Short Answer
Doors create large openings in the building envelope. Improper flashing can allow water to enter surrounding wall assemblies.
Detailed Explanation
An exterior door is one of the largest penetrations in a home's wall assembly. Like windows, doors require proper flashing to manage water at the opening. The flashing system at a door includes sill pan flashing (at the threshold, the most critical), side flashing, and head flashing, all integrated with the weather-resistive barrier. Without proper flashing, water that gets behind the siding can enter the wall assembly around the door opening, leading to rot, mold, and structural damage. Door flashing is particularly important because doors are at the bottom of the wall, where water from above naturally concentrates. A properly flashed door directs water away from the opening and out to the exterior.
Why It Matters
Door flashing failures can cause significant hidden water damage in the wall assembly before any visible signs appear.