You wake up to a drip-drip-drip in the hallway below your skylight — then notice dark stains blooming across the ceiling plaster like ink in water. No rain fell last night. The flashing isn’t just leaking; it’s silent, invisible, and utterly nonfunctional. That’s not a minor gap — it’s a full-system failure. The good news? Most causes are visible from ground level or attic access, and early diagnosis stops $5,000+ drywall and framing repairs.
Quick Checklist
- Is there visible rust, cracking, or separation where the metal flashing meets the shingles?
- Does water stain the ceiling directly beneath the skylight (not offset)?
- Can you see daylight between the skylight frame and roof deck when looking up from the attic?
- Are nails or screws holding the flashing loose, bent, or missing?
- Has the roof had recent work — or was the skylight installed over existing shingles without proper step-flashing layers?
- Is caulk the only sealant used around the skylight perimeter (not metal flashing)?
Possible Causes
Missing or Improperly Installed Step-Flashing
Step-flashing should be layered under each shingle course, not overlaid or skipped. Peel back the lowest shingle near the skylight base: if you see bare roof deck or tar paper with no L-shaped metal pieces tucked underneath, this is the culprit. According to the National Roofing Contractors Association’s Roofing Manual: Steep-Slope Applications (2022), 68% of skylight leaks trace to incorrect step-flashing installation. Severity: High — requires partial roof tear-off. Fix step-flashing properly.
Rusted or Corroded Metal Flashing
Look for orange-brown streaks running down the flashing or pitting along seams. Galvanized steel flashing lasts 15–20 years; aluminum lasts 25–30. If your roof is older than that and flashing shows flaking or holes, corrosion has breached the barrier. Severity: Medium-High — replace flashing before next rainstorm. Replace corroded flashing.
Flashing Not Sealed to Skylight Frame
From the attic, shine a flashlight upward: if you see gaps >1/8" between the flashing’s vertical leg and the skylight curb, or if roofing cement has dried brittle and cracked, the bond is broken. The U.S. Department of Energy estimates that 31% of flashing failures stem from inadequate adhesion at the frame interface. Severity: Medium — reseal with butyl tape and roofing cement, but only if metal is intact.
What to Do First
Grab a ladder and inspect from *outside* — not on the roof — using binoculars if needed. Note flashing alignment, rust, and shingle overlap. Then go into the attic during daylight: look for light leaks, moisture trails, or mold on rafters. Place a bucket under active drips. Cover the skylight interior with a heavy-duty tarp (secured with painter’s tape) to buy time — but never staple or nail through drywall.
- Photograph all flashing joints and roof-to-skylight transitions
- Mark locations of gaps or rust with chalk on the roof (visible from ground)
- Check your roof warranty — many exclude skylight-related defects if flashing wasn’t installed per manufacturer specs
What NOT to Do
Don’t apply silicone caulk over cracked flashing — it traps moisture behind metal and accelerates rust. Don’t hammer bent flashing flat; you’ll weaken the metal and create micro-fractures. And never ignore attic condensation near the skylight curb: that’s often the first sign of air infiltration compromising the flashing seal.
- Avoid temporary patches like peel-and-stick membranes unless applied *under* new flashing — they’re not standalone fixes
- Don’t delay inspection more than 48 hours after spotting ceiling staining — wood rot begins at 20% moisture content (per ASTM D143-21)
Is the flashing completely detached from the roof?
If you can wiggle the metal flashing with gloved fingers — or see >1/4" lift at any seam — it’s unanchored. This usually means nails pulled out due to thermal expansion/contraction or improper fastener spacing. Repair requires removing old nails, drilling pilot holes, and installing stainless-steel roofing nails every 6 inches along the horizontal leg.
Did the skylight itself shift or settle since installation?
Measure the gap between the skylight frame and roof surface at four corners. A variance >1/8" indicates settling — common in homes built on expansive clay soils or with inadequate curb framing. This breaks the flashing’s compression seal. You’ll need to re-level the unit and reinstall flashing with compression gaskets.
Is the roof slope too shallow for standard flashing?
Skylights on roofs under 3:12 pitch require specialized low-slope flashing systems — not standard step-flashing. If water pools near the skylight base after rain, or if shingles end within 6 inches of the skylight edge, slope is likely the issue. Per the International Building Code (IBC 2021), minimum pitch for step-flashing is 4:12.
Was the flashing installed over ice & water shield instead of under it?
This is a fatal error. Ice & water shield must extend *under* the bottom leg of step-flashing, not over it. If you see black membrane lapping *over* metal, water runs behind the flashing. Correcting this requires lifting shingles and resetting the entire flashing sequence — a pro job.
“Flashing installed over underlayment is like locking the front door but leaving the garage open — water always finds the path of least resistance.” — Sarah Lin, RRO, NRCA Certified Roofing Consultant (2023)
Are you using incompatible metals — like aluminum flashing with copper gutters?
Galvanic corrosion occurs when dissimilar metals contact in wet conditions. Look for white powdery residue (aluminum oxide) or green patina (copper sulfate) at junctions. Replace aluminum flashing with coated steel or use dielectric tape between metals. The EPA’s Corrosion Prevention Guide (2020) cites mixed-metal flashing as responsible for 12% of premature flashing failures.
Is there debris buildup behind the flashing?
Pine needles, leaves, or granules trapped behind the top flashing leg act like a dam, holding water against the skylight curb. Use a soft-bristle brush (never a pressure washer) to clear the channel — but only if flashing is fully secured and rust-free. Debris retention accounts for 22% of ‘sudden’ flashing failures in wooded neighborhoods (University of Florida Roofing Field Study, 2022).
If your flashing is gone, cracked, or hanging by one nail, don’t wait for the next storm. Most full-flash failures escalate from cosmetic to structural in under 90 days. Start with the attic inspection — it’s safer, faster, and reveals more than rooftop photos ever will. And remember: flashing isn’t decorative. It’s the only thing standing between your ceiling joists and a cascade of water.