If your rafters aren't insulated and are relatively open, the aluminized bubble wrap products do a pretty good job of keeping the heat outdoors where it belongs. With a pitch roof, you must provide an air path in the gap between the shingle roof and the reflective layer. Otherwise, the heated air will just soak through the bubble wrap. With a flat roof, you pretty much have to have a refractive (insulation) layer, as there is no convective (smokestack) effect to speak of.
With a 50 degree gradient, it won't take much air path at all to notice a difference. In one house I rented, I put up some of the cheapest stuff just to see how it would work: thin, white styrofoam with the mylar glued to one side stapled to the bottom of the rafters. Maybe 1/4 or 3/8 inch thick. 4 inch gap on top between the rafters and the ridge cap. The roof was 3 tab shingles, slate or charcoal color, over the pretty much uninsulated (somewhat) finished (bedroom-ish) attic. I stuck a solar (pv) vent fan in the downwind gable hole and used a 50 ft roll of the silver bubble wrap to fashion a ridge duct leading to the solar fan. The whole project paid for itself in one summer in Kentucky. I imagine it would work even better in Texas. Nobody could see it except my electric bill...
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