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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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.
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.
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.
See VERTICAL GLAZING DETAILS
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.
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
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
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.
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.
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.
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 (above 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. Below we see the sort of leak stains one may find around a problem-skylight.
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.
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
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
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.
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.
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.
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
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).
Andersen Windows and Doors
Skylights and roof windows with exterior sash clad with
Milgard Windows and Doors
Skylights with aluminum frames (thermal break optional)
with vinyl subframes on operable models; optional motorized
controls with rain sensor
Pella Windows and Doors
Wood interior, aluminum exterior, optional motorized
controls, and manual or motorized fabric-pleated shades
Roto Frank of America
Wood interior, aluminum exterior, optional motorized
controls, and manual or motorized fabric-pleated shades;
Sweet16 model fits 16 in. o.c. framing
Velux America Inc.
Skylights and roof windows with wood interior and
aluminum-clad exterior. Options include insect screens,
blinds, motorized controls and shades with rain sensor,
electrochromatic glass, and flashing kits for metal and tile
roofs and mulled units
Skylight Light Tube Manufacturers & Sources
Light tubes from 10 to 21 in. in diameter; options include
electrical lighting, daylight dimmer, and integral bath fan
Light tubes from 10 to 21 in. in diameter; options include
electrical lighting and multitube Spyder skylight
Velux America Inc.
Sun Tunnel light tubes from 14 to 22 in. in diameter with
flexible or rigid tubes
(June 23, 2011) Debbie G. said: Very helpful info. Thanks!
Question: is it condensation or a leaky skylight
(June 23, 2011) catherine said:
my skylight drips when it rains , I am not sure if it is condensation or a leak , the skylight is on my sun roof , it is a slightly raised slanted hardtop roof
If the skylight drips ONLY when it rains I suspect there is a leak in the roof flashing around the unit, or in the skylight itself. Try spraying water up on your sunroof in warm dry weather and see if you can reproduce the condition. Be careful about on-roof inspections on sunroofs as you don't want to damage other sunroof panels (nor slide off and fall).
Question: protective film to stop skylight leaks?
July 18, 2011) Jean said:
HIOw's about if the skylight itself has a leak? does a protective film would be useful to stop such a leak?
I don't think a protective film on skylights is going to do much to stop most skylight leaks, Jean. Most skylights leak at their surrounding flashing. Skylight films are plastic coatings applied to the glass.
Question: why is skylight leaking after new roof?
(Sept 1, 2012) Anonymous said:
skylight leaks and I wonder why? New roof put over the old roof and the the gap between the new roof and the skylight is now much less, would that cause a leak, the 3 leaking skylights are operable and the one non leaking skylight is in operable. When I leave the skylights open a crack they do not leak
I can't diagnose this or even speculate from just the information in your note. Check for improper flashing around the skylights or failing to even install flashing when the new roof was installed.
Question: leaks at skylight during hurricane
(Oct 30, 2012) Todd said:
skylight just started to leak from lower corner, huricane sandy hitting western Pa as of now. could this be from rain blowing, condensation from colder air, or acuall leak
Probably a leak
Question: light penetration around skylight - is this a leak?
(May 27, 2014) Debbie said:
What does light penetration into the attic mean in regards to a skylight?
Debbie I think what we're talking about is inspecting in an attic for points that may be leaking. Sometimes (not always) if you look up at a roof from inside an attic and see daylight, that's a hole or a leak point.
The reason I say not always is because light may be reflected indirectly up-roof through flashed openings or between wood shingles or slates nailed over spaced sheathing boards.
When you see a point of light or "a thousand points of light" in your roof, look there for water leak stains as well as checking the floor surface below that area for leak marks.
Question: risks associated with skylights
(June 2, 2014) Jen W-C said:
Hi and thanks so much for all this info!
My question relates to the risks associated with skylights. I am in the U.S. at the moment but lived in Denmark the last 8 years and there, you see skylights on almost every house. I am helping a friend get a new roof on her log cabin, and want to strongly recommend skylights. I know Velux has had excellent flashing that go with their windows and he slope is good. She has spoken with others (men, actually, whereas I am female but have a lot of experience with construction) and three have told her not to install skylights.
