Skylights: Types, Flashing, and Why Old Ones Leak

The three families: deck-mounted, curb-mounted, and tubular daylighting devices

Almost every residential skylight belongs to one of three families. Knowing which one is in your ceiling changes the entire repair conversation, because each family fails in a different place.

A deck-mounted skylight sits low, right on the roof deck. The frame and the flashing kit are engineered together as a system, and the flashing tucks under and over the shingles in a set order. A curb-mounted skylight sits on top of a wood box — the curb — that is framed up off the deck like a short chimney. The unit drops onto the curb and the flashing wraps the curb, not the unit. A tubular daylighting device (a TDD, sometimes called a sun tunnel) is a different animal entirely: a small dome on the roof, a reflective tube running down through the attic, and a flat diffuser lens in the ceiling. No view, no framing changes, just light.

In Ottawa County housing stock, all three show up. Detached single-family homes make up 76.7% of local housing units according to the U.S. Census Bureau's American Community Survey, and a lot of those homes have had additions, bathroom remodels, and hallway daylighting projects layered on over decades. The curb-mounted units tend to be the older or site-built ones. The tubular units tend to be the newest.

Fixed vs. venting vs. manual vs. motorized: what each adds and what each can break

A fixed skylight is sealed shut. It has no hinge, no gasket to compress, no motor, no wiring. It is the least likely of any unit to give you trouble, and if you only want light, it is the honest choice.

A venting skylight opens. That is genuinely useful in a market that the Oklahoma Climatological Survey's 1991–2020 normals put at 57 days a year above 90°F, with an August daily-high normal of 91.5°F. Hot air leaves at the highest point in the room, and a bathroom clears faster. But every opening unit adds parts that wear: a hinge, a perimeter gasket, a latch or crank, and a drainage channel in the sash that can silt up with grit and pollen.

Manual and motorized units

A manual venting unit opens with a crank or a pole hook. Fewer parts, nothing to lose power. A motorized unit — line-voltage or solar — opens on a switch or remote and usually carries a rain sensor that closes it automatically when the sensor gets wet. Rain sensors are a real convenience, but treat them as a convenience and not a guarantee. They react to water hitting the sensor, and a sudden hard blow of wind-driven rain arrives before the unit finishes closing.

The practical point: when a venting unit "leaks," check first whether it was actually closed and latched, and whether the gasket has taken a compression set. A gasket that has gone flat and shiny where it should be soft is a gasket that is no longer sealing.

Glazing: laminated vs. tempered, insulated glass units, and impact considerations

The glass in a modern skylight is not one pane. It is an insulated glass unit — an IGU — which is two lites of glass separated by a spacer, with a sealed cavity of dry air or argon between them. The spacer holds a desiccant that keeps that cavity dry. Everything about how an IGU fails traces back to that edge seal.

The two lites are usually not the same. The outboard lite facing the sky is typically tempered, which means it is heat-treated to be stronger and to break into blunt pebbles rather than shards. The inboard lite facing your living room is typically laminated: two thin sheets of glass bonded to a plastic interlayer, so that if it breaks, the interlayer holds the pieces in place instead of dropping them on the floor. Sloped glazing over occupied space is governed by the skylight and sloped-glazing section of the residential code — R308.6 in recent IRC editions — and the requirements for the inboard lite are the reason that section exists. Section numbering does shift between editions, so confirm the specific requirement against the current Oklahoma residential code rather than against whatever a manufacturer brochure printed years ago.

On performance, the unit-level standard is AAMA/WDMA/CSA 101/I.S.2/A440, which is what manufacturers certify skylights against for structural load, air leakage, and water penetration. The water-penetration side of that testing leans on ASTM methods — E547 for cyclic static pressure and E2140 for continuous water on the glazing. If you want to know how a unit was proven watertight, those are the test names to look for on the spec sheet. For energy numbers, U-factor and SHGC, the U.S. Department of Energy's ENERGY STAR skylight criteria are the reference point, and skylights have their own criteria separate from windows because a skylight faces the sky.

Condensation vs. leak: how to tell them apart before you call anyone

This is the single most useful thing on this page. A skylight that drips is not automatically leaking. Water gets into a room around a skylight in three completely different ways, and they call for three completely different responses.

