When we got our hens, we let them free range in our backyard while we prepped the side of the garage for there pen. When we were done, it was just bare dirt. In only a few days time, the flys became unbearable for us. It has been a record high summer here for us in the Pacific Northwest, but that aside, it was no excuse for the volume of flys we had. I researched around, and found posts like this threads original. We swithced to cleaned construction river sand (not play sand), and havn't looked back since.
The flys are hardly noticable now. Cleaning is a breeze in comparison to dirt. I've had other pets, and wood shavings were out. To bulky and annoying to get rid of it. Drainage is amazing as well. The sand will dry well before dirt would ever have dried.
To clean the pen, I took a pitch fork, and zip tied 1/2" x 1/2" wire mesh to it. Like a big kitty litter scoop. It works amazing to pick out the clumps. We put 2" of sand in the pen, and 1.5" in the coop itself. It make cleaning a breeze, and the smell is so much better as well.
Some people worry that sand will harbor bacteria, and be unhealthy. Well I look at it this way. If people have successfully used sand for more then 5 years with out outbreak, or incident, it's good enough for me. Also, because it drains so much better, there is less moisture for bacteria to grow in as well.
Sand!
I've been using sand for 3 years and agree. One reason it performs so well is that during the scooping some of the sand sticks to the poop and gets removed. More sand gets added as needed.
Just want to mention that outdoor sand needs to be kept dry. I learned the hard way that even with daily cleaning, wet sand + chicken poop = cackle spackle that's super smelly and then hardens to become cackle cement. During the first winter (in Western Washington) we quickly added a tarp to the top of their fenced run. It helped but by spring it was obvious it needed improvement. Now it has a translucent roof and everyone is much happier. It's the one and only time we had to shovel out a large amount and replace it with fresh.