Tarred leaky skylight (C) Daniel FriedmanHow to Diagnose & Repair Skylight Leaks

  • SKYLIGHT LEAK DIAGNOSIS & REPAIR - CONTENTS: Skylight leak prevention, diagnosis, and repair - How to inspect skylights from indoors and from the rooftop to find and fix leaks, When to make ceiling cuts to evaluate skylight and roof leak damage, rot, mold, Roof flashing cement failure modes, four types of flashing cement leaks, Sloped glazing inspection, diagnosis, and repair guide
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Skylight leaks: this article explains how to find the cause of skylight leaks and other sloped glazing leaks. The article continues with simple suggestions for skylight leak repair and leak prevention. Our photo of a leaky roof skylight that has suffered repeated and non-durable repair attempts using roofing cement can be seen at left.

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How to Diagnose & Repair Skylight Leaks

Sloped windows on barn silo (C) Daniel Friedman

[Click to enlarge any image]

Our photograph (left) shows an interesting sloped window installation on the roof of a barn silo that had been converted to living space. Conventional wood-frame double-hung windows were set into the sloped silo roof - this was not a successful installation and the windows rapidly rotted, leaked, and disintegrated.

Article Contents

As Mr. Bliss points out in SLOPED GLAZING DETAILS, a glazing system must perform many functions. It has to shed and drain water, support and cushion the glass to avoid mechanical pressure points, and seal against air and water leakage. It should be attractive and economical.

Yet many subtle and pervasive forces are working against you: thermal and structural movement, high UV radiation, wind and weather. A pretty redwood cross batten is no match for sliding sheets of ice.

The problems inherent in vertical glazing are multiplied in sloped glazing: higher levels of UV, water damming at the lower edges of sloped window frames or between the roof and the upper section of window and skylight frames, and structural loading.

Professionals in the large scale curtain wall industries, as well as residential contractors, have encountered problems with skylights and other forms of sloped window glazing. Fortunately, many new products and systems have been developed to beat the elements as well as to reduce the chances of installation errors leading to leaky skylights and related structural damage or even leak-related mold contamination in buildings.

Tarred leaky skylight (C) Daniel Friedman

Sloped glazing such as roof skylights probably has historically had more leaks into the roof structure due to improper roof flashing than other window failure causes.

Leaks at skylights, left unattended, can lead to costly structural damage, rotted roof sheathing, rotted roof framing, and wet, moldy insulation as well. These skylight leaks should not be blamed on faulty product design, although at some leaky skylights we find a combination of multiple failures.

Our photo of a leaky roof skylight that has suffered repeated and non-durable repair attempts using roofing cement can be seen at left. From the pattern of sealant application we suspect that the leaks at this skylight were around the window frame due to improper roof flashing at the time of installation.

Building owners, having trouble determining exactly where the skylight leak is occurring, sometimes simply slather caulk or roof cement all over everything in sight (an approach that is ugly but might work. But a roof-cement slathered skylight that is still leaking may be even more difficult to diagnose. Here are some tips that might help diagnose the actual leak point at a skylight:

Indoor Clues Help Diagnose Skylight Leaks

Indoor skylight leak stain (C) Daniel Friedman
  • Indoors, look very closely at where water stains originate inside the building - trace stains to their highest point. The leak will be at or up-roof from that spot.

    Our indoor photo of a skylight leak (left) shows a stain that is on top of the ceiling drywall; because this leak does not appear to be coming from within the ceiling cavity, we suspect that this stain is either caused by a leak at the skylight upper glass frame itself or the leak is from indoor condensation. A closer look (from a ladder) was needed.
  • Indoors, distinguish between a true skylight leak and indoor condensate stains that originate from indoor moisture condensing on the cool skylight surface, running down into, and overflowing a small condensate drip tray that some skylights include.
  • Cut a small ceiling inspection opening: indoors, if the ceiling drywall is only modestly leak stained, there is a chance that ceiling insulation has become wet and may even contain hidden mold. In this case, unless the stain is really trivial in size, it may be justified to cut a small opening near the leak to investigate the roof cavity for rot, visible mold, or wet insulation.
  • Remove ceiling drywall: indoors, if the ceiling drywall is badly leak stained it is appropriate to remove the damaged drywall material completely, remove any wet or stained fiberglass insulation, and clean the ceiling cavity. Leave the ceiling open around the offending skylight first to permit things to dry out thoroughly, and second to permit our next diagnostic steps just below.

Roof Flashing Cement & Other Rooftop Skylight Leaks

Leaky roof cement patch (C) Daniel Friedman
  • Outside, on the roof, (Watch out: don't try this if you are not able to access your roof safely, and don't work alone - falling off of a roof can be fatal), inspect the skylight for visibly obvious damage such as open seams, roof damage, or roof debris; also look for evidence of previous repair attempts such as in the photo above.

    Four Types of Roof Cement Failures: Our photo (left) of a roof cement "patch" on an asphalt shingle roof shows four types of roof cement patch failures that mean this patch is not really keeping water out of the building.