Her house is so dark that even in summer if you want to see anything in detail, you have to turn lights on during the day ($$). Assuming that one buys proper windows and uses highly capable contractors to install them when a new roof is being put on, do you think there is a substantial risk of failure down the line or think perhaps the people she spoke with were referring to window failures from the days when they were installed improperly, at a bad time (i.e. over the roof versus with the roof) or were less effectively designed products? Thank you for your response!
Thanks for the question, Jen.
I don't know who the men are with whom your friend has spoken but it sounds like a lot of bluffing and arm-waving to me.
Modern skylight products have evolved significantly from early (leaky) products. The manufacturers have a great interest in the success of their product, and to that end have designed raised curbs that bring the skylight above the roof surface, and pre-packaged roof flashing components that are very reliable and leak-proof if the installer follows the manufacturer's installation instructions.
The people with whom she was speaking could be
1. remembering the older products from 30+ years ago that were not so well designed
2. confusing indoor condensation with roof leaks
3. people who installed skylights willy-nilly without using the proper components nor reading the installation instructions. I was talking with such a builder about a leak on a home he had just finished constructing. "They all leak like that" he said, as if that meant it was OK. Well he was right, all of his houses leaked at that point because he built them all the same way - with the flashing installed backwards. Some builders know construction. Others are better at arranging financing.
The skylights shown there were installed in a truss-roofed cabin in Northern Minnesota, in a climate exposed to severe weather at the edge of Lake Superior. They don't leak.
Shown also on that page are the older "low profile" "bubble" type skylights that didn't have such a great success record.
Question: skylight leak sealant
(June 13, 2014) Gail said:
my skylight is leaking around the wood part of the skylight. what can i use on this to seal it
Clean the surfaces and try a silicone sealant
Question: many big skylights leak: we want to install a roof over the whole shebang
Larry and Kathy Boso said:
I have a question. We have a roof that has 16 double 5x8 windows. The leak during rain and they leaks are in different places. We have tried fixing all of them, but decided to just put a roof over them all. Our delema is, how to put a roof over them. We are thinking about a free staind roof. Cant put any weight on the windowed roof. Each window weighs 200lbs? It would cost an arm and a leg to get a crane operater to lift out the windows. Afraid of the whole roof colapsing. Any ideas? Help?
Wow what a dramatic fix for leaky roof windows. I understand the frustration. But before you decide on the roof-over option there are some considerations that lead to a bottom line: you need help from a design professional and from your local building department. Concerns include:
1. Even with a roof over, unless the edges are enclosed on all sides, blowing rain may still cause leaks at the skylights.
2. Special construction, perhaps trusses, would be needed to bear on the ridge and eaves walls to span over the existing roof.
3. With a "double roof" construction there may be fire safety and fire code issues. Firefighters don't want to have to chop through multiple roofs if fighting a fire.
4. Ultimately the cost of the roof over project, done properly, may lead to a different approach: eliminating most of the 16 double roof windows in the existing roof, replacing any roof windows to be kept with curbed units with proper flashing, and then re-roofing that surface.
Use our email found at our CONTACT link to send me some photos and I may be able to commnent further.
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Western Washington University Thermal Expansion is described and defined in a clear article that also gives both linear and volumetric coefficients of thermal expansion at 20 degC. for a variety of materials at http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/Thermal/ThermExpan.html
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Identification of Silicone Oil/PETN Interaction, PDF information from General Electric Corporation, Silicone Products Division, Product Information. Pamphlet RHB-4B. c A General Electric polydimethylsiloxane, document from the U.S. Department of Energy at http://www.osti.gov
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Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Analysis of Modern Paints, Thomas J.S. Learner, Research in Conservation, 2004 ISBN 0-89236-779-2 [Chemistry of modern paints, overview of analytical methods, pyrolysis-gas chromatography signatures of basic modern paints and their constituents, Fourier transform infrared spectroscopy for paint analysis, direct temperature-resolved mass spectrometry, and analysis in practice - technical reference useful for forensic paint science, focused on art works -DF]
Understanding Ventilation, John Bower, The Healthy House Institute, ISBN 0-9637156-5-8, 1995 [General building science-DF - ** Particularly useful text. Mr. Bower has retired from the field but his book continues to be important]
Certainteed Weatherboard fiber cement siding and trim products - see certainteed.com/ or see certainteed.com/resources/sidingandtrimspecsheet.pdf
"Moisture Control in buildings: Putting Building Science in Green Building," Alex Wilson, Environmental Building News, Vol. 12. No. 5. [Good tutorial, "Moisture 101" outlining the physics of moisture movement in buildings and a good but incomplete list of general suggestions for moisture control - inadequate attention given to exterior conditions such as roof and surface drainage defects which are among the most-common sources of building moisture and water entry.--DJF]
Paint Handbook: testing, selection, application, troubleshooting, surface preparation, etc., Guy E. Weismantel, Ed., McGraw Hill Book Company, 1981, ISBN-10: 0070690618, ISBN-13: 978-0070690615, [Excellent but a bit obsolete paint theory and practice, also a bit light on field investigation methods, out of print, available used-DF] How to select and apply the right paint or coating for any surface. The first major reference to help you choose the correct paint or other finish to do the job best on a particular surface exposed to a particular environment. Experts in the field give full advice on testing surface preparation, application, corrosion prevention, and troubleshooting. The handbook covers wood, metal, composites, and masonry, as well as marine applications and roof coatings. A ``must'' working tool for contractors, architects, engineers, specification writers, and paint dealers.
