Skylight Curb Flashing Before Modesto’s First Hard Rain
When a rectangular skylight starts dripping after a November storm, the glass is often fine. The weak point is usually the curb flashing where the skylight meets the roof. Here is what tear-off reveals on Modesto and Central Valley roofs, and how dry-in protects those openings before the first hard rain.
Mid tear-off on a Modesto-area roof: old shingles off around rectangular skylights, vents and a satellite dish, with the curb openings exposed for inspection.
Central Valley roofs take months of hard sun, then a sudden stretch of rain. Homeowners often point at the skylight glass when a stain shows up on the ceiling. On a curb-mounted rectangular skylight, water rarely comes through the pane first. It finds the flashing, the step metal, or the underlayment that wraps the curb—especially after years of heat have cooked the seals and the shingles around the opening.
Rectangular curb skylights vs. tubular sun tunnels
These are different penetrations. A curb-mounted rectangular skylight sits on a raised wood curb flashed into the roof plane. A tubular sun tunnel (daylight tube) uses a round dome and a pipe through the attic. Both need careful flashing, but the leak paths are not the same. We covered tubular units in our guide to sun-tunnel flashing in Modesto. This post focuses on the rectangular curb.
Finished charcoal architectural shingles with tubular sun tunnels (daylight penetrations)—a different detail than a rectangular curb-mounted skylight.
Why curb flashing fails before the glass does
- Step flashing and counterflashing pull apart. Heat cycles lift metal; fasteners loosen; water runs behind the curb instead of over the shingles.
- Underlayment stops short of the curb. If the wrap around the wood curb is incomplete or aged, the first wind-driven rain finds the gap. See our underlayment guide.
- Old mastic and seals dry out. Valley summers bake caulk and adhesives. What looked sealed in July can crack by November.
- Shingles cut too tight or too short upslope. Water needs a clear path over the flashing; a short course or a reverse lap sends it under.
- Neighbor penetrations pile up. Pipe boots, vents and satellite mounts near a skylight create more edges. Our post on pipe boots in Valley heat shows how those seals age too.
Chimney and wall flashings follow the same idea—water has to be directed back onto the roof surface. More on that in chimney flashing and Modesto leaks.
New OSB deck with GAF Tiger Paw underlayment started at the ridge and valley, and ridge vent openings cut—the stage where every penetration gets wrapped before shingles go on.
What tear-off shows around a skylight curb
When the old shingles come off, the curb is no longer hidden. Soft deck at the upslope corners, dark stains on the wood, rusty fasteners and brittle step metal are common finds. Sometimes the curb itself has settled or the old flashing was nailed flat with no room for movement. That is the moment to decide whether the curb gets rebuilt, the flashing gets replaced, or a small deck repair is needed before anything new goes on.
Leaving a tired curb “as is” under new shingles is how the same stain comes back after the next storm. Tear-off is the time to fix the opening, not to bury it.
Dry-in before Modesto’s first hard rain
After tear-off, the deck and every penetration should be covered so a surprise shower does not soak the house. Synthetic underlayment wrapped up the curb, ice-and-water style membrane where the design calls for it, and temporary covers on open skylight wells are part of that dry-in. We walk through the sequence in FeltBuster dry-in after a Modesto tear-off.
Dried in with GAF FeltBuster underlayment and GAF shingle bundles staged on the ridges—penetrations protected before the field shingles go on.
Once the deck is dry, permanent curb flashing and the skylight unit (or a solid deck patch if the opening is being removed) go in as part of the finished roof system. Skipping dry-in around an open curb is one of the faster ways a re-roof turns into an interior claim when the forecast changes overnight.
What to do if your skylight already drips
If water is already showing up under a skylight after rain, start with the same first steps we outline for a roof leak in the first fall storm: protect the room, note where the stain appears, and avoid climbing a wet roof. A quick look from the ground or attic can tell you whether the glass is cracked or the problem sits at the curb. Many repairs are flashing and underlayment work, not a full unit replacement—but that call belongs to an inspection, not a guess from the living room.
Quick answers
Is a rectangular skylight the same as a sun tunnel?
No. A curb-mounted rectangular skylight sits on a raised wood curb with step and counter flashing. A tubular sun tunnel uses a round dome and a reflective tube. Both need proper flashing; the details differ.
Why does my skylight leak only when it rains hard?
Light rain may run off over tired flashing. Wind-driven or heavier rain pushes water against open laps, dried seals and short shingle courses around the curb.
Can you re-flash a skylight without a full re-roof?
Often yes, if the surrounding shingles and deck are sound. If the deck is soft or the roof is near the end of its life, addressing the curb during a re-roof is the cleaner path.
Should an unused skylight be removed?
Sometimes. If the unit is failing and you no longer want the daylight opening, removing it and patching the deck with solid sheathing, underlayment and matching shingles eliminates that penetration for good. That is a scope decision after looking at the curb and the ceiling below.
Soft next step
If a rectangular skylight is staining the ceiling, or you want the curb checked before the next hard rain, Get a Quick Quote through our Contact page or call (209) 404-6110. More on roof repair and roof replacement in Modesto and the Central Valley.
Legend Roofing Company Inc · 4400 Gomes Rd, Modesto, CA 95357 · (209) 404-6110 · CSLB #1067374 · GAF Master Elite.