Skylights are on of the most common cause of leaks on a roof. There are many different things that can cause a skylight to leak, but in order to repair a skylight the correct way we first must understand what the problem is.
Diagnosing Skylight Leaks
There are 2 main areas of a skylight that can be leaking the first main area is the skylight itself.
This means the glass and frame. Below is a picture of the glass and frame. If the glass and frame leak it is usually due to a cracked glass or a failure in the gasket (the seal between the glass and the frame). The video below shows a failure in the gasket.
If the skylight itself is leaking usually the stain will be inside the red border in the photo below.
The second main area that can cause a skylight leak is the curb that supports the skylight. This is the part of the skylight known as the flashing. The skylight flashing consists of roofing material or metal.
The picture below left shows us a picture of a skylight curb/flashing. If the curb is leaking it is normally due to a failure of the roofing membrane around the curb.
Usually if it is the Flashing that is leaking the stain will appear on the ceiling around the skylight (outside the red border)
Indoor Clues Help Diagnose Skylight Leaks
By looking at where the water damage originates we can get a better idea of what is causing the leak and how to fix it.
Indoors, look very closely at where water stains originate.
inside the building - trace the stains to their highest point.
Our indoor photo of a skylight leak (left) shows a stain that appears to be originating from the skylight frame itself, therefore the first place to check would be the gasket and skylight glass.
Water Testing for Finding Skylight Leaks
If you are having trouble finding the source of your leak water testing using a hose can be a great help.
Garden Hose Skylight Test 1(here we are testing the first main area of the skylight, the curb):
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 the roof onto the curb of the skylight and around the unit . Look inside the building at the skylight for signs of new water penetration. If it leaks now we know the leak is due to a failure in the roofing membrane seal to the curb.
Garden Hose Skylight Test 2 (here we are testing the second main area of the skylight the glass and frame): Next allow hose-water to run onto the skylight surface itself. If the leak appears now we can conclude that there is an issue with the glass and frame.
It is important that we do the hose tests in this order because although when we test the cub the glass will remain dry when we test the glass water will run down onto the curb. so we first must asses the condition of the curb before we move on to the glass and frame.
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 curb roof flashing problem. But if it appears only when we soak the glass and frame then we will look to inspect the glass and gasket for the source of the leak.
Don't soak the building interior - just use enough water to see it appearing indoors.
If the water is entering through a failed gasket as in the video above the solution is simple. Apply caulk over the failed gasket to seal the skylight.
Simple Repairs for Skylight Leaks
1. Caulking
If the gasket has failed the best and easiest way to repair it is by caulking it.
For best results you want to use a polyurethane caulk. Other caulks may nit adhere to glass and will peel up or crack. Polyurethane sticks to both glass and plastic skylights and has an extremely long life expectancy.
If the gasket is dirty clean the area using a wet rag with water or alcohol.
If the skylight glass is cracked caulk can also be used as a temporary solution till you replace the skylight. Just put a bead of caulk over the cracks.
Below is a picture of a skylight gasket that has been repaired using caulking.
2. Flashing Cement
If the skylight leak is at the roof flashing/curb, it may be possible to make repairs using roof flashing cement around the perimeter of the unit.
Inspect the skylight for visibly obvious damage such as open seams, roof damage, or cracks.
Seal any suspicious areas using flashing cement.
The photo bellows an example of a skylight curb/flashing that was repaired using flashing cement.
Conclusion
Fixing skylight leaks can be a complicated and dangerous process. If possible we always recommend calling a roofing proffesional.
However if you are the DIY self type and are motivated to do this yourself we advise being safe and cautious. Follow all standard safety recommendations and guidelines for working on a roof.