Incomplete Houses And Feasts

The Tur (siman 560) cites the opinion of the Rambam (Taanis 5:12) that one may not line the walls of their house entirely from סיד which was the way aristocrats houses were done, rather one should first use a covering of טיט and סיד on top of that and then leave over a space unfinished opposite the door to express our lacking due to the churban.  The Tur disagrees and holds the unfinished space works to demonstrate our lacking due to the churban even in a house finished the way aristocrats do.  Rabbi Chagai Preschel (a previous Rosh Kollel in my kollel,) in בחגוי הסלע on Eicha wants to say the machlokes is in the geder of the takkanah to leave over an unfinished amah area.  The Rambam holds it is a takkanah on the cheftzah of the house.  Chazal said that one should not live in a house constructed like that of kings and therefore, one may not plaster their house in such a manner.  In addition, even when one does not plaster their walls like that for a king, one must still leave an unfinished area to show the house is not complete.  Hence one can't build a beautiful house but just leave something out for the house is still a beautiful house.  The Tur holds it is a din in the gavra that an action that brings simcha should be left incomplete to remember the churban and that is fulfilled by always leaving something of the house unfinished. 

With this he explains the change in the dikduk halashon of the Rambam and Tur in the next halacha.  The Rambam continues וְכֵן הִתְקִינוּ שֶׁהָעוֹרֵךְ שֻׁלְחָן לַעֲשׂוֹת סְעֻדָּה לָאוֹרְחִים מְחַסֵּר מִמֶּנּוּ מְעַט וּמַנִּיחַ מָקוֹם פָּנוּי בְּלֹא קְעָרָה מִן הַקְּעָרוֹת הָרְאוּיוֹת לָתֵת שָׁם, the Rambam puts the emphasis on the table being incomplete while the Tur writes ועושה אדם כל צרכי סעודה ומשייר בה דבר מועט אפילו כסא דהרסנא the emphasis is one the person, that he should leave something out of his feast to demonstrate his pain over the churban. 

The poskim discuss if leaving a picture of Yerushalayim or sign helps instead of the leaving an ammah unfinished (see Mishna Berurah #3 and sources cited in Pischay Teshuva.)  Such an argument would only make sense if leaving an ammah is a din in the gavra, if there is a din that the cheftzah of the house not to be properly finished then it would not help to hang a sign or picture. This chakirah also may play a role in determining who the obligation should be in the case of a rentor (see Igros Moshe volume 3 #86,) it is is a din in the gavra it should be an obligation on the one living in the house.  The poskim also discuss if one uses טיט with other things mixed in that it is lower quality and one won't be obligated to leave over an ammah (see Mishna Berurah #2, Aruch Hashulchan, Ritva Bava Basra 60b,) that  makes more sense if it is a din in the house not to be built nicely.  Then one can argue this type of cement is not good and it is not considered building too nice of a house but if it is a din in the gavra then it is more likely to say there still needs to be something recognizable that is left incomplete.