Blocked drains are normally cleared by rodding or jetting. Blockages are mostly caused by a build up of solid matter in a drain that is best removed by breaking up and flushing away. The most effective way of doing this is by high pressure water jetting. Although jetting is always preferable, in some cases this is not possible, and so the old fashioned practice of rodding the drain or sewer is the only alternative, and though less certain is normally effective.