Skylights are available in many styles and shapes with a host of different flashing systems for achieving a water tight seal. Skylight leaks can manifest themselves in a variety of forms. Many times homeowners will first realize they have a skylight leak by finding water stained ceilings or a puddle of water on the floor below a skylight. Taking the time for a
little detective work on leaking skylights can go a long way toward a permanent solution.
First lets look at where the water is showing up inside your home. Is the water and or water stain at the bottom or top of the skylight unit? Do you see water trails or stains in the skylight tunnel and where do they appear? Water stains give clues that can help locate the source of the water infiltration.
Standing below your skylight look up and find the highest water stain you can see on the tunnel wall in relation to the skylight unit for example near the bottom, near the center or at the top of the skylights unit – not how high up into the tunnel itself. If your leak
is near the top of the skylight unit we would recommend starting your diagnosis by having your roofing professional check the outside flashing of your unit. Have them pay special attention to the head flashing at the top of the skylight. Many times water running down the roof at speed will hit this area of the skylight and then run between the shingles and
skylight into your home. They should also check the sides of the skylight for proper flashing and seal.
Some very popular older skylights basically had a plastic/ vinyl flange that ran around the entire skylight border with the skylight glass sitting above this flange by about two inches. This flange was installed under the shingle roofing on the top and sides of the skylight with the bottom of the flange resting on top of the shingles. Many leaks on this
type of skylight occurred when water would run between the shingles and the skylight flange.
Resealing this area with a premium flashing cement can sometimes stem the flow of water into the home. Granted we are talking about properly resealing the unit between the shingles and flange area not brushing tar all over the sides of the unit. Most of the better and a lot of the newer skylights have metal flashing or a rubber gasket that requires little or no cement to achieve a water tight seal. Your professional roofer will want to make sure these type of units are flashed properly and all gaskets and flashing are secure.
Once the outside of the skylight has been inspected and any repairs made a check of the area surrounding the skylights exterior should be examined. This should include inspection of any roof flashing that are near the skylight such as roof vents, plumbing vent pipe collars and any other roof flashing. Many times a leaking roof flashing near a skylight
can be the cause of a skylight leak. We once found a roof ridge cap at the peak of a roof was leaking water. This water had been running between the shingles and roofing paper for 20-30 feet showing up as a skylight leak. We would suggest examining all flashing directly above a leaking skylight all the way up to the peak not just around the skylight.
After your professional roofer has finished with the out side you may want them to take a look at the inside of your skylight. Now if you have a water stain near the bottom of your skylight and the outside flashing is not leaking you may have a condensation problem.
First rule out ice dams see our article on Ice Dams.
Condensation forms on the inside of skylights during high humidity and sharp temperature variation conditions. This condition can occur as the humidity in your home rises or during a rain storm. You would be surprised how much condensation forms on skylights when cold rain hits them on the outside. Normally this condensation is captured by the skylights condensation channel. This hidden channel runs around the perimeter of the skylights glass inside your home. A skylight condensation channel is designed to capture condensation that occurs on the inside of the skylight glass and then allows the water to evaporate back into the air instead of running into your home. When to much condensation fills this channel it can overflow into your living area. If you are experiencing a leak on the lower end of the skylight have your roofer check and make sure your condensation channel is not over flowing.
A skylight tunnel can exacerbate a condensation problem since this causes less air movement and that means more condensation on the skylight (moving air absorbs moisture). It also has the effect of increasing the humidity in the tunnel since condensation that does occur and can not make it out of the tunnel is evaporating in the tunnel area where air is quite stagnate. If this is a problem you may have to better circulate the air up in your skylight tunnel along with measures to reduce the overall humidity in that area. Make sure your skylight tunnel is well insulated.
One last note about skylight condensation channels. Some builders and architects have mounted skylights as a “diamond” on the roof instead of “square” to the roof. If the skylight is not specially designed for this application it can create a problem with the skylights condensation channel. This can have the unintended effect of making a skylights condensation channel useless. Since the skylight is mounted on a roof with a slope the condensation runs to the bottom area of the skylight and would normally come to rest in the skylights condensation channel. The effect on this condensation channel of mounting a skylight as a “Diamond” to the roof slope is the same as filling a tray with water and then lifting only one side up. The water in the tray or condensation channel runs all to one side where it spills over the corner of the tray or channel. Combine this with the slope of a roof and just about all condensation that occurs on the inside of a skylight mounted this way is running into the home.
To finish up this article I want to mention a word about lexan bubble skylights. I’m not talking about curb mounted or raised lexan bubble skylights I am talking about flush mounted plastic lexan bubble skylight. Some roofing companies have a policy not to recommend
these types of skylights for any applications.
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