My tarp rides on the tent at the ridge pole, but a few inches above the at the junction with the side walls. I extend it about a 6-inches on either side. More would probably be better, but that's the size tarp I happen to have. Most of the time, air moves pretty freely between the tarp and the tent. The tarp rests on the canvas when it rains or under a snow load, but I don't leave a snow load on the tent very long. Just pound it off. My main reasons for using the tarp are to increase the longevity of the canvas by reducing the effects of the sun, and to keep pitch and other stuff from gunking up the outside of the roof.