It doesn't sound like a litter box problem (urinating) as much as a spraying problem. As pointed out upthread, spraying and marking territory is a different problem than just urinating.

I think you have too many cats for the size of the house. It would probably be more humane for them to not have to be so crowded in territory. Have you considered finding new homes for a few of them? I think that would be the best solution for the welfare of the cats.