It says in Parashath Ki These: If you build a new house, you must make a fence for your roof.
This can be understood on a different level. The month of Elul is a time for introspection and Teshubah (repentance). Our bodies are like the house where the soul dwells. Someone who repents properly becomes a new entity -- a new house for the soul.
The danger that anyone who repents faces, is the prospect of regressing. The best advice one could give someone who repents correctly not to regress, is to build a "fence for the roof". Our Hakhamim made fences for us around the Torah, by forbidding us to do something which is similar to a Torah prohibition but, nevertheless, permitted by the Torah, lest we should inadvertently come to do that which is forbidden by the Torah. We, in turn, should go one step further and make additional fences for ourselves by building fences to forbid even that which was permitted by the Hakhamim.
In this way, if a person stumbles and transgresses one of his self built fences, he does not transgress an actual law from the Torah or the Hakhamim, but only the fence that he built for himself.
(See Addereth Eliyahu, Parashath Ki These)
