Yes, you can use a tarp as a rainfly! That's why hammock tarps are there; to protect you against rain and sun! The purpose of this rainfly is to keep you dry under the hammock. As the Amazon market is filled with various types of hammock rainfly, you need to find the ones that are 100% waterproof! Otherwise, you lose the entire purpose of a tarp as a rainfly. You can either choose from poly tarps or canvas tarps and protect yourself in the best way possible while lying in a hammock.