How to Reseal Around Skylights
Skylights bring natural light into rooms that would otherwise rely on artificial lighting, but they also create one of the largest and most complex penetrations in your roof surface. The rectangular frame, multiple flashing pieces, glass-to-frame seal, and weep drainage system all need to work in concert to keep water out. When any component fails, the result is water stains on the ceiling, damaged drywall, and—if left unaddressed—structural rot in the surrounding roof framing.
Quick Summary
Time Required
45-60 minutes
Difficulty
Moderate to Difficult
Cost
$20-$80 for sealant; $500-$2,000 for full reflashing
Distinguishing a Leak from Condensation
Before climbing onto the roof, determine whether the water around your skylight is actually coming from outside. Misdiagnosing condensation as a leak leads to unnecessary flashing work that does not solve the problem.
Signs of a true roof leak
Water appears during or within hours after rainfall. Drips come from specific points along the frame edge rather than uniformly across the glass. Stains on the ceiling or drywall shaft may be discolored with brownish roof debris rather than clear water. The problem worsens during wind-driven rain from a particular direction.
Signs of condensation
Moisture forms as uniform fog or droplets on the interior glass surface, especially on cold mornings or after cooking, showering, or running a humidifier. Water drips from the lowest point of the glass rather than from the frame edges. The problem disappears when you reduce indoor humidity or improve ventilation around the skylight.
The definitive test
Wait for a dry, warm day. Have a helper spray the skylight with a garden hose from the roof while you watch from inside. Start at the bottom of the skylight and work upward, spending at least two minutes at each level. If water appears inside, you have confirmed a leak and identified its approximate location.
Inspecting the Flashing-to-Frame Junction
The joint where metal flashing meets the skylight frame is the single most common point of failure. This junction endures constant stress from thermal expansion—the metal and the frame expand and contract at different rates—and UV degradation of the sealant that bridges the gap.
What to Look For
- Hardened or cracked sealant: The bead of sealant between the flashing lip and the frame should be flexible when pressed with a fingertip. If it feels hard, crumbly, or has visible cracks, it has failed and needs replacement.
- Gaps between flashing and frame: Even a hairline gap at this junction will allow wind-driven rain to enter. Run a thin feeler gauge or credit card along the joint—if it slides into a gap, sealant has pulled away from one of the surfaces.
- Lifted flashing edges: Wind or ice can pry flashing edges away from the roof surface. If you can slide your fingers under the flashing, water can follow the same path.
- Corrosion at the frame contact point: Galvanic corrosion can develop where dissimilar metals touch (for example, an aluminum frame with steel flashing). Look for white powdery deposits or pitting on either surface.
Understanding and Clearing Weep Holes
Many skylight frames include small weep holes along the bottom rail that serve as drainage outlets for condensation that forms inside the frame. These tiny openings are easy to overlook but play a critical role in preventing water buildup.
Locate the weep holes
Check the bottom (downhill) rail of the skylight frame for small slots or holes, typically 3 to 5 millimeters in diameter. Some manufacturers place them on the exterior face of the frame, while others position them on the underside. Consult your skylight's installation manual if you cannot find them.
Clear blockages
Use a thin piece of wire, a straightened paperclip, or a toothpick to probe each weep hole and dislodge any debris. Paint overspray from interior remodeling frequently seals weep holes shut. Blow compressed air through the holes to flush out fine particles.
Verify drainage path
Pour a small amount of water into the skylight's interior channel (between the glass and the frame). Water should flow out through the weep holes and onto the roof surface below. If it pools inside the channel instead, the holes are still blocked or the drainage path is obstructed by debris or sealant.
Step-by-Step Re-Sealing Procedure
Once you have confirmed a flashing leak and identified the failure point, re-sealing is straightforward if the metal flashing itself is still in good condition. Work on a dry day with temperatures above 40 degrees Fahrenheit for proper sealant adhesion.
Follow these steps in order for a durable, watertight re-seal:
- Remove all old sealant: Use a putty knife and utility knife to scrape and cut away every trace of old caulk and roofing cement. New sealant bonds to clean surfaces—never layer new sealant over old.
- Clean the surfaces: Wipe the metal flashing and frame with rubbing alcohol or mineral spirits to remove oils, grime, and oxidation. Let the surfaces dry completely before applying new sealant.
- Apply primer if recommended: Some polyurethane sealants require a primer on metal surfaces for optimal adhesion. Check the sealant manufacturer's instructions and apply primer if specified.
- Lay a continuous bead of sealant: Apply polyurethane roofing sealant in a continuous bead along the entire flashing-to-frame junction. Do not leave gaps or start-stop points—water will find any break in the bead.
- Tool the sealant smooth: Use a wet finger, a plastic spoon, or a caulk-smoothing tool to press the sealant firmly into contact with both surfaces. This eliminates air pockets and ensures the sealant bridges the full width of the joint.
Pro Tips
- •Photograph the flashing layout before touching anything: Skylight flashing kits have precise layering orders (bottom piece goes on first, sides overlap it, top piece overlaps the sides). If you need to remove any piece for repair, your photos will show exactly how to reassemble the system correctly.
- •Address condensation with ventilation, not sealant: If your skylight has a condensation problem, adding more sealant makes it worse by reducing air circulation. Instead, improve bathroom exhaust fans, add a small fan near the skylight shaft, or install a dehumidifier during winter months.
- •Never seal over weep holes: It is tempting to caulk every opening you see when chasing a leak, but sealing weep holes traps moisture inside the frame and causes internal corrosion. If you accidentally seal one, reopen it immediately.
- •Consider a skylight replacement if the frame gasket has failed: The glass-to-frame seal is factory-installed and cannot be reliably reproduced in the field. If water is entering between the glass and the frame (not at the flashing junction), the skylight unit itself needs replacement rather than re-sealing.
Frequently Asked Questions
How do I tell if my skylight is leaking or just condensating?
The key distinction is timing and weather conditions. A true leak will produce water during or shortly after rainfall, and the water will appear at specific points along the frame edge or flashing. Condensation typically appears on cold mornings or during high-humidity periods regardless of rain, and it forms as uniform moisture or droplets across the entire glass surface. If you see water only during cold weather with no rain, condensation is almost certainly the cause.
Can I reseal a skylight from inside my house?
Interior sealing can address condensation-related dripping by improving the seal between the skylight frame and the ceiling drywall, but it cannot fix an actual roof leak. If water is entering through failed flashing or a compromised seal between the flashing and the skylight frame, you must address it from the roof side. Sealing the interior without fixing the exterior source can trap moisture in the roof structure and cause hidden rot or mold.
How long does skylight flashing sealant last?
Polyurethane roofing sealant applied to skylight flashing joints typically lasts 7 to 12 years depending on UV exposure, temperature extremes, and the quality of the sealant. Skylights on south-facing roof slopes tend to need re-sealing sooner due to greater sun exposure. Plan to inspect and re-seal skylight flashing every 8 to 10 years as part of routine roof maintenance.
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