If you are reading through a building story and think of some comment or point not specifically uniquely related to that particular topic, it would be appropriate to start a new topic and reference back to the original thread if you think appropriate.
Some examples from this page:
post with some examples of replies that would make good stand along topics]
Topics are:
Why so much cement is used when laying floor tiles
and Cultural Aspects of Building
The topics can be split off by the moderator, but I didn't do it this time since the replies reference back to the original thread.
When to Start a New Topic
Moderators: MGV12, BKKBILL, Sometimewoodworker