Residents of a metropolis live in a world with artificially constructed boundaries between floors. Each floor is built on top of a previous one, forcing residents to navigate stairwells to move between them. A pair of wings is found on certain buildings, and these allow residents to glide from floor to floor.
The primary objective of Flat Flapp is to control a resident who must move from one floor to another by flapping these wings. The flight style presented in the game simulates the surreal experience of a living being with wings in a real-world environment. To progress through the map, players must locate reachable floors and reach them in a logical and efficient manner.
Flapping in the game can be done with either a steady rhythmic motion or in bursts of frequency. Gameplay integrates environmental elements such as unlocked wings, wind resistance, and high-rise buildings that affect air resistance and the resident's flight speed. The game may be played directly in a web browser without requiring any downloads or registrations, offering players flexibility in their gaming preferences.