Question: Where should one affix the Mezuzah to the doorpost of one’s porch: On the right of the entrance from the house to the porch or on the right of the entrance from the porch to the house?
Answer: The Gemara (Yoma 11b) states that our Sages derived from the verse “On the doorposts of your house” that the Mezuzah must be placed on the side of the door through which one usually enters the house and one usually starts walking with one’s right foot. Thus, the Mezuzah must be affixed on the right side of the doorpost.
The Maharil writes in his responsa: “Regarding a doorpost between a house and a courtyard (meaning that the courtyard is attached to the house and there is a doorpost between the house and the courtyard), if the courtyard does not have an opening to the public domain (meaning that it is sealed and one can only enter the courtyard through the house), the Mezuzah should be placed on the right of the doorpost when entering from the house to the courtyard which is the way one must enter the courtyard.” The Bet Yosef (Yoreh De’ah, Chapter 289) quotes his opinion as Halacha. Based on this, it would seem that one should place the Mezuzah on the right side of the doorpost when entering from the house to the porch since the only way to access the porch is from the house.
Nevertheless, the Bet Meir disagrees with the ruling of the Maharil and writes that one must always place the Mezuzah on the right when entering from the courtyard to the house, for the determining factor is always the entrance to the house. Hagaon Chazon Ish rules likewise.
Thus, this issue is subject to a disagreement among the Poskim. Maran Rabbeinu Ovadia Yosef zt”l points out that this issue had already been a subject of dispute among the Tannaim in Masechet Mezuzah and based on the words of the Baraita, it seems that the Halacha follows the opinion of the Maharil and not that of the Bet Meir. Nonetheless, he writes that the Halacha does not necessarily follow the Baraita in Masechet Mezuzah, for it is possible that these Baraitot were not taught in the Batei Midrash of Rabbi Chiya and Rabbi Oshaya and any Baraita which was not taught in their Bet Midrash cannot be relied upon as Halacha. Indeed, the Responsa Ginat Veradim writes that there are Baraitot in Masechet Sofrim and other minor Masechtot which are difficult to understand and cannot be relied upon for their print contains many errors.
Halachically speaking, Maran zt”l writes that there is good reason to exempt porches from having a Mezuzah affixed altogether, for most porches do not have a ceiling and the Gemara (Menachot 33b) states that a house without a ceiling is exempt from having a Mezuzah affixed. Additionally, even if the porch would have a ceiling, porches do not usually have an area of four Amot by four Amot (approximately 6.5 feet by 6.5 feet) and when a house does not have this area, it is likewise exempt from having a Mezuzah affixed. Even if the porch has the square footage of four Amot by four Amot (i.e. there are sixteen square Amot, such as two Amot by eight Amot), according most Poskim, among them the Rosh and his disciple Rabbeinu Yerocham, the areal must be in the specific form of four Amot by four Amot and sixteen square Amot is insufficient. Thus, halachically speaking, as long as a porch does not have an area of four Amot by four Amot or if it is not covered by a ceiling, it is exempt from having a Mezuzah affixed to it according to the letter of the law. Nevertheless, one who acts stringently and affixes a Mezuzah to it in any case is truly praiseworthy and the Mezuzah should be affixed on the right side of the doorpost when entering from the house to the porch in accordance with the Maharil’s opinion, as it is the primary opinion according to Halacha.
Summary: A porch without a ceiling is exempt from having a Mezuzah affixed to it according to the letter of the law. Moreover, a porch that does not have an area of four Amot by four Amot (approximately 6.5 feet by 6.5 feet) is exempt from having a Mezuzah affixed. One who acts stringently and affixes a Mezuzah to a porch is especially praiseworthy. If one does so, the Mezuzah should be affixed without a blessing and on the right side of the doorpost when entering from the house to the porch (see Responsa Yechave Da’at, Volume 4, Chapter 51).