When a door leaks the most common suspect is the bottom corner of the unit.
Patio door seal leaking.
Moisture could be seeping into your home from the top.
Leaks in wooden roofs are commonly caused by a gap between the panels which move during expansion and contraction.
I ve got a sliding patio door on the back of my home there s a moisture problem directly under the door in the basement.
If you have a water leak at your threshold you must watch this how.
Identify the location of the leak.
Http youtu be hg2mfndacfw waterproofing how to weather seal door threshold water leak.
Whether your door is sagging or bowed or just not making proper contact with the weatherstrip to seal out air and water the upper corners of your frame could be the problem.
Aim a garden water hose with a sprayer set on mist or low at the outside of the sliding door.
Step 2 fill a caulking gun with a watertight polyurethane or oil based sealant and hold it at an angle to insert the product directly into the gap.
Test the door for the location of the leak.
If you cannot re create the leak problem inspect the interior of the door for stains or rust.
However in some instances the top corner of the door is the true culprit.