Can I use an old aluminum road sign as a replacement floor for my hot water heater closet? The old floor is rotten and has some give to it right where a portion of my hot water heater sits, and my dad told me to "cut a peice of plywood about the same size and shape of the existing closet floor and just lay it on top of the old floor. That should probably last a good 5 or 10 more years. That's all I've ever done." But I want to know if an old aluminum or galvinized thin sheet metal road sign would be hazardous, dangerous, and/or illegal in any way whatsoever? I already have a rough cut of the old sign/new floor and plan on finishing the whole project within a day or two so thought I might do a little research to make sure I'm not doing something stupid that I don't already know.
I'm sorry, but that made me laugh.
First thing you should do, is find what is causing the floor to rot and fix the leak. What is "spongy"? The floor or the joist? If the joists are rotten, you need to replace them, then put a new floor down.
After you fix the leak problem and install a new floor, you could install a Water Heater Drain Pan under your water heater, which will catch any leaking water and drain it away (with the proper plumbing hookups)