Condensation forms when warm, humid indoor air touches a cold surface. A skylight is the coldest surface in most ceilings, and the Oklahoma Climatological Survey's normals put Miami at 89 nights a year below freezing. On those nights, the glass and the metal frame get cold enough to pull moisture straight out of the air in your house. A shower running, a pot boiling, a bathroom fan that dumps into the attic instead of outside — any of those loads the air. The water then runs down the glass, collects in the frame track, and eventually spills over onto the shaft. It looks exactly like a leak. It is not one, and no amount of caulk on the roof will fix it.

A true leak follows the rain. It starts during or shortly after a storm and stops when things dry out. And an IGU seal failure is neither — the water is sealed inside the glass unit itself and never touches your ceiling at all.

Why old skylights leak: flashing kits, curb height, sealant-dependent installs, and IGU seal failure

Four causes account for most of what we find, and only one of them involves the glass.

The flashing kit was wrong, missing, or improvised

A deck-mounted skylight is a system. The manufacturer's published installation instructions pair a specific unit with a specific flashing kit and a specific order of assembly. When a skylight is reused on a new roof, or when a unit from one maker gets a kit from another, or when someone builds the flashing out of coil stock on the tailgate, the layering logic breaks. Water only has to find one lap running the wrong way.

The curb is too short or was never counterflashed

On curb-mounted units, the curb has to be tall enough that the flashing rides well above the water running down the roof, and the unit's own flange has to counterflash the top of that curb so water is shed outward. The NRCA Roofing Manual's skylight and curb details show that relationship clearly. Site-built curbs are the usual offender — framed a couple of inches proud of the deck, flashed as an afterthought, and asked to survive 30 years of runoff.

The install depends on sealant to stay dry

This is the big one. Flashing is supposed to work because of geometry — because each piece overlaps the one below it and gravity does the rest. Sealant is a supplement, not a system. But a lot of skylights, especially ones that were retrofitted into an existing roof without lifting shingles, are dry only because someone ran a bead of caulk. Caulk has a service life, and under Miami's UV load it is a short one. When you see a skylight ringed in black roof cement, you are not looking at a repair. You are looking at a countdown.

The glass unit failed internally

IGU seal failure has nothing to do with your roof. Every day the sun heats that sealed cavity, the air expands and pushes on the edge seal; every night it contracts and pulls. Do that a few thousand times and the seal eventually breathes. Once it does, humid air enters, the desiccant saturates, and you get permanent fog. The unit still keeps rain out. It just no longer performs, and it looks bad from the couch.

The plastic-dome problem: UV degradation, crazing, and why domes yellow

Acrylic and polycarbonate domes were everywhere on 1970s and 1980s roofs, and a lot of them are still up there. They do not age gracefully. Ultraviolet light slowly breaks the polymer chains at the surface. The dome goes amber, then develops crazing — a spiderweb of fine surface cracks that scatters light and turns a bright room dim and yellow. Underneath the cosmetics, the plastic is getting brittle.

That brittleness is what matters here. The Oklahoma Climatological Survey's normals give Miami 4 to 5 days a year with hail larger than three-quarters of an inch. A new, flexible dome absorbs a hit like that. A 25-year-old crazed dome does not — it shatters or punches through, and now you have an open hole. Add 89 freezing nights a year, because cold plastic is stiffer plastic, and a January hit is harder on a dome than a June one.

There is no repair for a crazed dome. Replacement is the whole menu. The good news is that a tubular unit or a small modern glass unit will usually drop into the same footprint, and you get back the light you have been losing a little at a time for two decades.

Flashing kits and step flashing: matching the kit to the roof covering

Step flashing is the L-shaped metal that ties a roof plane into a vertical surface. One piece per shingle course, each one bent over the corner, each one lapped by the shingle above it. Around a skylight, step flashing runs up both sides. At the bottom there's an apron or base flashing that lays over the shingles. At the top there's a head flashing or saddle that goes under the shingles. That sequence is not stylistic. It is the whole reason the assembly works.

