A Leak-Free Skylight Is Built in Layers

A skylight is less a single product than a stack of materials arranged around a hole in the roof. Structure, curb, membrane, metal flashing, glass and an interior shaft each handle a different job, and water only reaches the ceiling when one of those layers is missing, reversed or poorly joined to the next. That is why two identical skylights can perform completely differently on neighboring houses.

Most homeowners judge a skylight by its glass, yet the glass is rarely what fails. Leaks, drafts and condensation almost always trace back to the layers you never see once the job is finished. Understanding how each one works makes it much easier to evaluate a quote, question an installer, or diagnose an older unit that has started to drip.

This is also why skylights reward specialist experience. Homeowners looking into Kirkland Skylights Services will find that C&A Roofing treats skylights as part of the roofing system rather than an add-on, installing VELUX units with the manufacturer’s own flashing kits, rebuilding curb framing where needed, and specifying Low-E glass. One of their Redmond projects replaced a skylight another company had installed that leaked within six months; the rebuilt pair has stayed dry through two rainy seasons. If an aging skylight is on your list, ask them to price its replacement alongside any roof work so both are sealed together.

The Structural Opening

Everything starts with the cut in the roof deck. In a rafter-framed roof, the rafters interrupted by the opening need headers at the top and bottom to carry the load around it, and wider units may require doubled rafters on each side. Trusses are a different matter. They are engineered as complete units, so cutting one without an engineer’s approval can weaken a whole section of the roof. Many manufacturers size skylights to fit between standard rafter or truss spacing for exactly this reason.

The opening also needs to be square and sized precisely to the unit. An oversized hole leaves gaps that the upper layers are then forced to bridge, which is where improvised fixes usually begin.

The Curb or Deck Mount

Next comes the base the skylight sits on. Curb-mounted units rest on a raised box, typically built from 2x lumber, that lifts the frame several inches above the roof surface. Deck-mounted units attach directly to the sheathing and rely on an integrated frame and flashing for height.

Curbs work well on low-slope roofs and when replacing older skylights, since the existing curb can sometimes be reused if it is sound. A rotted or out-of-square curb, though, undermines everything above it. Deck-mounted designs sit lower and cleaner, and they tend to be the default on new installations on pitched shingle roofs.

The Membrane Layer

Before any metal goes on, a self-adhering waterproof membrane wraps the opening and runs up the sides of the curb or frame. This is the same ice and water shield material roofers use in valleys and at eaves, and it acts as the backup for everything above it. If wind drives rain past the flashing, the membrane keeps it from reaching the wood.

This layer is also where shortcuts hide. Skipping it, or lapping it the wrong direction so water runs behind the seams instead of over them, is a common reason skylights fail years later rather than right away.

The Flashing System

Flashing is the primary water barrier. A proper kit includes a sill piece at the bottom, step flashing interwoven with each course of shingles along the sides, and a head flashing at the top that diverts water around the unit. On larger skylights or heavy rainfall roofs, a small saddle above the unit helps split the flow.

Manufacturer kits are engineered for the specific frame and roof material, which is why they outperform flashing bent on site or sealed with roofing cement. VELUX, for example, builds its current deck-mounted units around a three-part approach combining an adhesive underlayment, engineered flashing and a deck seal. Metal roofs need their own kit profiles to match standing seam or panel shapes.

The Unit and Its Glazing

The skylight itself adds its own protection through gaskets, a sealed frame and multi-pane glass. Modern units typically pair a tempered outer pane with a laminated inner pane, so a cracked pane stays in place instead of falling into the room. Low-E coatings cut heat gain in summer and heat loss in winter, and they reduce fading on floors and furniture below.

Venting skylights have grown more popular, particularly solar-powered models that open by remote and close automatically when a rain sensor detects moisture. They release trapped heat and humidity from kitchens, bathrooms and upper floors. Tubular skylights occupy a separate niche for hallways and closets where a full shaft is impractical.

The Interior Light Well

The final layer is the shaft that carries light from the roof to the ceiling. It needs to be insulated like an exterior wall, with an air and vapor barrier on the warm side. When that step is skipped, warm indoor air meets a cold shaft and condenses, producing drips that look exactly like a roof leak.

Shaft shape affects the result as well. A splayed well that widens toward the ceiling spreads light across a bigger area than a straight vertical box, and painting the shaft a bright, flat white improves diffusion without glare.

How the Layers Compare

Why the Order Matters

Each layer depends on the one beneath it being right. Flashing cannot compensate for a warped curb, and a premium glazing package does nothing if the shaft is sweating. That is also why skylight replacement is best timed with a re-roof. With shingles already off, the membrane and flashing can be rebuilt as one continuous system instead of being patched into a roof that is staying in place. A skylight around 20 years old sitting in a roof due for replacement is almost always worth swapping at the same time.