Our group is having a "discussion" on this. We're on the wet coast and usually use a tarp over our wall tents to keep the rain and /or snow off the canvas.  If you use a wall tent and have a tarp stretched over it do you try to keep the tarp completely off the tent by suspending it on a rope or do you use the tent to support it.

JW

JW, I use a pole or rope & come along and keep the tarp up off my tent. I leave a tent in all winter this way and the snow slides off. The tent has a interanal frame thats independant of the tarp.
The tarp extends three feet past the sides of the tent and this makes it safe for snow loads up to 2 1/2 feet.

For rain I'd still like a small airspace if possible.

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.

A plastic tarp over your canvas wall tent is desirable in most cases because it protects it from sap, bird doo, and other gunk that falls on it.  

To keep your tent breathable and  working as it should, try to keep the plastic at least 4" off the canvas.

Yeah absolutely keep the tarp off the canvas if you use one.

I find if the tarp is 3' or more above the tent ( rope hung between 2 trees )and left to hang down ALL snow will just fall off! No worry about the tent caving in.