City Girl,
Doubled plywood for a floor sitting on cinder blocks at the corners, if I understand you correctly. Do you ever plan to step into the house to clean it or whatever? If so, I would put a framework under that floor, build it just like a real house. I'll explain this in a minute. But first, do you have a handle on setting those cinder blocks? They must be level and stable. They are your foundation. If they rock, the coop will rock. Need detailed directions? Just ask.
Here's what I would do for the floor. I would go to Lowe's and buy 3 treated 8 foot 2X6's. I'd have the guys there cut them to length for me. That probably won't be exactly 4' on all of them. Probably 4' on two of them and 3'-9" on four. We'll measure for this later. First let me tell you what you're doing, so it will make sense. (I hope) I'm going to pretend that you will use screws and a screw gun to put this together. You could nail it, too. I couldn't, because I can't hammer a nail. I'm very spoiled with a big old nail gun, which most folks don't have. Anyway....
Take one of the longer boards, set it on your cinder block foundation, on the outside edges of the blocks. Set it on end, so it is 6" tall. Put a shorter board up against it at the corner, on the inside. Screw them together. Use at least two screws, 3" long. We are making a box here, so the shorter board plus the width of the two longer boards will equal your 4' measure. Go to the other end of your second board and use the other longer board at the other corner, on the outside of the board. Okay, now you've used both of your longer boards, opposite of each other, so you can't go wrong from here. Attach another board at the remaining corners. You should have a square. These are your rim joists. They support the outer walls of the coop. You will want this to be pretty square. If you don't have a carpenter's square, you can use something that you know is square like a nonflexible box to check the square of the thing as you are building it. Just stick it in the corners and see if it fits tightly. If not, move the boards until it does. This doesn't have to be perfect, but it shouldn't be too far off.
Now for the floor joists, which will support you should you step into the coop. Take the other two boards, on end, and screw them in at about 16" and 32" inside your box. Now you have a very sturdy floor framed up.
Take a piece of plywood and screw it onto this framing, one screw every 8 inches or so along the outside edges and along the floor joists. These will be shorter screws. You can use the other piece of plywood on top if you don't need it for anything else. You don't have to worry about hitting your joists on this one. Just screw it in around the edges and wherever else in the middle.
Then construct your coop on top of this platform. I think of building in terms of 2X4's, and I would screw the bottom 2X4 of the wall panel directly into the plywood floor. But with 2X2's, I worry that they might split. So I would buy a drill bit slightly smaller than my screws and drill pilot holes all the way through the 2X2 first. Drill 4 or 5 holes down the length of the 2X2, then screw it into the floor. Then go to the next panel, attach it to the floor the same way, then attach the wall panels together at the corner, and so on.
Now for the correct measurements on the 2X6's. You want these to be the exact measurements of the bottom 2X2's in your wall panels. These might all be the same length, but I doubt it. Normally, two sides will be slightly shorter, to give you finished dimensions that are equal. So measure them, and add 2 more of the shorter length for your floor joists.
I hope that made sense If I were near you, I would come over with a truckload of tools and pop that baby out in 15 minutes. My nail gun would sound like an M-16. Pop pop pop pop! Pop! Oh, I've had a lot of coffee this morning!
But seriously, if you think you want to give it a try, I'm sure you can handle it. If you have any questions, ask me. Of course, you may think that I'm going a little overboard, and you could be right. It wouldn't be the first time!
