1) "HANG THE DOORPOSTS FIRST"
OPINIONS: The Gemara relates that when the Reish Galusa built a house and requested from Rav Nachman that he affix the Mezuzah, Rav Nachman said that the "Dashei" should be hung first. What is the "Dashei," and why was it necessary to erect it before affixing the Mezuzah?
(a) RASHI explains that "Dashei" refers to the door frames. They must precede the affixing of the Mezuzah, because if the Mezuzah is affixed to the entranceway first and then the doorposts are erected, the Mezuzah will be invalid because of the rule, "Ta'aseh v'Lo Min ha'Asuy." The Mezuzah must be affixed to the entranceway of a house that is already built. If the Mezuzah is affixed before the house is completed, it is not valid.
(b) While TOSFOS does not argue with Rashi's application of the Halachah of "Ta'aseh v'Lo Min ha'Asuy" to the laws of Mezuzah, he claims that this explanation is not consistent with the wording of the Gemara. According to Rashi's explanation, Rav Nachman should have told the Reish Galusa to hang the "Sifei" (doorposts) first, and not the "Desheh" which means "door." (Perhaps Rashi understands the word to mean doorposts, because in his Girsa, as in our texts, the word is "Dashei," in the plural form.)
Tosfos explains instead that it was necessary to attach the door in order to know which direction the door would open (the "Heiker Tzir"). Tosfos understands that this story is a continuation of the previous Gemara which discusses a case in which it was not clear which direction was the "Derech Bi'ascha," the way from which one entered the doorway. Rav Nachman told the Reish Galusa that it is necessary to attach the door first in order to discern the side towards which the door opens, so that the Mezuzah would be placed on the right of the side which is "Derech Bi'ascha."
According to the explanation of Tosfos, why was it necessary for Rav Nachman to tell the Reish Galusa to attach the door? If Rav Nachman merely needed to ascertain which direction was "Derech Bi'ascha," he should have simply asked the Reish Galusa on which side he planned to attach the door. With that information, Rav Nachman would have known on which side to affix the Mezuzah.
RAV MOSHE FEINSTEIN zt'l (Igros Moshe YD 1:176) derives a practical Halachah from the fact that Rav Nachman did not merely ask the Reish Galusa on which side he planned to attach the door. Rav Moshe rules that a doorway that has no "Heiker Tzir" -- that is, either there is no door in the doorway (according to the Rishonim who maintain that a doorway does not need a door in order to be obligated to have a Mezuzah), or there is a door that opens in both directions (such as a swinging door) -- does not require a Mezuzah. Since the "Derech Bi'ascha" cannot be established, there is no obligation of Mezuzah at all. This is why Rav Nachman required that the Reish Galusa attach the door and thereby establish the "Derech Bi'ascha" before he affixed the Mezuzah. Without knowing for certain which way was "Derech Bi'ascha," there was no obligation to affix a Mezuzah there. The Reish Galusa's mere intention was not able to establish the direction of the passageway, the "Derech Bi'ascha."
Rav Moshe adds that this exemption applies only when the "Heiker Tzir" is needed in order to determine the direction of "Derech Bi'ascha." If the "Derech Bi'ascha" is obvious from the general direction of the public traffic through the entranceway, then the obligation to affix a Mezuzah takes effect without the need for a "Heiker Tzir."
Not all of the Poskim agree with Rav Moshe's ruling that a doorway without a "Heiker Tzir" (when one is necessary) is not obligated to have a Mezuzah. Rav Moshe himself quotes the SHE'EILAS YA'AVETZ who maintains that in the case of such a doorway, a Mezuzah should be affixed to each side of the doorway out of doubt. Others (see MIKDASH ME'AT) maintain that since there is no clearly discernable "Derech Bi'ascha," the Mezuzah may be affixed on either side of the doorway. (See a summary of the opinions cited by SEFER CHOVAS HA'DAR 8:5.)
(c) The RAMBAM agrees that it is sometimes necessary to have the "Heiker Tzir" in order to ascertain on which side of the doorpost the Mezuzah should be placed, but he claims that this explanation is not consistent with the wording of the Gemara. The Rambam, in a Teshuvah to the Chochmei Lunil (cited by the KESEF MISHNEH, Hilchos Mezuzah 6:5), argues with Tosfos' assertion that this story is a continuation of the previous Gemara. (In a witty remark, the Rambam says, "Ein Dorshin Semuchin" -- we do not expound the meaning of one Sugya based on an inference from an adjacent Sugya.)
The Rambam explains that the Gemara is discussing the main entranceway into the house, where there is no doubt about the direction of "Derech Bi'ascha." Why, then, did Rav Nachman tell the Reish Galusa to erect the door first?
The Rambam learns that the obligation to affix a Mezuzah applies only to an entranceway that actually has a door. He derives this from the verse, "Al Mezuzos Beisecha uvi'She'arecha" (Devarim 6:9). The Rambam understands that "uvi'She'arecha" ("and upon your gates") refers to the gates of our houses and the gates of our courtyards, and that a gate always has a door. Therefore, the obligation to affix a Mezuzah applies only to an entranceway that has a door.
