Pipe lining slightly reduces the inside diameter of the existing pipe because a new epoxy-saturated liner is installed within it. However, the reduction is generally minimal—typically only a few millimeters—and rarely affects normal drainage performance.

In many cases, water flow actually improves because the cured liner creates a smooth, seamless interior surface that reduces friction, eliminates rough corrosion, and helps prevent debris from catching inside the pipe. The result is often better overall flow despite the slightly smaller internal diameter.