There are pre formed plastic flashing pans that you can use under the door or you can create one with thin sheets of lead or the modern rubberized asphalt based flashing tape.
How to fix sliding door water leak.
How to fix a sliding door leak.
Http youtu be hg2mfndacfw waterproofing how to weather seal door threshold water leak.
Check the weatherstripping or flashing.
Weatherstripping and flashing are rubber insulators located on the.
Press it tightly against the jamb including the corner.
There are countless videos online that show you how to install these flashings under a door or window.
How to stop water leaks under a door frame.
Test the door for the location of the leak.
Place the end of a strip of adhesive backed aluminum tape on one of the side jambs in the doorway 3 inches above the threshold.
If you have a water leak at your threshold you must watch this how.