I can attest to the truth of what Dawn just said! I put sand in the run when I built it last spring. It worked great all summer and fall. Easy, easy, easy. I thought there was enough of a slope to the ground to handle any run-off. But the girls spent those months scratching and digging to make it more level! hahaha The sand needs to be dry to be most effective.
When the winter rains came, as they do in the Pacific NW, I was really unprepared. My run was only partially covered. It's narrow and long rather than a big square, which meant the rain blew in easily. I bought a clear tarp which helped, but not enough to keep the ground (sand) dry. To top it off, I don't have the coarse sand Dawn recommends, it's from a landscaping company but it's fine and compacts when wet. When we got heavy rain for several days on end, it was a mess, I wasn't able to use the litter scoop easily, so the poop mixed with the overly wet sand and it all compacted. And it smelled bad, too. I ended up scooping a lot of it out and replacing it with dry sand I'd had left over. I did that 2 - 3 times. For now I break up the compacted sand every day as I'm cleaning so it will dry out & air out as much as possible before the next rainy day. (The girls tend to stomp down certain areas, so those are really the only spots I have to break up.) Keeping it loosened every day makes cleaning much easier.
This summer I'll make changes. The clear tarp will be replaced with a more durable material and extended to keep the run drier. And I'll be looking for sand that's coarser and sharper so it will drain better.
The sand inside the coop has been perfect! Dry and super easy to keep clean. I love it!