March, 2016
Inspection News and Views from the American Society of Home Inspectors



Skylights are popular architectural features in homes, on both flat (low-sloped) and sloped (steep) roofs. They may be single-, double- or triple-glazed, and they may have flat or curved glazing. Skylights may be installed on curbs, or they may be flush mounted. Most templates are manufactured units, but they also may be site-built.

A Word About Glazing
Glazing may be glass, polycarbonate, fiberglass or acrylic. Some are a combination of these. There are many types of glass used for skylights. Because they are not vertical, like windows are, they need to be stronger. Glass may be tempered or laminated, both of which are stronger than regular glass. Some manufacturers offer a hybrid, with laminated glass on the inside and tempered glass on the outside. Laminated glass stays cleaner and is more noise-resistant than tempered glass. Glazing may be Low E and gas-filled for improved energy efficiency.

The Challenges
Skylights are more susceptible to problems than windows for several reasons. Because they are not vertical, they are more exposed to rain, wind, hail and, in some climates, snow and ice. Skylights are more likely to catch and hold water. They also see far more direct sunlight, including ultraviolet light, than windows. Here are some common problems to watch for with skylights:

  • Leaks: As always, water is the No. 1 enemy of homes.
  • Rot: Typically, rot stems from long-term minor leakage and often is concealed in the framing system.
  • Mechanical damage: Animal damage and falling branches are common culprits.
  • Patches: Evidence of previous repairs is a red flag.
  • Cracked or broken glazing: These problems may be caused by mechanical damage, structure movement or thermal stresses.
  • Loss of seal on double-glazed units
  • Installation problems, including the following:
    • No curb or low curb (although some are designed this way)
    • Improper or incomplete flashings
    • Wrong application (for example, a system designed for steep roofing is installed on a flat roof or vice versa; a system designed for asphalt shingles is installed on a tile roof)
    • Window being used as a skylight
    • Skylight that is poorly secured to the roof

Inspecting Skylights
Again, water is the issue. Skylights should be inspected from above and below, as well as from the attic, if possible.

On the exterior, inspectors should look for a curb and proper flashing details. On sloped roofs, check the area above the skylight carefully for evidence of water collecting there. Ice damming around skylights is a common cold-weather issue. Check the glazing for damage and the area around the skylight for evidence of previous repairs. Roofing cement or caulking on a skylight is a temporary repair at best. Inspectors can lift up (gently) on skylights to ensure that they are well secured.

From the interior, we look for stains and damage to the ceiling or skylight well finishes. We use a pin-less moisture meter (which goes beyond the ASHI Standard of Practice) to help determine whether the problem is active. If it is wet, the problem is active. If it is dry, it is inconclusive. Even if it shows dry in the summer, it may be active in winter due to ice damming. If it is dry in winter, it may be wet during spring or summer rains.

Condensation may also be an issue, especially in bathrooms and kitchens in cold climates. Many skylights have an interior tray around the perimeter to collect condensation. Trays may leak or overflow. Condensation problems often look like leaks. Allow for both possibilities in your report, where appropriate.

Leaking skylights can damage interior finishes, but can also cause rot and mold in concealed areas, and can damage the structure.

Good, Fair, Poor
Here is our ranking of skylight types:

1. Curb mount: Good
2. Integral curb: Fair
3. Flush mount: Poor

We’ll look at each one.

1. Curbs preferred
We have found that skylights set on wood curbs are the most successful. Curbs should be a minimum of 4 inches high on sloped roofs and 8 inches on flat roofs. Curbs should be installed and flashed independently of the skylight. Different roofing materials require different flashing systems.

The skylight is simply set onto the top of the curb and screwed into place. The skylight may have a gasket to ensure a tight fit on the curb.

The illustration below shows a double-glazed, curb-mounted skylight on a sloped roof with a 4-inch curb. It shows a head flashing at the top, and an apron flashing at the bottom. There would be step flashings and counter flashings along the sides, just like a roof/sidewall flashing system.

