Flashing, curbs and why skylights leak: the water path explained

Rain rarely defeats a skylight's glass. The usual trouble is at the edges, where a flat, sealed unit meets a sloping roof built from overlapping layers. This page explains the vocabulary (flashing, curb, underlayment, water path) and the general reasons that junction is demanding. It is background, not a way to work out why a particular roof is leaking, and it is not a guide to doing roof work. Those jobs belong to qualified professionals who follow local code and the product's instructions.

How a roof sheds water in the first place

A sloped roof does not keep water out by being waterproof in one piece. It works by shedding: each course of shingles overlaps the one below, so water moves downhill over the surface and never has a path inward. Beneath the shingles, underlayment (a protective sheet over the roof deck) acts as a second line of defense. The U.S. Department of Energy summarizes the principle for flashing work this way: layers go on from the bottom up, so each upper layer overlaps the one below and sheds water off the roof (U.S. Department of Energy: Flashing around skylight with roof removal).

A skylight breaks that pattern. Cutting an opening and setting a unit in it creates a penetration through the roof's water control layer, and the DOE describes it as a weak spot in the roof's water management protection system. Weathering and poor installation can weaken the protection around the unit and let water into the attic. The rest of this page is about how the industry tries to rebuild the missing layer.

What flashing is, and what it does

Flashing is thin material, either metal or a flexible self-adhered membrane, that bridges the joint between the skylight and the surrounding roof covering. The DOE notes that flashing may be metal strips or self-adhered membrane and must integrate with the roof's other layers, whether new or existing: sheathing, membrane, underlayment, rigid foam and roof cladding (U.S. Department of Energy).

The Pacific Northwest National Laboratory's Building America guide describes flashing on a curb-mounted unit in distinct pieces, each with its own job: sill flashing at the bottom, step flashing woven into the shingle courses along the sides, and head flashing across the top. The guiding rule is that water should drain down and out, never behind the layers (PNNL Building America Solution Center: Existing skylights repaired or replaced and fully flashed). The same page calls for sealing screw heads on flashing to prevent leaks and corrosion, a reminder that even fasteners are potential entry points.

Curbs: raising the unit above the water

A curb is a short raised frame that the skylight sits on. The Efficient Windows Collaborative distinguishes three mounting approaches: flush-mount units that sit on the deck with no curb and are overlapped and sealed by the roofing; curb-mount units on a separately supplied curb; and integral-curb units where the curb comes with the skylight (Efficient Windows Collaborative: Skylights).

The PNNL guide says curb-mounted units offer better bulk moisture control and are recommended when the roofing is not being replaced or extra insulation is added above the deck. Deck-mounted units have a lower profile and are more common in new construction. Neither is automatically safer; each depends on the flashing sequence built around it.

Codes set a floor for low-slope roofs. The IRC requires unit skylights on roofs with a pitch below 3 units vertical in 12 horizontal to sit on a curb extending at least 4 inches above the plane of the roof, unless the manufacturer's installation instructions specify otherwise (ICC: 2021 IRC Chapter 3). Local adoption and amendments vary, so a building department has the final word on what applies.

Commercial and low-slope roofing guidance goes further. In a 2007 summary of changes to its membrane roofing manual, the National Roofing Contractors Association (NRCA) lists a recommended minimum flashing height of 8 inches for its prefabricated metal curb detail, optional removable sheet-metal counterflashing, and sealing material between the curb and the frame. NRCA also recommends that designers consider permanent fall-protection devices at skylights (Professional Roofing (NRCA): Technical Elements). That detail concerns membrane roofs, so read it as an example of how much thought goes into one curb, not as a rule for a house with shingles.

Where the water path tends to be interrupted

The sources point to a handful of general weak points. They are described here so the problem makes sense, not so a reader can pinpoint the cause of a particular stain.

• Layer order. If a lower layer sits on top of an upper one anywhere along the joint, water can run behind it. This is why the sequence is described from the bottom up.
• Fasteners and cuts. The PNNL guide calls for sealed screw heads and for taping any cuts in the underlayment.
• Aging materials. Weathering reduces the effectiveness of the water control layers around the skylight, according to the DOE.
• The unit itself. PNNL lists condensation between glass panes, leaking frame joints and worn gaskets as signs of a failing unit, separate from any flashing question.

Sealant is sometimes assumed to be the main defense. In these sources it is one component of a layered system, not a substitute for flashing. The related guide on skylight installation, leaks and lifespan looks at what repair or replacement decisions rest on.

Leak, or something else?

A damp skylight is not always a leaking one. The Efficient Windows Collaborative points out that water drops on a skylight are often condensation from moist indoor air, because skylights are often the first surface where condensation forms. Remedies it lists include insulating the skylight well, improving air movement, and using highly insulating glazing with a well-designed frame. Our page on skylight condensation and moisture explores this, and skylights, attics, light shafts and insulation covers the shaft side. Telling condensation from a true leak takes an inspection of the whole assembly.

Why this is left to professionals

Several risks combine: working on a roof, altering structure, and building a layered weather system whose errors may not show until the next heavy rain. PNNL advises that water testing, which some people try to find leaks, can damage interior finishes and cause leaks that would not occur in normal weather if overdone, and that it is best done by someone experienced in enclosure forensics. Reading about flashing helps you ask better questions of a roofer; it does not replace one. See choosing a roofing or window contractor for questions worth asking, and permits and codes for roof openings for how code review fits in.

The short version

A roof sheds water through overlapping layers, a skylight interrupts those layers, and flashing and curbs exist to put the protection back. Leaks tend to start at the joints, not in the glass, and some wet-looking problems turn out to be condensation. Local code, the product instructions and a qualified professional have the final word.