If an existing main is to be replaced with a smaller diameter main it may be possible to slip line the old main. This involves inserting the new main up the old one.

Sliplining is one of the oldest methods for trenchless rehabilitation of existing pipelines. Sliplining is used to repair leaks or restore structural stability to an existing pipeline. Sliplining is completed by installing a smaller, "carrier pipe" into a larger "host pipe".

Sliplining is generally a very cost effective rehabilitation method. It is also very easy to install and requires tools and equipment widely available to any pipeline contractor. Segmental slipining may not require bypassing of the existing flow.