The photo below shows the bottom corner of a curb mount installation. There is a self-adhering modified bitumen membrane (“ice and water shield”) on the roof surface, which runs up the sides and over the top of the curb. You can see the step and counter flashings on the side. The frame is set on the curb and screwed onto the curb from the sides. You can also see the curved, double-glazed acrylic panes.

A curb-mounted skylight

The skylight in the photo below is flashed into the concrete tile roof with lead flashing. Irregular roofing materials require a different approach. Many skylights have proprietary flashing details for different roofing materials.

Skylight on tile roof

Curbs done wrong
Although we like skylights on curbs, the installation still has to be good. A skylight on a curb is like a short chimney and it should be flashed the same way. The photo below shows an incomplete installation with step flashings, but no counter flashings. The wood curb is visible above the flashing and below the skylight. This also reveals the fact that there is no self-adhering underlayment on the sides of the curb.

Incomplete flashing

2. Integral curb
The integral curb skylight is a hybrid between the curb and flush-mount approach. The illustration that follows shows how the raised skylight frame sits on the roof. The frame typically has an apron that sits flat on the roof. A self-adhering modified bitumen membrane covers the apron.

The photo below is an integral curb skylight. Note the white apron and the self-adhering modified bitumen underlayment, which seals it. There is no step or counter flashing here, although some systems use them.

An integral-curb skylight installation

The side flashings may be step flashings interwoven with the roofing material. The counter flashings ideally extend onto and are secured to the top of the curb.

The photo below shows a problem at the top of the skylight, where the roofing material is incomplete. This creates a vulnerable spot in a critical location.

Roofing material missing

3. Flush mount
We have mentioned our preference for curbs. The illustration below shows a flush-mounted skylight. These skylights are more prone to leakage in our experience.

The skylight below has failed and endured temporary repairs, which were not successful. The acrylic glazing has discolored. The good news is the roof needs to be replaced, presenting a good opportunity to replace the skylight and then make interior repairs. By the way, is there any chance the valley flashing has been an issue as well?

A skylight in distress

General Comments
Now we will look at some issues that apply to all skylights.

Check the glazing: Look carefully at all of the glazing for cracks or movement within the frame. Remember that many skylights are double- or triple-glazed, and you have to look at each piece of glazing. Observe whether the edges of the glazing are set in a gasket or terminate in a tray that allows for condensation to drain. Sealed units should never have condensation, whereas the vented and drained skylights will have condensation from time to time.

Look carefully—cracks are easy to miss.

Sealed units should not have condensation.

Patches: Most skylights leak eventually. If you see evidence of patching on the outside, you can assume that the skylight has leaked.

Patched at top

Windows used as skylights: Watch for windows that are used as skylights. Windows should be vertical. Some people say that windows should be completely vertical. Many authorities require that any glass more than 15 degrees off vertical must be strengthened by tempering, laminating, wiring, annealing or equivalent. This is one of the reasons that acrylic may be used rather than glass on skylights.

Strengthening glass is fairly expensive. The acrylic is strong, although it does scratch easily.

Go inside the building: From the interior of the building, look for evidence of leakage, particularly at the bottom corners of the skylight opening. A stepladder often provides a better view.

Look from the inside for damage and leaks.

Be careful: Almost all skylights will leak eventually. Problems may be concealed by recent decorating. We suggest you let your clients know that these are susceptible to leakage.

Light wells or tunnels
Skylights installed on steep roofs often have large wells around the skylight extending down through the attic to the ceiling level. We described that in the first illustration in this article. These wells often widen as they get closer to the ceiling to allow better light disbursement. Skylight wells should be insulated and, in cold climates, there should be a vapor barrier on the warm interior side of the well.

In this article, we have introduced skylights and some of the common issues. In the ASHI@HOME Training Program, we explain the details of other common conditions and their associated causes and implications, along with strategies for inspection.