Timber slabs design is currently limited to grid layouts derived from prefabricated rectangular panels. The lack of adaptability of timber slabs to accommodate multiple span directions makes it difficult to compete with reinforced concrete slabs constructed on site. This paper describes an adaptive slab system composed of thin Cross-Laminated Timber (CLT) panels and robot-fabricated beam networks for reinforcement. The beam network was developed through intricate negotiation of structural optimization and fabrication constraints, which can adapt to changes in slab span and directions. A mobile robot platform that allows for on-site assembly of timber sticks into continuous beam networks was developed. The robot platform and slab system were tested with a case study pavilion. The co-design of the robot platform and the slab system fills the gap in on-site robotic timber construction and expands the design freedom of timber buildings.