Paint and Surface Coatings, Theory and Practice, R. Lambourne & T.A. Strivens, Ed., Woodhead Publishing Ltd., William Andrew Publishing, 1999 ISBN 1-85573-348 X & 1-884207-73-1 [This is perhaps the leading reference on modern paints and coatings, but is a difficult text to obtain, and is a bit short on field investigation methods - DF] Provides a comprehensive reference source for all those in the paint industry, paint manufacturers and raw materials suppliers, undergraduate and postgraduate students, and industrial paint users. R. Lambourne was in the Research Department at ICI Paints Division and the Industrial Colloid Advisory Group, Birstol University, UK.
Seeing Through Paintings, Physical Examination in Art Historical Studies, Andrea Kirsh, Rustin S. Levenson, Materials in Fine Arts, 2000 ISBN 99-051835 [ forensic science, technical reference, focused on art works - DF]
Sealants, Durability of Building Sealants (RILEM Proceedings), J.C. Beech, A.T. Wolf, Spon Press; illustrated edition (1995), ISBN-10: 0419210709, ISBN-13: 978-0419210702 This book presents the papers given at the RILEM Seminar held at the Building Research Establishment, Garston, UK in October 1994. The book provides an opportunity for researchers to review up-to-date progress towards the achievement of the objectives of the standardisation of laboratory techniques of sealants in the variety of service conditions to which they are exposed.
Soiling and Cleaning of Building Facades (RILEM Report), L.G.W. Verhoef (Editor), Routledge; 1 edition (November 3, 1988), ISBN-10: 0412306700, USBN-13: 978-0412306709 The report of a comprehensive investigation by RILEM which examines all aspects of the cleaning of facades, subject to soiling by both biological and non-biological agencies. The contributors are international authorities working in this field giving essential advice to all those who need to know how to approach the problems connected with the soiling and cleaning of building facades.
Staining, Prevention of Premature Staining in New buildings, Phil Parnham, Taylor & Francis; 1996, ISBN-10: 0419171304, ISBN-13: 978-0419171300 The appearance of ugly staining early in a buildings life, ruins an otherwise pleasing appearance, tarnishes the image of the owners and gives rise to costly refurbishment works. In this book Phil Parnham raises a number of questions that should be considered whenever a new building is being designed or built. These are: * why has staining become so prominent; * what causes premature staining; which parts of new buildings are likely to be affected; * how can it be avoided? By using a number of highly illustrated case studies, the author answers these questions and ends by suggesting measures that should be taken by all design and construction professionals to prevent premature staining.
"Weather-Resistive Barriers [copy on file as /interiors/Weather_Resistant_Barriers_DOE.pdf ] - ", how to select and install housewrap and other types of weather resistive barriers, U.S. DOE
The ILLUSTRATED HOME illustrates construction details and building components, a reference for owners & inspectors. Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
TECHNICAL REFERENCE GUIDE to manufacturer's model and serial number information for heating and cooling equipment, useful for determining the age of heating boilers, furnaces, water heaters is provided by Carson Dunlop, Associates, Toronto - Carson Dunlop Weldon & Associates Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on any number of copies of the Technical Reference Guide purchased as a single order. Just enter INSPECTATRG in the order payment page "Promo/Redemption" space.
The HOME REFERENCE BOOK - the ENCYCLOPEDIA of HOMES, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume.
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