DELTABEAM® Composite Beam represents an excellent solution for creating a slim floor structure with timber slabs. Hybrid structures are nowadays increasingly popular as the combination of steel, concrete and timber enables the optimization of the design by taking advantage of the strong points of each material. However, the combination of different materials and the use ofinnovative design details set the challenge for assessing the safety and reliability of the solution. Therefore, Peikko investigated the load bearing capacity of the joints between DELTABEAM® and crosslaminated timber slabs both at ambient temperature and in fire situation. Load transfer tests, a charring test and a loaded fire test were carried out in order to prove the effectiveness of the transverse reinforcement, which ties the beam and slab together and secures the load transfer. As far as DELTABEAM®, the fire resistance is ensured by inner rebars so that no additional fireproofing is needed. In particular, the full-scale loaded fire test confirmed the satisfactory performance of both edge and intermediate DELTABEAM®s with cross-laminated timber slabs. The results allowed for developing test-based design recommendations and showed how the typical timber slab details can be used in a new and more efficient way with DELTABEAM® slim floor solution.