A modular room unit forming a room space, the modular room unit comprises: a base slab forming a bottom of the modular room unit, the base slab having a top surface facing toward the room space, and a side surface extending transversely to the top surface in a direction away from the room space, a wall element forming at least part of a side wall of the modular room unit, the wall element having a first wall surface facing toward the room space, a second wall surface opposite to the first wall surface on the other side of the wall element, and an end wall surface which is arranged to extend at least between the first wall surface and the second wall surface, the wall element being arranged on the base slab such that the end wall surface of the wall element is arranged on the top surface of the base slab, and a waterproof layer provided on the top surface of the base slab such that the waterproof layer is arranged to extend on the top surface of the base slab and between the top surface of the base slab and the end wall surface of the wall element for providing a uniform waterproof layer to the top surface of the base slab and between the base slab and the end wall of the wall element; and wherein the wall element is formed from a metal thin sheet having a flange extending in a direction away from the room space; characterized in that the flange comprises a first flange portion which forms part of the end wall surface of the wall element and a second flange portion extending from the first flange portion in a direction away from the top surface of the base slab, the modular room unit further comprises a connecting element connecting the base slab and the wall element together, the connecting element is connected to the second flange portion and to the side surface of the base slab. a base slab forming a bottom of the modular room unit, the base slab having a top surface facing toward the room space, and a side surface extending transversely to the top surface in a direction away from the room space, a wall element forming at least part of a side wall of the modular room unit, the wall element having a first wall surface facing toward the room space, a second wall surface opposite to the first wall surface on the other side of the wall element, and an end wall surface which is arranged to extend at least between the first wall surface and the second wall surface, the wall element being arranged on the base slab such that the end wall surface of the wall element is arranged on the top surface of the base slab, and a waterproof layer provided on the top surface of the base slab such that the waterproof layer is arranged to extend on the top surface of the base slab and between the top surface of the base slab and the end wall surface of the wall element for providing a uniform waterproof layer to the top surface of the base slab and between the base slab and the end wall of the wall element; and wherein the wall element is formed from a metal thin sheet having a flange extending in a direction away from the room space; characterized in that the flange comprises a first flange portion which forms part of the end wall surface of the wall element and a second flange portion extending from the first flange portion in a direction away from the top surface of the base slab, the modular room unit further comprises a connecting element connecting the base slab and the wall element together, the connecting element is connected to the second flange portion and to the side surface of the base slab.