the curb
• poor air sealing

A skylight is particularly exposed to this problem because its glazing and frame separate a conditioned interior from the exterior.

NFRC-certified skylights can carry a condensation-resistance index, in addition to U-factor, SHGC and VT ratings.

True Water Leakage

A roof leak involves liquid water entering through the exterior weather barrier or roof/skylight interface.

Possible sources include:

• flashing
• roof membrane
• underlayment
• curb
• frame-to-curb seal
• drainage path
• fastener penetration
• adjacent roof condition

Water can then travel along framing or roof layers before appearing inside.

Therefore, “water at the skylight = failed skylight” is not a sufficient diagnosis.

Why Skylight Flashing Matters

A skylight creates an opening in the roof assembly.

The roof covering and weather-control layers therefore have to transition around that opening without creating an uncontrolled path for water.

Flashing is a major part of that transition.

The 2024 IRC requires flashings at roof openings and intersections, and requires flashing to be installed so that moisture is prevented from entering through the roof at those locations. The code also ties installation to applicable manufacturer instructions.

The practical water-management sequence is generally:

roof surface → flashing → underlayment/membrane → curb/frame interface → drainage path

The exact arrangement depends on the roof system and skylight manufacturer.

That means “replace the flashing” is not automatically the correct repair. The flashing must be examined as part of the entire roof penetration.

The Most Common Skylight Leak Areas to Inspect

Rather than assuming one cause, inspect the complete interface.

1. Head Flashing

The upslope portion of the flashing has to manage water flowing toward the skylight.

Look for:

• discontinuous flashing
• damaged metal
• incorrect laps
• failed sealant
• debris
• incompatible materials

2. Side Flashing

Side flashing manages water running alongside the skylight.

Check for:

• open joints
• poor laps
• corrosion
• punctures
• improper integration with roofing materials

3. Sill / Apron Condition

The downslope section needs to direct water away from the opening.

Problems here can include:

• poor drainage
• damaged flashing
• incorrect overlap
• sealant failure
• water accumulation

4. Curb-to-Roof Interface

The curb is the structural/weather interface between the roof and skylight.

Inspect:

• membrane termination
• flashing
• sealant
• fasteners
• counterflashing where applicable
• movement-related separation

5. Frame-to-Curb Seal

A skylight can be properly flashed and still have leakage at the interface between the skylight frame and curb.

This is one reason simply replacing visible roof flashing may not solve the problem.

Step 1: Determine Whether Weather or Humidity Triggers the Problem

Record when the moisture appears.

ConditionMore likely direction of investigationOnly after rainfallRoof/flashing/interfaceOnly after wind-driven rainWeather-exposed joints/interfacesCold mornings without rainCondensationDuring very humid periodsCondensationDuring both rain and cold weatherCould be either or bothPersistent regardless of weatherInvestigate broader moisture sources

This is not a diagnosis by itself. It is a way to determine where to investigate first.

A roof leak can occur during a dry period if water entered earlier and accumulated somewhere in the assembly. Likewise, condensation can occur during rainy weather, so “it happened while raining” does not prove a roof leak.

Step 2: Inspect the Interior

Start with the inside because this gives you the visible symptom.

Document:

• exact drip location
• staining
• wet drywall
• wet insulation
• moisture around the frame
• water on the glass
• water on the sill
• recurring patterns

Do not immediately remove interior finishes.

The objective is to establish the location and behaviour of the moisture before disturbing the assembly.

Step 3: Inspect the Roof Side

The exterior inspection should cover the skylight and the surrounding roof.

Check:

• head flashing
• side flashing
• sill/apron
• curb
• roofing materials
• underlayment where accessible
• membrane transitions
• sealants
• fasteners
• nearby roof penetrations
• drainage conditions

This matters because the skylight may not be the only possible source.

NRCA guidance specifically emphasizes that investigators should consider multiple potential roof and building-envelope sources before attributing interior water to a particular component.

Step 4: Inspect the Frame and Curb

If water appears around the skylight frame, inspect the interface carefully.

