Layback rolls require considerable lower back flexibility, most of us sit hunched up at desks for ours on end or slouched on a couch and as we age we generally loose flexibility in our spine. Gently stretching backwards over a bosu ball can greatly help you reach the goal of being able to rest your head on the back deck of your kayak during the recovery phase of a layback roll.
Forward finishing rolls become easier as your center of gravity is kept close to the center of the kayaks rotation. If you can improve your hamstring flexibility (and lower back to an extent) you will be able to get lower onto the front deck and keep your body n the water for a longer portion of the roll. Please note my posture in this picture is very bad, it could cause injury to my lower back I will replace it with a picture of the correct posture as soon as I can get it taken.
The standard Greenland roll and many others require considerable torso rotation in order to achieve the preferred position with your shoulders flat on the water surface while your body is sticking our sideways from the kayak. Gently stretching your back, glute and abdomen muscles will help you achieve this position.
With all these stretching exercises it is important to do it gently. aim to start two times a day and spend five minutes stretching each time. Don’t push it, don’t bounce your body to place extra stress on the muscles. First time you do a stretch simply see how far you can turn or bend before you can feel tension in your muscles, then relax and repeat. Expect to make very slow gentle progress over months not days. Once you have reached your flexibility goals don’t stop, continue daily to maintain your gains. And remember – stretching should never hurt.
All the stretches can be done in your kayak. I suggest you should warm up this way prior to any rolling training or practice. 
Initially you can practice the stretching exercises while holding onto the edge of a dock, the bow of a friends kayak, or the edge of a pool if you are in one. Remember it is important not to overdo the stretches in your desire to roll, you body needs to recover from all the years of neglect and become accustomed to the new expectations you have of it. Whether stretching out on land or afloat it is important to warm up your muscles first, whether that isa just bouncing arround or joggin, or even a warm up paddle, make sure not to be strecthing cold muslces.
You may find you are more flexible in one direction that the other, this is one reason why we all tend to favor one side rather than the other when rolling, strength variation being the another. If you can over time become flexible in both directions of rotation, and legs, you will find your on and off side rolls will both considerably improve as you balance your muscle tension.
Once you are confident in your kayak with support you can graduate to stretching with just the paddle as a support. This will allow you to spontaneously stretch out when you are on a long paddle or in the middle of a lake or sea and just feel the urge to roll.
When practicing these stretches with just a paddle be ready for the inevitable. One day you will accidentally roll in and be provided the opportunity to practice a combat roll. If you do not yet have a strong reliable roll then the use of a paddle float can reduce the probability of a messy wet exit. Especially when you are laying on your back deck you are very low in the water other kayaks may not see you so make sure you are wearing something highly visible.
When you are fully able to rotate your core, layback on your back deck and get your nose on the foredeck you will be in great shape for rolling. Some would argue that this level of flexibility is unnecessary, and they are correct, it is possible to roll without being flexible. however to roll with the best possible Greenland style a high degree of flexibility is a prerequisite.
Your flexibility should no longer inhibit your ability to roll, instead you can now focus on your body position, knowing now that you have the flexibility to get ito the position and it is (just) a matter of training your muscle memory. In my opinion you should not start learning to roll until you have gained sufficient flexibility for the roll you want to start with. If you are not flexible enough you will be fighting your bodies resistance and probably end up using too much force and either fail or damage yourself.
