IF this is happening on Earth, then the acceleration of gravity wherever the captain and the roof are is about 9.8 m/s² downward.
IF his shield is not affected by air resistance, and it's free to behave under the influence of gravity alone, then its acceleration is 9.8 m/s² downward, from the moment it leaves his hand until the moment it hits a building, a tree, a car, a bus, an evildoer, or the ground.
Whatever speed it has when it leaves his hand, in whatever direction, makes no difference.