Potential defects include:

• failed sealant
• displaced gasket
• damaged frame
• improperly installed fasteners
• failed curb connection
• movement between components

Do not automatically apply new sealant over an existing joint.

First determine:

Why did the joint fail?

Possible reasons include movement, substrate contamination, incompatible materials, incorrect joint design or deterioration.

Step 5: Check for Condensation

When no obvious water-entry path is found, assess condensation.

Useful observations include:

• indoor relative humidity
• indoor temperature
• approximate exterior temperature
• location of moisture
• time of day
• surface temperature of glass/frame where measurable

Condensation occurs when the relevant interior surface falls below the dew point.

That means a condensation solution can involve:

• reducing excessive indoor humidity
• improving controlled ventilation
• improving glazing performance
• reducing thermal bridging
• improving curb insulation
• improving air sealing

Simply adding roof sealant will not solve a condensation problem.

Skylight Leak vs Condensation

ObservationMore likely causeWhat to checkMoisture on broad glass surfaceCondensationHumidity + surface temperatureWater appears after rainfallPossible roof leakFlashing + roof interfaceWater concentrated at framePossible interface leakCurb + frame sealWater stain below skylight after stormPossible leak pathFull roof/skylight assemblyMoisture mainly on cold morningsCondensationIndoor RH + glazingMoisture continues despite dry weatherCould be retained water or condensationBroader investigation

The table is for diagnosis direction, not a substitute for inspection.

Skylight Flashing Repair

The correct repair depends on the failure.

Failed Sealant

If the sealant joint itself is the confirmed failure:

1. Remove failed sealant.
2. Clean and prepare the substrates.
3. Check compatibility and joint design.
4. Install the specified sealant.
5. Tool the joint correctly.
6. Allow curing according to manufacturer requirements.

Do not simply apply a new bead over deteriorated material.

Damaged Flashing

If the flashing is damaged:

• determine whether the damage is local or systemic
• inspect adjacent laps
• inspect the underlayment/membrane connection
• replace the damaged flashing
• restore the intended drainage path
• reinstall the roof materials according to the roof/skylight system

The repair should restore the system, not just cover the hole.

Incorrect Flashing Installation

This is more serious than a small sealant defect.

Examples include:

• incorrect overlap direction
• missing components
• poor integration with underlayment
• incorrect termination
• drainage blocked by roofing material
• incompatible roof/skylight interface

In these situations, partial resealing may only delay the next leak.

The correct repair may require disassembling part of the roof-to-skylight connection and rebuilding the flashing sequence.

Do Not Seal the Leak From the Inside

Interior sealant is tempting because the interior stain is easy to see.

But it can create three problems:

It may hide the entry path.

It may redirect water into another cavity.

It may trap moisture inside the assembly.

The objective should be to restore the exterior water-management path.

That is consistent with the basic roof-flashing principle: flashing exists to prevent water intrusion at roof penetrations and intersections.

When Should the Skylight Be Replaced?

Not every leak requires replacement.

Repair may be appropriate when:

• the glazing is in good condition
• the frame is structurally sound
• the curb is sound
• the flashing can be correctly restored
• replacement parts are available

Replacement becomes more reasonable when:

• the frame is deteriorated
• the glazing has failed
• insulated-glass seals have failed repeatedly
• the operating mechanism is beyond economical repair
• the product is obsolete
• the roof/skylight interface has repeated failures
• thermal performance is significantly below current project requirements

A flashing failure does not automatically mean the skylight itself has reached the end of its service life.

Thermal Performance Can Affect Condensation

A skylight’s thermal performance can influence condensation risk.

For U.S. fenestration, U-factor is determined under NFRC 100, while SHGC and VT are determined under NFRC 200 when labeled values are used under the applicable energy-code provisions.

Current NFRC-certified skylight products demonstrate why the complete product matters: certified listings can show U-factor, SHGC, VT and condensation-resistance values for the same skylight configuration.

So the specification should not simply say:

“Use Low-E glass.”

