I have a downstairs bonus room in which the previous owners of my home installed VERY porous flagstone. The flagstone is absolutely filthy from the previous owners. You can see outlines on the floor of clean areas where their furniture sat. It is so porous that the couple of times my cats have thrown up hairballs on the stone it has left permanent stains no matter how much scrubbing I've done. I have mopped, scrubbed, and even washed it with an acid solution from Lowe's intended for outdoor stone and while that helped somewhat, it still looks filthy. I can't understand why they would install flagstone indoors. Anyway, my question is can carpet be installed over this? The flagstone is uneven and somewhat lumpy. Would I need to just get an extra thick pad? Does a moisture barrier of some kind need to go under the carpet since this room is partially basement? Also, there is a natural gas fireplace in the room. I am not using the fireplace, but would it be against code to take the carpet right up to the fireplace or would there need to be a tiled area in front of it?