Here is what we built. We live in the woods, so it has to be predator proof - well that's true pretty much wherever you live, and probably that, along with a place to keep dry and sheltered are the most important aspects. The hardware cloth goes about a foot under the ground and a foot out. We built this for 6 ducks but soon it will house 8. I wish I had made the inside sleeping space a little larger - it is about 4x12. It has a plywood floor covered by stick on linoleum tiles. On top of that I put shavings and clean those every day. No food or water goes in that area and I have a little door that I shut between that and the run when the ducks go in at night. In the run it is just bare ground, half covered with a tiny pond and pea gravel and the other half covered in shavings that get wet and have to be scooped and replaced quite often. It's not the best, but I've tried straw and same thing, only harder to keep clean and dry. No matter what ducks are messy and a lot of work to take care of - they must be worth it! We used a tin roof. I live in the Pacific NW and it rains here a lot. My ducks go out and free-range in the yard once or twice a day, and I also have a tractor that I can put them in out there so they can be in grass several times a day - because that's what they really want to do - be outside and forage. Due to predators I have to always be outside watching my ducks when they're in the yard, or have them in the tractor. Everyone's situation is so unique, you'll find a lot of different houses that make sense for different people. Good luck!
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