Q. I once visited a friend in a small apartment and I saw that he had his zecher lechurban (Remembrance of the Destruction of the Bais Hamikdosh), on the ceiling. Is that correct?
A. Poskim try to be melamed z’chus (find some merit) on the ones who don’t have a zecher lechurban in their homes in our days. Mishna Berura (560: 2) mentions that it may be due to the lesser quality of the building materials used in our days, but asserts that that is not a seemly reason. Others maintain that it depends on the thickness of the plaster used and it is thinner in our days. (Hisorerus Teshuva 1: 164, see also Aruch Hashulchan 560: 3-4 ).
Due to the above and other reasons there is room for one to be lenient to some degree in this important mitzva.
Mishkenos Haroim (18) does indeed permit leaving the zecher lechurban on the ceiling when no other option is available.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller Shlit’a