this was me. I tried it on my popup roof but was constantly nervous I would tighten them too much and crack the roof. I also didn't feel comfortable I wasn't able to put a bowline on my kayak on and the sternline to me wouldn't do much good because the kayak hung off the roof by 3 feet and if I was in an a accident the kayak could slip right through my car window. That and when I got to the campground I realized I had to put the kayak on my car because the boat ramp wasn't near the campground. Much less hassle keeping the kayak on my car roof and in my mind safer, because I can safely use the bow and stern lines and tighen the straps tighter on the car. Yes, I do take a hit on mpg but not terribly significant. When I tow I calculate gas using my city numbers to err on the conservative side. Even though I usually do better than that.