The Rambam continues to explain the Gemara here. Did the Reish Galusa, who asked Rav Nachman to affix the Mezuzah before the door was attached, maintain that an entranceway is obligated to have a Mezuzah even without a door? From the fact that Rav Nachman responded, "hang the door first," and not merely, "hang the door," it seems that the Reish Galusa agreed that only an entranceway that has a door is obligated to have a Mezuzah, but he maintained that the door may be attached after the Mezuzah is affixed. Rav Nachman told him that the door must be attached first, because otherwise there is a problem of "Ta'aseh v'Lo Min ha'Asuy," as Rashi explains. (Mordechai Zvi Dicker)
2) A MEZUZAH ON AN "ACHSADRAH"
QUESTION: Rav Chisda states that an Achsadrah (a structure covered by a roof and enclosed on three of its sides) is exempt from the obligation to have a Mezuzah, because it has no doorposts ("Petzimin"). This exemption is based on the verse that says that a Mezuzah must be placed "Al Mezuzos Beisecha," "on the doorposts of your houses" (Devarim 6:9), which implies that a doorway that has no doorposts is exempt.
Rav Chisda's statement implies that if an Achsadrah does have doorposts, then it is obligated to have a Mezuzah. The Gemara questions this ruling: when doorposts are erected at the entranceway of an Achsadrah, they are merely for the purpose of holding up the ceiling and not for the purpose of enclosing the Achsadrah. Therefore, they cannot be considered "Mezuzos Beisecha" to be obligated to have a Mezuzah.
The Gemara answers that this indeed was Rav Chisda's intention. When Rav Chisda said that an Achsadrah is exempt from a Mezuzah, he meant that it is exempt even if it has doorposts, since the doorposts are merely for the purpose of holding up the ceiling.
The Gemara continues and questions Rav Chisda's statement from a Beraisa that says that an Achsadrah is obligated to have a Mezuzah. The Gemara answers that the Beraisa refers to a different type of Achsadrah, a "Roman Achsadrah" (see Rashi).
Why does the Gemara question only Rav Chisda's ruling from the Beraisa's statement? The Gemara should question the Beraisa's statement by itself, even without Rav Chisda's ruling. The Gemara initially assumes that an Achsadrah is exempt from a Mezuzah even when it has doorposts, since those posts are only to provide support for the ceiling. The Gemara, therefore, should challenge the Beraisa -- which says that an Achsadrah is obligated to have a Mezuzah -- even without Rav Chisda's ruling. Why is the Beraisa's statement problematic only according to Rav Chisda? (CHAZON ISH)
ANSWER: RAV CHAIM KANIEVSKY shlit'a (in MASECHES MEZUZAH #19) answers this question based on the opinion of the ROSH. The Rosh (#14) writes that the "heads of walls" can be obligated to have a Mezuzah, since they are considered like doorposts. That is, if a house has three walls and the fourth direction is entirely open, one must affix a Mezuzah on the wall at the side of the opening. This is because the heads of the walls (which one faces as he enters the open house) on each side of the opening are considered "doorposts" ("Petzimin"). (The Acharonim explain that when the walls on each side of the entranceway also serve as walls for additional rooms on each side, the Rosh agrees that additional doorposts must be built at the entranceway between the two walls in order to affix a Mezuzah.)
The PERISHAH asks that if, as the Rosh says, the "heads of walls" can obligate an entranceway to have a Mezuzah, then why is an Achsadrah exempt? Even though the posts serve to support the ceiling, the heads of the walls should obligate the Achsadrah's opening to have a Mezuzah!
Rav Chaim Kanievsky answers that there is a basic difference between a house and an Achsadrah. The walls of a house are built for the purpose of enclosing the house from all directions. The Rosh maintains that when the two facing, parallel walls end on one side with no fourth wall attached to them, we view the thickness of the head of each wall as though it is meant to demarcate and close off the fourth direction.
An Achsadrah, in contrast, is generally built with only three walls, and the owner has no intention to entirely enclose the area inside. The fourth side is left open for air and light. Therefore, the two facing walls on the two sides of the opening are meant only to serve their own respective sides, and not to demarcate and close off the fourth side. This is Rav Chisda's logic for exempting an Achsadrah from a Mezuzah, even though a similarly-constructed house would require a Mezuzah (according to the Rosh). The underlying principle of his ruling is that a wall that is not meant to close off an open side of a structure is not considered a "doorpost" for that open side.
This is also the intention of the Gemara when it asks that according to Rav Chisda, an Achsadrah with doorposts should not be obligated to have a Mezuzah, since the posts serve only to support the ceiling. The Gemara is asking --according to Rav Chisda's own logic -- that posts that are not meant to demarcate and close off an open side do not obligate the opening to have a Mezuzah.
Without Rav Chisda's ruling, the Beraisa that says that an Achsadrah needs a Mezuzah is easy to understand. One would assume that the heads of the walls are considered doorposts, just as the Rosh says with regard to a house. Only according to Rav Chisda's own logic, that a wall or a post that is not meant to close off an open side of a structure is not considered a "doorpost" for that open side, is the Beraisa's ruling problematic. Even if there are doorposts, they serve only to hold up the ceiling, and they should not add any obligation to the Achsadrah, just as their "heads of walls" add no obligation. This is why the Gemara asks its question only according to Rav Chisda. (Mordechai Zvi Dicker)