I have playground sand (3-4") over 3-4" of gravel. We put gravel down because the whole setup is on a slight slope and we wanted water draining from uphill to pass through without getting blocked by the coop. However I think the gravel wasn't necessary and it's not ideal, as it mixes in with the sand and I end up scooping out a lot of gravel with the poop (I clean it like a giant kitty litter box) and then it goes into my compost bin. I'd rather not have a bunch of gravel in my compost bin, but that's the way it is. No biggie. Bottom line is I agree with others, playground sand is great, and you don't need the underlayment material.