Abstract: There is provided herein a roof for a reactor vessel that utilizes a weighted floating cover. The cover is free to move up and down within the vessel as the fluid level changes during the operational cycle. In some embodiment, the cover itself is not directly connected to the reactor vessel or its sidewall, but instead is held in place and supported from above by truss-like support members that permit movement of the cover in concert with the changing fluid level in the vessel. In an embodiment, the support members are anchored proximate to the top of the reactor vessel.