First, we will deal with the “under one roof” claim. In the context of discussing the eating of the Korban Pesach within a certain area, the gemara in Pesachim[1] questions whether those who are within the doorway of the boundary are considered to be inside the area. The gemara notes that the same rule would apply to tefilla (i.e., tziruf[2] for a minyan). The gemara in Eruvin,[3] in discussing a minyan split between adjacent courtyards of different sizes, records different rulings based on the relative sizes of the areas and of the groups located in them. Neither gemara suggests that it suffices to solve the quandary if the areas are under one roof. Although one might argue that these sources are simply not referring to cases in which the areas are under one roof, it is clear from Rishonim, the Shulchan Aruch,[4] and many poskim that the guidelines for separate rooms inside a building are much the same as those that apply to separate courtyards.
In one relevant discussion, the Rashba[5] asks why it is permitted for a chazan to stand on the bima when its dimensions make it a separate domain, thus separating the chazan from the tzibbur. He offers two answers: 1) A bima specifically functions as an integral part of the shul; 2) If some people in one domain see some people in the other, the two groups constitute one unit (as they do in the context of a zimun for bentching[6]). The Shulchan Aruch[7] cites the Rashba’s first answer as halacha concerning tziruf for a minyan, while mentioning an opinion that this is valid only on condition that the bima’s partitions do not reach the ceiling. From this ruling, we see that although a bima is obviously under the same roof as the rest of the shul, other reasons are needed to justify the tziruf for a minyan. This source and those cited above are among many that debunk the claim that you heard.
Most practical cases depend on the extent to which we accept the Rashba’s second answer – that a visual connection between the two groups suffices.[8] The major question is whether the parameters for connecting groups regarding zimun (i.e., visual) also apply to forming a minyan for tefilla. The Mishna Berura[9] is not conclusive on the matter. Therefore, when possible, it is best to have ten men in one room.
Once a minyan is achieved in one room, most opinions assume that those in the overflow room enjoy the benefits of a minyan, regardless of the visibility connection. The Radbaz,[10] however, writes that those in the small room are deemed as davening with a minyan only if the small room can be accessed exclusively through the main room. In any case, those who are not in the room with the minyan may participate in the parts of the tefilla that require a minyan.[11] The logic is that ten men in one room create the setting (i.e., attract the Shechina[12]) for the matter of kedusha, at which point partitions do not prevent the sanctity from flowing beyond.[13]
It seems that this logic allows for leniency in the following common scenario. Ten men are davening in the main room, but not all of them have finished Shemoneh Esrei when the chazan is ready to start chazarat hashatz. In a previous response,[14] we prefer the view that it is necessary for eight people to have finished, not including the chazan, in order to begin chazarat hashatz. Some poskim do not require that many, as the presence of the ten men suffices to bring the Shechina, but others counter that chazarat hashatz requires a minyan that relates to the repetition. In the case of adjacent rooms, we should be able to combine factors. The presence of ten men in the main room brings the Shechina; at that point, we only need ten men who are connected to chazarat hashatz. Since those in the small room can fulfill their obligation via the chazan,[15] they count toward the quorum needed to start.