Flashing kits are also covering-specific. A low-profile kit for asphalt shingles will not work on a standing-seam metal roof or a profiled covering, and vice versa. The kit has to match both the unit and what the roof is made of.

Shafts and light wells: the hidden place moisture shows up

Unless the skylight is in a vaulted ceiling, there's a shaft — a boxed tunnel from the roof deck down through the attic to the ceiling opening. Nobody thinks about the shaft until it stains.

The shaft passes through unconditioned attic space, which means its walls need insulation and air sealing on the attic side, exactly like an exterior wall. When they don't have it, warm house air rises into the shaft, hits cold surfaces, and condenses inside the drywall cavity. You get a stain that appears in dry weather, gets worse in January, and drives homeowners to caulk a skylight that was never leaking.

Shafts also disguise real leaks. Water entering at the head flashing can run down the outside of the shaft framing and show up on drywall two feet from the skylight, or drop onto a ceiling in the next room over. That is why the attic view matters more than the roof view — from the attic you can see the shaft exterior, the framing around the opening, and where the water actually tracks. It's a standard part of what we look at during a [roof inspection](/services/roof-inspections).

Replace with the roof or wait? The economics of doing it during a tear-off

Here is the arithmetic that decides most of these jobs. A skylight's flashing lives inside the roof covering. To replace a skylight properly, you have to open the roof around it, remove and rebuild the flashing, and re-lay the shingles. That work happens anyway during a full tear-off. Doing it separately means paying to open the same square of roof twice.

There's an Oklahoma-specific piece here too. Oklahoma amended the residential code's roof-covering section — IRC R908.3.1.1 — to require tear-off rather than shingling over the old covering in defined conditions. Where that applies, the deck around your skylight is going to be stripped bare regardless. That's the moment when a new flashing kit can be integrated with fresh underlayment the way the manufacturer's instructions intend, instead of being threaded into a roof that's already built.

So the rule of thumb is simple. If the roof is within a few years of replacement and the skylight is aging, wait and do them together. If the roof has a decade left and the skylight is actively leaking, fix the skylight now and expect to pay for the roof work around it. And if the skylight is fogged but not leaking and the roof is new, that's a glass replacement, not a roof job — the two are unrelated. Whether the work belongs on a [roof repair](/services/roof-repair) ticket or gets folded into a [roof replacement](/services/roof-replacement) is usually obvious once someone has looked at the attic.

When removal is the honest answer

Not every skylight deserves to be replaced. Some of them are just holes somebody put in a roof in 1984.

Decking over an opening and shingling it in is cheaper than any new unit, and it's the right call more often than the industry admits. Consider it when the skylight lights a hallway or closet nobody notices, when it sits on a low-slope section where it will always be the wettest detail on the roof, when the shaft has a chronic condensation history that no reasonable amount of insulation has fixed, or when there are four aging units on one roof and you're really being asked whether to buy four new ones.

Tubular units are the middle path worth knowing about. If what you actually want is daylight in a dark interior room, a TDD delivers it with a fraction of the roof penetration and none of the shaft. Removing two failing glass units and adding one tubular unit is a perfectly sensible outcome, and it's the kind of trade we'd rather talk through than sell around. Trees matter here too: on the east side of Ottawa County, where the Ozark Plains bring rolling, wooded ground, a shaded skylight already collects leaf litter and moss and delivers less light than the brochure promised. On the flatter Neosho Lowlands to the west, open wind fetch means more wind-driven rain hitting the head flashing. Same county, two different arguments.

What a skylight line item on a quote should specify

"Replace skylight — $X" is not a scope. Here's what a real line item covers. Bring this list to whoever is quoting your [skylight work](/services/skylight-services), including us.

On warranties: units and glass carry manufacturer coverage that varies widely by brand, by product line, and by whether the unit was installed per the published instructions. Read the actual document. A verbal number from anyone — installer or salesperson — is not the warranty. The paper is.

If a skylight in your house is dripping, fogged, or ambered over, the first useful step is figuring out which of the three problems you actually have, because they don't share a fix. We'll look at it from the inside and from the attic, tell you what we see, and put it in writing. Free inspections, and no charge for the quote — call (209) 758-8550, or start with our [Miami, Oklahoma](/locations/miami-ok) service page.