Many homes include a fiberglass shower stall. Sometimes these units develop a water leak around the drain unit. This, of course, causes rotting of the sub-floor.The least expensive repair method attacks the problem from the underside. The following instructions assume that the crawlspace beneath your home is sufficient for performing this task.Tools and MaterialsTo replace a rotted sub-floor, you will need the basic carpentry and Plumbing tools plus a rotary or spiral cutting saw.The specific design of the installed shower stall, the layout of the plumbing pipes, and the thickness of the installed sub-floor govern the material requirements.Physical RepairsTag the shower stall as off limits for current use.Examine the current shower drain. Perform any necessary upper area disconnects.Enter the crawlspace and examine the work area.Complete the bottom side disassembling of the plumbing.To provide a dry workspace, spread polyethylene sheeting on the ground beneath the work area.Exceeding the area of the damaged wood by one to two inches, mark out a rectangular cutting line.Working at a controlled depth setting, use a rotary saw to cut away the marked wood. Be careful not to fully penetrate the existing flooring.Using a hammer and pry bar, remove the cutaway.To trim rotted wood that extends directly above a floor joist use a hammer, wood chisel, and pry bar.Measure and notate the size of the cutaway, including any necessary dimensions concerning the location of the drain hole.Use a 7" circular saw and a hole saw to cut replacement flooring.Construct a 2 by 4 frame that can be fitted to the existing floor joist in a manner that will support the union of the replacement flooring and the existing flooring along the seam that is perpendicular to the floor joists.Screw the replacement flooring to the topside of the 2 by 4 frame.Install the new floor to match the level of the existing flooring.Secure the floor by nailing or screwing the new frame into the matching floor joists.Using two inch long decking screws, draw the existing flooring flush to the 2 by 4 frame via a toenail approach. Be careful not to fully penetrate the existing flooring.To complete the job, restore the plumbing.