Modular building cleanrooms having integrated mechanical rooms with removable, walkable floor panels above their ceilings are disclosed. The attic area above the sealed cleanroom ceiling and below joists of the modular building's frame provides an interstitial volume through which ducts, piping, electrical conduit, etc. run from the mechanical room to dampers, valves, and junctions boxes. The walkable floor panels can be bar grating through which maintenance people can see components below. The floor panels can have individual sections that may be removed for access to the components. Along with walkable floor panels, the modular building roofs can have foldable electrical panelboards that are pre-wired at the factory and then rotated upright once delivered to their destination.