Do you see the issue in the picture?
Well, there are a couple, but let's focus on the main sewer pipe.
To ensure proper drainage, horizontal drain pipes should slope downward at 1/4" to 1/2" per foot. At a recent home inspection, we captured this picture of the main sewer line in the crawlspace. The direction of flow is from right to the left. Starting from the right, the slope is good up until the 'Y' connection. After the 'Y' connection, the pipe does not appear properly sloped before it exits the foundation. In fact, there appears to be a low spot at the 'Y' connection. Further visual investigation located pipe supports securing the pipe in place, so this is not an issue related to sagging from improper support. This issue may have been created during original installation.
In this instance, the low spot appears to be minor, and we can't confirm there is an issue. We can only report on what we see. So what did we tell the buyer? "[In the main sewer line] there is a dip in the pipe creating a low spot. Debris may collect in the low spot and over time may result in a blockage. Recommend inquiring of the homeowner of any past blockages that needed to be removed, and their frequency of occurence. If blockages are frequent, recommend evaluation and repair by a licensed plumbing contractor."