If I had to come up with a SQ Ft for a roosting coop only I would say 1 sqft would be plenty to possibly keep from getting to much body heat from the flock during hot summer nights, but keep in mind the coop needs to have plenty of air movement. If you were to build a TIGHT 10x10ft coop and made roost for 100 chickens and put that many in it---they would probably pass out or worse from the heat build up on a real hot summer night. That's why I either build them with a lot of gaps between the boards or with windows on each side to allow a cross wind.
In the pic when I have had 70 or 80 in this coop roosting on those poles---they all will be roosting on the poles in about a 5x7 area----that's only 35 sq ft less than 1/2 sq ft each. But In the winter I would have no problem roosting 70 chickens in 35 sq ft But that coop would be to small in my opinion for summer with that many.
People can say what they want-----but look at my roost pole pic----at 3 sqft each(18 chickens)----if I had 18 chickens inside this coop they ALL would be on the top roost pole with 3 or 4 on the next lower----the coop would look almost empty. AGAIN If they were living/eating/laying in it most all the time---sure I could see allowing them that much space.
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