DE can cause respiratory problems if kicked up in a dust. What is your purpose of using it in the coop? If to deter internal parasites, you're throwing your money away, since any oocysts they pick up they will get from outside. If to deter flies, perhaps, though I'd wait till there was an issue before using it. You could sprinkle it on the roosts where they meet the wall of the coop, and perhaps rub some into the perches. Or you could use some pyrethrin/permethrin poultry dust in those cracks as a deterrent. DE is hard on the lungs, both for the chicken and for the poultry owner. You may own chickens for years and never have an issue with mites or internal parasites. Wishing you the best with your flock!