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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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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.
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 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:
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.
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.
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
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
Also see SKYLIGHT VENTILATION DETAILS and
-- Adapted with permission from Best Practices Guide to Residential Construction.
Continue reading at SKYLIGHT VENTILATION DETAILS
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Frequently Asked Questions (FAQs)
(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!
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.
Please also see
on skylight flashing.
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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