Entry andpatio doorsa window you walk through.

The flashing logic is identical to a window: a pan under the unit, jamb flashing lapping the pan, head flashing lapping the jambs and tucked under the barrier. The difference is that the pan is at the threshold, where it is walked on and where any water that gets past the sweep collects, so it has to drain outward and the sub-sill has to be sound. A rotten sub-sill under a patio door is one of the most common things we find, and it is invisible until the door is out.

Then there is the mechanics. A door has to stay square for years while being pulled on, and that means a properly supported head, a sill that is bedded and shimmed so it does not flex underfoot, and hardware adjusted at installation rather than left at factory settings. Sliding units need their track level and their rollers set; swing units need the hinges and the strike aligned so the weatherstrip compresses evenly all round. A door that drags or lets light through at a corner is almost always an installation that was never adjusted, not a product fault.