Just to clarify a little more, I have both a 4x20' mat that is on a flat rack made from plywood and 2x4's. Mat lays right on the wood. My field heater is 1200' of 1/2" black poly pipe and because there are significant gaps between the pipes the roofing does help by keeping the entire surface they're layng on hot so the hot roofing radiates and heats the pipes on the sides where the sun doesn't hit directly. At least that's my thinking. Also prevents weeds from growing.
Al