A tile system for use on a grid of a suspended ceiling comprises a plurality of tiles, each said tile having a main body having an exposed side and a concealed side, the exposed side defining a main surface, the concealed side defining a rear plane, and four side surfaces between the exposed side and the concealed side of the main body, wherein two of the four side surfaces have at least one tongue in the rear plane, and two other of the four side surfaces have at least one groove open to the rear plane. Fasteners, such as clips, are configured for attaching the tongues of one of the tiles to upside-down T channels of the grid, such that the rear plane of the tile is against a downwardly facing surface of the T channels of the grid. The tiles are arranged such that, for a pair of the tiles side by side and sharing a same channel, one of the tiles has its at least one tongue connected to the channel by said fasteners, and the other of the tiles has its at least one groove receiving the at least one tongue of the adjacent tile. A method of installation is also provided.