A ceiling joist formwork system (1) comprises a ceiling table (2), a vertical support (6), an inner board (3), a ceiling joist base (4), and an outer board (5), the outer board (5) and the ceiling joist base (4) being interconnected, especially rigidly interconnected. The vertical support (6) is rigidly fixed to the ceiling table (2), and the ceiling joist base (4) is fixed to the vertical support. The system is characterized in that the ceiling joist base (4) and the outer board (5) are rotatably mounted about a first articulated point (GP1) on the vertical support (6), a guide (7) is rigidly fixed to the inner board (3) and the guide (7) and the inner board (3) are rotatably mounted about a second articulated point (GP2) on the ceiling joist base (4). The inner board (3) is guided by means of an edge (14a, 14b) in relation to the ceiling table (2) along a direction comprising at least one component vertical to the formwork upper plane of the inner board (3). The ceiling joist formwork system can be used to simplify the dismantling, especially of the inner board, and the risk of damage to the inner board is reduced.