Skylights are on of the most common cause of leaks on a roof. There are many different things thatch cause a skylight to leak, but in order to repair a skylight the correct way we first must understand what the problem is.
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.
1. 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.
2. 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.
3. 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.
Water Testing for Finding Skylight Leaks
There are 2 main areas of a skylight that can be leaking the first main area = 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.
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)
The second main area that can leak 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 above.
Garden Hose Skylight Test 1(here we are testing the first main area of the skylight, the curb): 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 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
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 on a pitched roof removing shingles near the skylight, installing proper head, side, and foot flashing around the unit, as you reinstall new shingles in the area. and on a flat roof redoing the base flashing around the curb.
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.
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.
Although from afar the flashing cement seams to be intact we know from stains around the skylight inside the building (as seen in photo below) that the skylight is leaking.
If we look closely at the flashing cement repair around the skylight we can see the reason for the water stains.
In the photo (left) we can see that the flashing cement around the skylight failed because of cracking and poor bonding. This was probably due to the sun drying and shrinking the tar or because of failure to properly clean the area before performing the repair.
Also reinforcing fabric would help prevent cracking in the tar.
From here we learn 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.
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 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.
Also remember that a skylight with a curb is always better because it reduces the chance that running water will butt up against the skylight causing a leak.