I made a thread about this same question the other day.
I've made my 10'x10' coop using part of our old deck as the foundation since it was built so strong and the size I wanted already. The floor is wood planks 2'x8' to be exact and they each have about a 1/4" between them. I should've just sheeted the floor to begin with but now it would be a real pain. Instead, I've got some real thick rubberized tarping material (Army surplus) that I'm going to cover the coop floor with. I will prob. use 2 layers of it and then add 8" or so of pine shavings/straw above it. This way, I can remove the whole bit and clean out really good. If I hose of the inside, the water etc. can just drip down thru the cracks. I'll give it a shot and if it doesn't work out, I'll sheet the floor with ply later.