The Rama (635) cites the Hagahos Maimonies that when one is building a succah they should be careful to first make the walls of the Succah so that at the time they place the sechach on top, the placing of the sechach will render it a kosher Succah. This seems to be against the gemara that says that as long as there is a חלל טפח when one placed down the גדיש, pile of hay, they can then be חוטט בגדיש (carve out a space in the pile) and it would be a kasher succah. In this case the sechach is being placed prior to the walls being formed, yet it is okay. The Rama deals with this question by qualifying the din - ואם עשה טפח סמוך לסכך מותר לסכך קודם שיעשה שאר הדפנות כמו בחוטט בגדיש. Meaning, that so long as one has a 1 tefach wall at the time they place the sechach, it is permitted to construct or carve out the rest of the wall after placing the sechach. The justification for this is that the hagahos maimonies understands that the succah doesn't need to be kosher the moment the sechach is placed on top, rather the sechach must be placed on top of a succah that is considered to have walls, even if the walls are just a tefach.
The Magen Avrohom cites the Bach who says that it is only a din lichatchila, but bidieved even if one placed the sechach before forming the walls it would be kasher, and the Levush disagrees and holds that it is passul. The source of the Bach is a Rashi in the Mishna 16a that is cited by the Bach. Rashi describes one who is making the walls of the Succah מלמעלה למטה - שהתחיל לארוג המחיצה אצל הסכך ואורג ובא כלפי מטה. It seems from rashi that the sechach is already up on the Succah and he is starting to construct the walls "אצל הסכך", implying that as of yet there are no walls, yet by forming the walls, the Succah will be kasher. The Taz rejects this proof because Rashi is not intending to describe the sechach being on the succah, he is merely trying to describe the location where he is starting to make the wall, and that is near where the sechach would be. Rashi is not intending to say that the sechach is already there. Therefore, the Taz rules like the levush, against the Bach, and holds that if the sechach is placed before the walls are formed, the Succah will be passul m'doraysa.
To me it seems that although there may not be a proof from the Rashi cited by the Bach, there is another Rashi that clearly holds that the argues on the Hagahos Maimonies who is the source of this chumrah. The Hagahos Mamimonies must understand that the case where one piles a גדיש on a חלל טפח is where there are already walls that are a tefach in width on which he is putting the gadish, and then carving out the other 9 tefachim to form 10 tefach walls. According to this approach the focus should be on the fact that there was a tefach of wall that exists prior to the placing of the gadish (which serves as the sechach). However, when Rashi explains this gemara, he doesn't focus on the walls at all. Rashi seems to understand the the requirement of having a חלל טפח at the time that one places the gadish is because without airspace of a tefach, the gadish cannot qualify as sechach. It sounds from Rashi that even if there were no planks of wood under the gadish that can serve as walls, so long as the gadish will be lifted from the ground by a tefach and form a tefach of hollow space underneath, one can then be חוטט בגדיש to create 10 tefach walls. This is clear from Rashi who writes that the problem without a חלל טפח is - דעכשיו נעשה שם סכך מאליו ולא היה עליו שם סכך מתחילה, and the solution of having a חלל טפח is -
נמצא שם סכך על אותו גדיש, וכשחוטט מלמטה למעלה עד שמגביה את החלל לשיעור גובה עשרה אין זו עשייתו שהרי אינו מתקן אלא הדפנות, ובדפנות לא אמרינן תעשה ולא מן העשוי
Rashi does not at all focus on whether or not the boards under the gadish that create a chalal tefach are placed in a way that they are fit to serve as walls. The entire focus is that the gadish be placed so that at the time it is placed, it creates a chalal tefach. So long as it creates a chalal tefach, the entire construction of the walls can take place AFTER the placing of the sechach. Being that this sugya is the source of the Hagahos Maimonies, and Rashi seems to understand the gemara differently and holds that there is no requirement for walls to exist at all prior to placing the sechach, there is no source for this din. This is all to develop the shitah of Rashi but l'ma'asah, we are bound by the Rama who paskens like the Hagahos Maimonies.