In our Mishna, we are told that the minimum quantity of thick sand that one is liable for transporting on shabbos is the amount needed to put on top of a trowel filled with plaster (and mix with it) to strengthen it before use in building .
The Gemara suggests that this Mishna must be the opinion of Rabbi Yehuda who holds that sand indeed does strengthen plaster. ( otherwise this would not be considered something of importance to be considered a punishable act .)
This view is reflected in a Beraisa which says that one is not permitted to plaster his house, as a sign of mourning for the Temple, unless one mixes the plaster with straw or sand to make it less effective .
Rabbi Yehuda holds that it is forbidden even if one mixes it with sand, as sand strengthens it .
The Gemara refutes this suggestion and says that it is possible that even the Chachamim who argue with Rabbi Yehuda would accept the ruling in the Mishna, as the act of weakening the plaster is considering for its benefit so that it can be used for plastering one’s home.
It follows from this discussion that everyone agrees that there is a prohibition against plastering one’s house even during the week without adding something to the cement that weakens it , and this prohibition was made in order to make us remember that we are in a constant state of mourning for the destroyed Temple , שיבנה במהרה וימינו .
The truth is that this prohibition is also brought in Bava Basra 60b, and there is a different Beraisa that is also brought there that says that plastering the house is only permitted if one leaves a square of 1 armlength unplastered.
We need to clarify whether this is a second condition, both of which are required , or whether anyone of the two is sufficient. A third option is that the authors of the two statements actually disagree with each other, and only one of the two conditions would work at all.
This is also part of a list of decrees that Chazal made after the destruction for the same reason , including refraining from serving one dish at a meal, putting ash on the head of a חתן (groom), leaving out a stone from women’s jewelry , and based on Sotah 48a and Gittin 7a, refraining from playing musical instruments (a topic for another post, Hashem willing .)
Although the custom to break a glass at a wedding might be a derivative of these customs, and there are some pious people in Yerushalayim in particular that do indeed have unplastered squares in their homes, it does not seem to be common practise to follow all these things, and given that these are based on explicit rulings in the Talmud, this requires some serious explanation.
One possibility is that these decrees never actually caught on and were not accepted by the majority of Jews at the time, due to their being too harsh.
Although this might seem surprising, the Rambam does in fact rule (Mamrim 2/6-7) that if Chazal make a decree and it is not accepted by the majority of the people at the time, it is null and void!
He even rules further than even it appears to have caught but a later court din finds that this is not so, they may annul it, even if they of lesser stature than the court that enacted it .
However , we see from our daf ( and the sugyas in Bava Basra and Sotah ) that this rule was discussed many years later by the Amoraim of the Gemora, and no mention is made of the possibility that it didn’t catch on.
It is clear from the same Rambam (Mamrim 2/2) that once a decree was accepted , the decree remains in force and cannot be annulled ( though see Kesef Mishna there who toys with the idea that if the decree indeed was later dropped by the people because they could not handle it, it may also be annullable- he rejects the idea mainly because of a Rashi that implies otherwise , itself an interesting point regarding his methodology .)
Furthermore, the Rambam himself brings this rule, together with the others ( Taaniyos 5/12) and rules that it is only permitted if one leaves a square Ama unplastered – he doesn’t seem to clearly mention the heter of mixing the plaster with sand or straw ( why this is so requires an analysis of the sugya and Rambam outside the scope of a daf post, but see the נוסעי כלים on the Rambam for a detailed discussion about this, or preferably try work it out yourself first!)
However, he does make it clear, based on the Gemora, that if one buys a house already plastered , there is no need to remove a square Ama’s worth of the plaster .
As most of our houses are bought already plastered, this could explain why we do not see these unplastered squares in most people’s homes.
However , it seems that if one builds a home from scratch, as people certainly do still do, one would be required to leave the square unplastered, as per the Gemara and the Rambam.
Yet many religious people do not seem to do this either, and many or all of the other decrees mentioned in the Gemara AND brought by the Rambam also do not seem to be universal normative practise .
This is even more bizarre given that these laws are brought by the Tur and Shulchan Aruch as well( O.C 560.)
Although discussed by various Poskim, including the Chayei Adam, Mishna Berura and the Tzitz Eliezer regarding music (15/33) , I am not aware of any halachically convincing explanation for this .