Foam rolling on the lumbar area is ill-advised since it is difficult to relax the lower back during the process. Without relaxation taking place, any tender spot or trigger point is not likely to be "released". Secondly, foam rolling directly on the lower back tends to apply excessive pressure on the lumbar spine and intervertebral discs. There is an abundance of nerve fibers that run down the length of the spinal column. Direct pressure on nerves is not a good idea. Thirdly, the application of direct pressure on the lower back is not safe for some organs such as the kidneys and liver without rib cage protection. For these reasons, it is not recommended that you use the foam roller for your lower back. Instead, foam roll on muscles such as your glutes, hamstrings and hip flexors. It's much safer and you may actually target the source of your back pain.
Brian Danley, CFT
"Motivation is what gets you started. Habit is what keeps you going."