Weathertight Doors are primarily located above the waterline of the vessel. They are designed to prevent the ingress of water from outside to inside. This generally includes a small head of water. Their main use is to prevent the ingress of green seas into the space of the ship they are designed to protect. Most of the doors on deck/accommodation on tanker ships are weathertight doors. These doors are designed to open outwards, thereby ensuring a positive pressure should there be a draught of water acting on them.
Watertight doors, on the other hand, are designed to prevent water ingress from both sides, thereby ensuring that the watertight integrity of the neighboring compartment of the ship is not lost. Watertight doors are located below deck level and are designed to open and close upwards or sidewards (usually by automatic means). Most of the doors on car carrier vessels and Ro-Ro decks are watertight. A remote indication of the status (open / close) of watertight doors is required by regulations.