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The vent height above the roost was a concern of mine, I thought they might be too low compared to the roost. How high above the roost should the bottom of the vents be minimum? Right now there is about a foot of space.
The safe answer, from what I have read, is that the venting should be as high as possible from the roosting chickens. I seem to recall reading 18 inches was a good distance, but maybe 12 inches will be fine. A number of people close up their vents in the harsh winter weather to prevent drafts, but then you have the danger of humidity and frostbite.
I have seen some of these slide out tray designs on coops sold at the farm stores. I would only suggest that you actually try it out first as a full tray may not pull out as easy as an empty tray. A full tray may sag from the weight, snag on housing, and make your life miserable. Typical complaint on that design I have heard from others. I opted to make a drop down clean out panel and a solid floor (but my coop is larger and built for deep litter).
Nice drawings. Wish I was that organized when I was making my coop.