A boiler in the form of a furnace comprises at least two side walls, at least two end walls, a roof, and a bottom. A distance between the at least two end walls defines a depth (D) of the furnace, and a direction between the at least two end walls is a depth direction (Dd). At least one beam extends in the depth direction (Dd). The beam is positioned under the bottom for supporting the bottom, and the beam is also positioned between the at least two side walls at a distance from each side wall. The ends of the beam extend under the vertical portions of the at least two end walls. At least one rod is between each end of the beam and the corresponding vertical portion of the side wall. The rods transfer a load from the beam to the vertical portions of the end walls.