A better approach is:

Specify the required U-factor and SHGC, then select a rated skylight system that meets the project requirements.

Water Leakage and Condensation Can Happen Together

This is an important point the original article missed.

A poorly insulated skylight can develop condensation.

A poorly flashed skylight can leak.

The same skylight can have both problems at the same time.

For example:

• rain enters through a failed flashing detail;
• cold weather lowers the frame temperature;
• indoor humidity creates condensation;
• both forms of moisture appear around the same opening.

Therefore, finding condensation does not automatically rule out a roof leak.

Likewise, finding water after rain does not eliminate condensation as a secondary problem.

Skylight Leak Diagnosis Matrix

FindingPossible causeNext actionWater only after rainFlashing / roof interfaceExterior inspectionWater after wind-driven rainWeather-exposed jointInspect flashing + sealsMoisture on glass in cold weatherCondensationCheck RH + surface temperatureWater at curbCurb/frame interfaceInspect seal + flashingWater below skylightWater migrationTrace path above stainRepeated leak after resealingUnderlying installation defectReinspect complete interfaceOld skylight + failed IGUProduct deteriorationEvaluate replacementLeak + interior condensationMultiple mechanismsDiagnose both

A Professional Skylight Leak Repair Sequence

For contractors and building owners, the workflow should be:

1. Document the moisture

↓

2. Establish when it occurs

↓

3. Inspect the interior

↓

4. Inspect the exterior roof/skylight interface

↓

5. Check flashing, curb, seals and roof materials

↓

6. Assess condensation conditions

↓

7. Trace the actual water path

↓

8. Repair the failed component

↓

9. Restore drainage and weather protection

↓

10. Verify the repair

This is more reliable than simply “re-caulking the skylight.”

What to Ask a Skylight Manufacturer

When a leaking skylight needs technical review, provide:

• skylight model
• installation age
• dimensions
• roof type
• roof slope
• photos of the exterior
• photos of the interior
• location of water entry
• weather conditions when the leak occurs
• flashing detail
• curb detail
• glazing type
• previous repair history

For replacement projects, also request:

• current product performance
• U-factor
• SHGC
• installation instructions
• flashing requirements
• curb requirements
• product certification
• available replacement components

NFRC’s certified product directory can be used to verify labeled performance data for certified skylight products.

U.S. Skylight Flashing and Product Specification

For U.S. projects, separate the roof installation requirements from the fenestration product requirements.

AreaWhat to verifyRoof weather protectionApplicable adopted codeFlashingRoof/skylight manufacturer instructionsCurbProduct + roof requirementsUnit skylight performanceApplicable NAFS ratingU-factorNFRC 100 where applicableSHGC / VTNFRC 200 where applicableGlazingApplicable safety and structural requirementsCondensationProduct rating + building conditionsInstallationApproved manufacturer detail

The 2024 IRC requires roof flashings at roof openings and ties installation to approved manufacturer instructions; unit skylight provisions also address installation conditions such as crickets/saddles.

For unit skylights, current North American fenestration requirements are based on the NAFS framework; NAFS-26 is the current 2026 edition.

Frequently Asked Questions

Why is my skylight leaking?

A skylight leak can come from flashing, the curb-to-frame interface, roof membrane or underlayment, seals, fastener penetrations or another nearby roof condition. Water may travel before appearing indoors, so the visible drip location is not necessarily the entry point.

How do I know whether my skylight is leaking or just condensing?

Condensation is generally associated with cold interior surfaces and elevated indoor humidity, while a roof leak is associated with liquid water entering through an exterior weather-control layer. Timing, location and weather conditions provide clues, but neither should be diagnosed from one symptom alone.

Why does my skylight leak after rain?

Possible causes include failed flashing, incorrect flashing installation, failed curb seals, roof membrane problems or leakage at another nearby roof interface. Inspect the complete roof penetration rather than replacing only the visible seal.

How do you fix skylight flashing?

