Tegulae are flat oblong tiles with raised sides.  Imbrices are half pipes, slightly enlarged at one end. They fit together so that the tegula slightly overlaps the one below  and on each side the gaps between the tegulae are covered by the imbraces. This is similar to tiled roof construction  used around the world today.

roof roof  tiles
These tiles were attached to wooden roof beams running horizontally.                      The Roman version.
      Reconstructed tiles of the Parthenon. Notice the vertical wooden beams.