Answer: The Shulchan Aruch (CM 427:1) teaches us the details of the mitzvah to build a maakeh, a fence around a flat roof. The Meiras Einaim (CM 427:1) notes that this chapter, the last in Choshen Mishpat, does not exist in the Tur and is in fact a copy of Rambam (Rotzeach Ushemiras Nefesh 11).
The Gemara (Kiddushin 41a) teaches that while one can appoint a sheliach to perform certain mitzvos on their behalf, it is preferable for one to perform them oneself. Thus, Rambam (Berachos 11:13) writes that if one asked a Jewish worker to build a maakeh on their behalf, the worker recites the beracha.
The Shulchan Aruch (CM 427:1) writes that one cannot appoint a non-Jewish person to act as a sheliach to perform a mitzva on one’s behalf. Nonetheless, the Machaneh Ephraim (Sheluchin Veshutfin 11) argues that one would still say the beracha if one’s non-Jewish worker built the fence. Firstly, the worker is working on behalf of the house owner rather than independently. Additionally, it is the practical result that matters more than the act of building (See Aruch Hashulchan CM 427:3).
The Nesivos Hamishpat (188), Minchas Chinuch (546) and Shoel Umeshiv (1:2:110) challenge this, however, writing that building a maakeh is no different to any other mitzva for which one cannot appoint a non-Jewish sheliach.
In conclusion, if one employs a Jewish worker to create the fence, then the worker can recite the beracha as a sheliach. One doesn’t say a beracha, however, if the builders are not Jewish.