Skylight Leak Flashing vs Curb vs Replace Decision Tree

Why this matters

A skylight leak has four possible origins: the flashing kit around the skylight, the curb (whether site-built or factory-integrated), the glass-to-frame seal, or condensation forming inside the home that is misread as a leak. Each has a different repair scope. Replacing the flashing kit on a 20-year-old skylight delays the inevitable; replacing the skylight on a 2-year-old unit with failed flashing wastes the unit. The decision is structured: read the leak path from inside, inspect the flashing kit and curb from outside, age-check the unit, and then choose between flashing repair, full skylight replacement, or condensation correction.

Symptom presentation

Four patterns:

• Water dripping from the corner of the skylight ceiling well: flashing failure at that corner, water entering at the curb-shingle interface.
• Water staining at the head (top) of the skylight well: head flashing failure or chimney-effect from poor head flashing integration.
• Water sheeting down the inside of the glass: condensation forming on the cold glass surface, not a leak.
• Water at the ceiling well perimeter without correlation to a specific corner: curb-flashing system failure or the skylight is past service life.

Quick checks before going on the roof

• Observe the interior leak path. Photograph during or immediately after rain or hose-test.
• Check the skylight age. Most residential skylights have 15 to 25 year service life; over 20 years and original is replacement territory.
• Identify the skylight type: factory curb (Velux, Fakro, Wasco) vs site-built curb.
• Identify the flashing kit type: deck-mounted, curb-mounted, low-profile.
• Look for matching shingle granule loss around the skylight; suggests roof age or installation defects.

Isolation tree

Step 1: condensation vs leak.

• Water on the interior glass surface only, no water on the ceiling well drywall, no exterior signs: condensation. Not a leak. Recommend humidity control, bath fan use, supply register adjustment.
• Water on ceiling well drywall or at the corner: leak. Continue tree.

Step 2: locate the leak entry by interior observation.

• Drip at the head (top): inspect head flashing.
• Drip at the side or corner: inspect step flashing or apron flashing at that location.
• Drip at the foot (bottom): inspect apron flashing or sill flashing.

Step 3: exterior inspection of the flashing kit.

• Step flashing pieces: woven into shingle courses on the sides.
• Head flashing: continuous piece at the top that diverts water around the skylight.
• Apron or sill flashing: at the bottom, sheds water down the slope.
• Inspect for damage, lift, separation, or sealant degradation.
• Inspect the curb-to-flashing integration. The flashing should be under the skylight curb on the uphill side and over the skylight curb on the downhill side.

Step 4: curb inspection.

• Factory curb (integrated): inspect the curb-glass seal. Failed factory seals are skylight replacement, not repair.
• Site-built curb: inspect lumber for rot, flashing integration, and dimensional accuracy.

Step 5: glass-to-frame seal.

• Insulated glass unit (IGU) seal failure shows as fogging between the panes. The IGU is replaced as a unit on most modern skylights.
• Single-pane skylight (older units): glass-frame seal can leak; replace gasket or replace skylight.

Step 6: age and condition.

• Under 10 years: flashing repair is usually the right answer unless the leak is at the IGU seal.
• 10 to 20 years: case-by-case based on condition. Flashing repair if curb and glass are sound.
• Over 20 years: replacement is usually the right answer. The skylight is at end of service life.

Confirming diagnosis

Three confirmations:

• Hose-test the suspected entry point. Water targeted at the specific area produces an interior drip in the same location, confirming the source.
• Inspect the skylight age stamp or curb stamp (most manufacturers label).
• Inspect the curb wood for rot if site-built. Probe with awl or screwdriver.

The decision matches the diagnosis: flashing repair for flashing failures on a sound skylight; full replacement for end-of-life or IGU failure.

Remediation by path

Flashing repair (sound skylight, flashing failure):

• Lift surrounding shingles to expose the full flashing assembly.
• Remove failed flashing pieces.
• Install new flashing kit per manufacturer specification. Use the same brand flashing kit as the skylight when available; aftermarket kits exist but factory kits integrate best.
• Integrate new flashing with new shingle courses; weave step flashing one per course.
• Test water-tightness with hose if practical before close-up.
• Document.

Curb repair (site-built curb with rot or damage):

• Disassemble the skylight to access the curb.
• Remove and replace rotten curb lumber.
• Re-frame the curb to original dimensions; out-of-square curbs leak at the flashing.
• Reinstall flashing kit per spec.
• Reinstall skylight or replace if damaged during disassembly.

Skylight replacement (end of service life or IGU failure):

• Order matching size; most factory skylights have standardized sizes that allow direct replacement.
• Coordinate roof access and weather window for the install.
• Remove existing skylight, flashing, and curb (if site-built).
• Install new skylight per manufacturer instructions, with new flashing kit.
• Match interior well drywall and finish.

Condensation correction (not a leak):

• Reduce interior humidity to 30-50% per ASHRAE 62.2.
• Improve bath fan ventilation if the skylight is in or near a bathroom.
• Verify HVAC supply or return is not blowing directly on the skylight (cold-air supply on warm glass creates condensation).
• Consider insulated skylight upgrade if condensation persists in a non-bathroom location.

References

• IRC International Residential Code R908.5, Roof Openings (skylight requirements)
• AAMA/WDMA/CSA 101/I.S.2/A440, Standard for Windows, Doors, and Skylights (NAFS)
• ASTM E2112, Standard Practice for Installation of Exterior Windows, Doors and Skylights
• Velux installation instructions for factory-curb skylights with deck-mounted flashing kit
• Fakro and Wasco manufacturer installation instructions
• ASHRAE Standard 62.2, Ventilation and Acceptable Indoor Air Quality in Residential Buildings (humidity control)