Livingston, Morna (2006). "Yemen's Cisterns at High Altitudes". Traditional Dwellings and Settlements Review. 18 (1): 51. JSTOR23565982. (Hypertraditions: Tenth International Conference, December 15-18, 2006 --- Bangkok, Thailand). Dr. Selma Al-Radi relates that in her restoration of the Amiriya Madrasa in Rada'a, the nearly 500-year old existing qadad was so strong that they had to use a sledgehammer to break it apart.
Rathjens, Carl (1957). Jewish Domestic Architecture in San'a, Yemen. Jerusalem: The Israel Oriental Society, affiliate of the Hebrew University. p. 74 (note 9). OCLC48323774.
