One problem with a tarp-covered dog pen -- and I speak from sad experience -- is that raccoons can carve through it like a knife through butter.
We started that way and, after animals cut through the tarp or got between it and the top of the pen, finally built a roof (wood frame, topped with that green corrugated plastic stuff). And, at the advice of someone on this forum, sunk concrete blocks under the pen's frame to stop predators from digging beneath. (My husband, who did this digging in cement-hard Carolinas red clay, still talks about what a brutal job that was.)
The other thing we were forced to do to our pen was completely line the interior with hardware cloth, which has smaller holes than a dog pen has. We did this after the night we and our neighbors were awakened by the horrible screams of our barred rock being eaten alive from outside the pen by a raccoon. The raccoon reached through the pen's large holes, grabbed the sleeping/roosting hen, who was flush against the side of the pen, and ate her face off. I know this is a gruesome thing to read, but I just throw it out there was a warning. It was AWFUL and though I know a coon is just being a coon, it forever changed the way I think about them!
Anyway, since our retrofitting, the pen has not been compromised by a predator. Wish we'd done it right at the start.