Wow 20 chickens sounds like a lot to me. I have six hens in a 4 X 6 coop with a 9 foot x 20 foot run plus the underneath of the coop and still think that's to small for my girls... I gave up after three days with pine shavings in the coop. There was no way to keep it clean. I cleaned it every day. I've since switched over to using sand and pdz in the coop. The girls and I love it! Wow what a difference cleaning the coop it has been. I just use a kitty litter coop on an extended stick. Takes me maybe two minutes a day to clean.
I'm new to raising chickens and want nothing more than to keep my girls dry and clean.
Do you have lots of ventilation above your girls when they roost for the night? Poop trays under the roosting boxes? Windows to open during the daytime?