Why is the roof of the Lincoln Memorial flat?

The roof of the Lincoln Memorial is flat to create a sense of monumentality and to emphasize the classical architectural style, specifically the use of a temple-like structure. This design choice aligns with the Greek Doric columns that support the memorial, enhancing its grandeur and symbolism. Additionally, a flat roof allows for the incorporation of a large central chamber that houses the statue of Abraham Lincoln, making it a focal point of the memorial.