Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 184
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 184, with English translation by Maimonides (Rambam).
לעשות מעקה, שנאמר: "ועשית מעקה לגגך" (דברים כב,ח).
To construct a guard rail, as [Deuteronomy 22:8] states: "And you shall construct a guard rail for your roof."