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.

Site-built skylight in a Palapa style roof, San Miguel de Allende, Guanajuato, Mexico (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 at page top 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. The photo just above illustrates a site-built skylight using a corrugated fiberglass panel in a palapa-style roof in Mexico.

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

Roof Flashing Cement & Other Rooftop Skylight Leaks

Leaky roof cement patch (C) Daniel Friedman Patched skylight flashing (C) Daniel Friedman

Leaks at a skylight show up inside (C) Daniel Friedman

Still older bubble type skylights that were mounted flush with the roof surface (shown below) 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.

Low profile plastic skylights are likely to leak (C) Daniel Friedman

Water Testing for Finding Skylight Leaks

Near-flat skylight leaks (C) Daniel Friedman

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 above) 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 such as the skylight shown below on a home in Port Angeles, Washington in the U.S. 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.

Curbed skylight, Port Angeles WA, USA (C) Daniel Friedman

Our skylight photo below illustrates a curbed skylight in a brand new installation in on a nearly-flat EPDM roof in Poughkeepsie, New York ca. 1999.

Curbed skylight on a flat roof (C) Daniel Friedman

Below is this same skylight, now more than fifteen years old. It has not leaked, but as it is installed over a bathroom, condensation on the interior of the skylght glass can still be an annoyance.

Curbed skylight on a flat roof (C) Daniel Friedman

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 Bliss

As 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.

Skylight Manufacturers & Product Sources


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