According to Sheetal Werneke of The Money Pit, leaking above the glass door is likely a sign of damaged flashing. Flashing is the seal placed around the glass that keeps the door from rubbing against the surrounding wall. Any time a wall has an opening, that opening becomes a vulnerable spot. The opening weakens the wall, letting in cold air, hot air and water from snow or rain. In some cases, the damage occurs as the result of weakened wood above the doors. The wood breaks down and absorbs moisture, which slowly drips through the top of the doors.
A simple method of solving the solution is with a product that works like flashing. Clear silicone caulk works like a sealant and stops water from leaking inside the doors. The caulk creates a protective barrier around the eaves and soffits. Hardware stores and home improvement stores sell do-it-yourself flashing kits that include everything you need for installing new flashing around doors. The kits work with a variety of wall surfaces, including brick, concrete and wood.
The longer you leave the leak untreated, the worse the problem becomes. As the water pools around the doors, it causes damage to the wall surrounding the door. On the outside of the home, the water can seep under your siding, leading to mold, mildew and rotting on the wood. You might also notice rotting and moisture damage inside your home, especially around the doors. Moisture damage affects plaster and drywall, causing peeling of paint or wallpaper.
Depending on the installation of your sliding glass doors, the surface might feature existing caulk. Any damage to the caulk, including cracks or chips can lead to leaks. Examine the door carefully, looking at the caulk. A putty knife scraped along the door frame should have enough pressure to loosen the existing caulk and leave behind a smooth surface. Applying new caulk to old caulk is risky because as the material expands, it might block the door from opening properly.