First identify which part of the flashing system has failed. A proper repair can involve restoring laps, replacing damaged flashing, reconnecting underlayment or membrane, restoring drainage and reinstalling roof materials. The manufacturer’s installation detail should be followed for the specific skylight and roof system.

Should I reseal a skylight from the inside?

Generally, interior resealing should not be the first repair. It may hide the symptom without correcting the exterior water-entry path. Diagnose the exterior source first.

Why does my skylight condense?

Condensation forms when an interior surface becomes colder than the indoor air’s dew point. Indoor humidity, outdoor temperature, glazing performance, frame temperature and thermal bridges can all contribute.

Can better skylight glazing reduce condensation?

It can reduce condensation risk by increasing interior surface temperature, but it does not guarantee zero condensation. Building humidity and outdoor conditions still matter.

How much does skylight leak repair cost?

The cost depends on the failure. A localized seal repair can be relatively limited, while rebuilding flashing or correcting a roof/skylight interface can require substantially more work. A proper diagnosis should precede pricing.

When should I replace a leaking skylight?

Consider replacement when the unit itself has deteriorated, the glazing has failed, parts are unavailable, the operating mechanism is beyond practical repair, or repeated failures indicate that repairing the existing system is no longer economical.

Can an old skylight be repaired instead of replaced?

Yes, when the frame, glazing and supporting roof conditions remain sound and the failed component can be correctly repaired. Flashing deterioration alone does not automatically mean the entire skylight must be replaced.

Do skylight leaks always come from flashing?

No. Flashing is a major part of the roof penetration, but leaks can also involve curbs, frame seals, roof membranes, underlayment, fasteners, drainage or nearby roof conditions. The actual source should be traced before repair.

Final Takeaway

The wrong approach to a skylight leak is:

See a stain → add sealant → wait for the next storm.

The better approach is:

Diagnose → trace → repair → verify.

First determine whether the moisture is:

condensation, exterior water intrusion, or both.

Then inspect the complete roof/skylight interface:

flashing → underlayment/membrane → curb → frame seal → drainage → surrounding roof.

For U.S. projects, roof-flashing requirements should be checked against the locally adopted code and the skylight manufacturer’s installation instructions. The 2024 IRC, for example, requires flashing at roof openings and specifically addresses unit-skylight installation conditions.

The most important rule for a building owner or contractor is:

Do not repair the visible water stain. Find the water path first.Start inside and work outward. Note exactly where the stain appears, how large it gets, and when it shows up (only during wind-driven rain, after snow melt, or on cold mornings). If moisture appears during temperature swings without rain, suspect condensation: warm indoor air reaching a cold skylight frame, uninsulated light wells, missing vapor control, or blocked weep channels causing water to collect and drip.

If it appears during storms, assume exterior intrusion until proven otherwise. Check the basics first: cracked glazing, failed gaskets, loose fasteners, damaged cladding, and clogged drainage paths. Then move to the roof details that actually keep water out: step flashing and head flashing, ice-and-water membrane continuity, correct shingle/lap direction, and a curb that is tall enough and properly wrapped. Pay special attention to the uphill side of the skylight, where water and debris concentrate, and to any seams in membrane or flashing that rely on sealant instead of laps.

Trace the leak methodically. Use a controlled hose test (low flow, one area at a time, starting downhill and moving up) while a second person watches inside. Stop as soon as water appears; the last area sprayed is the likely entry point. If the roof is complex or the leak is intermittent, consider a moisture meter, inspection camera, or temporary water-dye testing to confirm the route.

Repair only what the diagnosis proves. Replace damaged flashing rather than “re-caulking” it. Restore correct overlaps, integrate membrane with the curb, and clear drainage/weep paths. If condensation is part of the problem, improve air sealing and insulation at the light well, manage indoor humidity, and verify the skylight’s weeps and thermal breaks are functioning.

Finally, verify. Re-test with water, then re-check after the next real storm. Document what was found and what was changed so the next person is not guessing. A skylight that is properly flashed and drained should not depend on sealant to stay dry.