"When you build a new house, you must make a guard-rail for your roof...preventing one who falls from falling off the roof." (Deut. 22:8)

The obligation to build a parapet around the roof applies to all houses, not just newly built ones, and devolves on the individual whenever a house comes into his possession, not only if he builds it! The reason the Torah nonetheless chooses to couch this law in the case of a newly built house is in order to allow the following, allegorical interpretation:

When you get married...you are commencing a new period in your life...

"When you build a new house": When you get married and begin to build a new household within the Jewish people, you are commencing a new period in your life, with new responsibilities and tasks that you have never yet had to face. Therefore...

"Make a parapet for your roof": The stringencies you have relied on until now to safeguard your observance of the Torah and its commandments are no longer sufficient. The demands of your new lifestyle will undermine your spirituality unless you take on new precautions.

"Preventing one who falls off from falling the roof": For the new, additional involvement with the exigencies of life in the physical world that are associated with marriage is a descent, and you are therefore poised to fall from your previous spiritual level unless you take preventative measures.

Another, more general interpretation:

"When you build a new house" means, "when you set out to make the physical world into a home for G‑d." This house is considered "new" because spiritualizing the physical is a reversal of the order of creation. G‑d made the physical world appear consummately physical; we reveal its inner Divine essence and make it a vehicle for the spiritual.

...our involvement in the physical world is not for our own betterment or indulgence...

"Make a parapet for your roof": In order to succeed in this mission and avoid being dragged into the materiality of the physical world, we must be sure to remain sufficiently aloof from the world. This we accomplish by setting appropriate boundaries, red lines that we do not cross. This shows that our involvement in the physical world is not for our own betterment or indulgence, but for selfless purposes. In turn, this selflessness opens us up to greater insights into Divine reality and to higher levels of the Divine consciousness that we seed to disseminate.

Adapted from Likutei Sichot, vol. 2, pp. 384ff; vol. 19, pp. 208ff
© 2001 Chabad of California/www.LAchumash.org