The most basic part of rolling a kayak, the most important part, is being able to orient yourself to the boat before you start the motion. In whitewater the paddler can get pulled in any direction, and needs to be able to assume a protected, turtle-like tuck when they flip over. This forward tuck makes it possible to get your paddle situated parallel to the boat at the water line, for a proper roll. These days it is modern and cool to be able to roll from any position. Playboaters master the back deck roll because it is integral to the moves they do. For the regular whitewater kayaker, a regular forward tuck leading into a basic sweep or C to C roll is all you really need. Getting the offside is great, and then explore. First, get a good tuck and set up position, which requires hamstring flexibility to touch your toes and them some, and crunch strength to pull your body to the front deck no matter what the river wants to do to you. If you have that strength, you've no excuse, save the panic of being upside down underwater, which happens to almost all of us. Stop going for that rip cord, and TUCK. From there it will be much easier.