The Torah obligates a person to affix Mezuzot to the doorposts in his home, and before affixing a Mezuza one recites the following Beracha: "Baruch Ata Hashem Elokenu Melech Ha'olam Asher K’dishanu Be'Mitzvotav Ve'tzivanu Li'kbo'a Mezuza."
The question was asked if it would be permissible for a fellow to invite a Rabbi to affix the Mezuzot in his home for him? Some people, for example, might wish to have a Rabbi place a Mezuza in their new house in order to bring Beracha into the home. Is this permissible, and if so, what Beracha is recited, and by whom?
Chacham Ovadia Yoseph addresses this question in his sefer ‘Halichot Olam’ (vol. 8), and he writes that it is indeed permissible to invite a Rabbi to affix the Mezuzot, but in such a case the text of the Beracha changes. Before placing the Mezuzot, the Rabbi would recite a slightly different Beracha. The Rabbi would say, "Baruch Ata Hashem… Al Kevi'at Mezuza", rather than the standard text of "Li'kbo'a Mezuza". So one who affixes a Mezuza on behalf of another recites the Beracha of "Al Kevi'at Mezuza" (with regard to the affixing of a Mezuza) instead of "Li'kbo'a Mezuza" ("to affix a Mezuza").
A similar Halacha applies with regard to the obligation of Ma'akeh. One who lives in a home with an accessible flat roof bears the obligation to place a Ma'akeh, (a parapet), around the roof so that nobody will fall from the roof. When one places the Ma'akeh, he recites the Beracha, "Baruch Ata… La'asot Ma'akeh" ("to make a parapet"). If however, one hires a Jewish worker to build the Ma'akeh for him, then the worker recites the Beracha, "Baruch Ata… Al Asiyat Ma'akeh" ("with regard to the making of a parapet"). In this case too, the text of the Beracha depends on who performs the Mitzvah. If the homeowner performs the Mitzvah, then the text of the Beracha reads "La'asot…," whereas if somebody else performs it on his behalf, then the text becomes "Al Asiyat…"
To review, a person may invite a Rabbi to affix the Mezuzot in his home, in which case the Rabbi recites the Beracha, "Al Kevi'at Mezuza," as opposed to the text of "Li'kbo'a Mezuza" when the homeowner performs the Mitzvah himself.