    • Flashing cement gaps: first, at upper center you see that the flashing cement did not bridge the gap between two shingles where the applicator was trying to make a seal.
    • Flashing cement bonding failures: second, the roof cement did not bond to the shingle material below - probably it was applied over dirt or over a wet shingle - notice the raised cement surface around the hole in the cement in the center of the photo.

      Also see Avoid Adhesion Window Sealant Failures for details.
    • Thermal Expansion/Contraction sealant failures: third, the roof cement has not survived thermal expansion and contraction as temperatures changed on the roof and in the roof materials. The result was a stretching motion that pulled the roof cement apart in a diagonal pattern of holes running from upper left to lower right in the photo.

      See Temperature effects on windows and also for technical data

      See THERMAL EXPANSION of MATERIALS for the effects of temperature changes on materials.
    • Flashing cement pinholes: the fourth roof cement patch is harder to see unless you (click to) enlarge the photograph. Small openings in the roof cement at the lower center of the photo show that this patch material has not only dried and lost its elasticity, but it is developing pinhole perforations.
Patched skylight flashing (C) Daniel Friedman
  • If the skylight is relying on roof cement to prevent roof leaks, look very carefully at the flashing cement for cracks, openings, or for failure to bond to the skylight or to the roof. Often something that looks "sealed" is actually an open leak.

    Our photo (left) shows an older bubble-type skylight that was heavily patched around its perimeter using roof flashing cement. This unit has a two-inch raised perimeter to get the plastic bubble above the roof deck.

    Still older bubble type skylights that were mounted flush with the roof surface are notorious leakers because they relied entirely on sealants between shingles and the skylight. Newer curbed skylights (having a raised perimeter to get the sloped glazing above the roof surface) are much more reliable.

Water Testing for Finding Skylight Leaks

  • Garden Hose Skylight Test 1: Outside, and from on the roof surface : in dry weather, try pulling a garden hose to the rooftop to permit live water testing of the skylight for leaks. First allow water to run down-roof onto the up-roof end of the skylight and around the unit. Look inside the building at the skylight for signs of new water penetration.
  • Garden Hose Skylight Test 2: Next allow hose-water to run onto the skylight surface itself. If the leak appears around the glass frame, visible from inside, then the problem includes at least a glass sealing problem. If leaks around the skylight appear only when water is flowing down the roof surface and not onto the upper surface of the skylight, there is a skylight roof flashing problem.
    Don't soak the building interior - just use enough water to see it appearing indoors.

Simple Repairs for Skylight Leaks

Sealed glass failure at skylight (C) Daniel Friedman

If the insulating glass unit of the skylight has failed, you can seal the unit against further water leakage, but only replacing the unit will remove condensation, opaque skylight glass, and a failed window unit.

Our closeup photo of the down-roof corner of a leaky skylight (left) shows what is probably a double failure.

Leaks at the insulated-glass frame permitted water to enter the window structure where the freeze-thaw climate at this New York home continued to damage the window by forcing apart and losing the seal of the insulated glass itself.
See Sealed Window Joint Failures for more on this skylight failure.

If the skylight leak is at the roof flashing, it may be possible to make temporary repairs using roof flashing cement around the perimeter of the unit, but a proper repair will require removing shingles near the skylight, installing proper head, side, and foot flashing around the unit, as you reinstall new shingles in the area.

Avoiding or Preventing Skylight Leaks

Debris can cause skylight leaks (C) Daniel Friedman\In addition to installing skylights properly, using the methods discussed in this article and following the manufacturer's instructions, a period inspection for evidence of leaks into or around the glazing unit can avoid costly building damage by early detection and repair of any problems.

If debris collects on or around a skylight (see our photo at left) the water held in that location combined with the working action of extra winter ice (if the building is in a freezing climate) will reduce the roof life around the glass unit, leading to early leaks in this area.

Try gently brushing debris away from and off of your skylights when performing a roof inspection. Don't walk carelessly on a debris-covered roof - it's like walking on ball bearings.

Modern factory-built skylight units have been designed by their manufacturers to make the window as idiot-proof as possible, including factory-made skylight flashing kits and simple, clear instructions.

Still if the contractor is inexperienced with skylight installation, if the skylight was installed later in the life of the building as a retrofit, and especially if the installer did not read the instructions provided by the manufacturer, leaks at the skylight are likely.

Seal Skylight Flashing at Time of Skylight Installation to Prevent Future Leaks

Figure 2-5: Flashing at Skylights, details (C) J Wiley, S BlissAs discussed in Best Practices Guide to Residential Construction, chapter on BEST ROOFING PRACTICES:

As a backup to prevent leaks at skylights, during skylight installation and even though modern skylights are usually provided with a factory-built flashing and counterflashing, it is always a good idea to seal the skylight curb and surrounding roof area with a bituminous membrane (see Figure 2-5 at left).

For proper skylight and other sloped glazing installation details,

Additional examples of skylights are found


see Skylight Glass Breakage.

-- Adapted with permission from Best Practices Guide to Residential Construction.


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