Nesting Boxes

My girls haven't begun using their nest boxes yet (any day now, hopefully!), but I've read posts suggesting that they shouldn't be higher than the roosts so as to discourage the hens from roosting in, and fouling, them. Mine are actually at floor level with a 2 x 4 across the front as a lip. Also on a BYCer's suggestion, I bought an inexpensive "astro-turf" type mat and cut pieces to fit across the bottoms, and partially up the sides, of each box. This way when it's cleaning time you just lift out the mat, hose it off, let it dry and return it. I have them filled with brown paper bags that have been put through the shredder, but until they're actually laying I guess I won't know if this is a good filler or not.

 

As already mentioned, nesting boxes can be any height, although they should be lower than the roosts. If they're higher than 18 inches or so, I'd make sure there was a perch of some kind in front so that it's easier/more comfortable to get inside the boxes, and if they were super high (36 inches or more) I'd probably provide a chicken ramp to get up to them (depending on whether you have light flyers or heavy breeds. My nest boxes sit at about 24 inches.
If you're saying the bottom has rotted out of your nest boxes, I'd just replace it with 1/2 inch plywood or something, if that's what your question was asking...