Remembering Yerushalayim All Year - 2
After the destruction of the Bais Hamikdosh, Chazal decreed that a person who builds a new house, should leave an unplastered area of one אמה (cubit) by one אמה in the home, as a constant reminder of the golus of Klal Yisroel. In those days, the walls were made of stone with one layer of mud smeared over for insulation from the elements, and a second layer of plaster for added insulation and beauty. By leaving a specific area without plaster, the mud underneath would then show, and this created a זכר לחורבן - Remembrance of the destruction (1).
In today's day and age, homes are built differently, and we do not insulate with mud in the walls. We generally use "drywall" underneath with an overcoat of paint or something decorative. Thus, our reminder would be to leave an unfinished or unpainted area of drywall, although in some cases where the drywall itself has an attractive finish, it might be desirable to scrape the unfinished area to make it appear less decorative (2).
Others, however, rely on the fact that our drywall is unlike the old-time plaster and don't leave any øëæ at all (3). Another way would be to simply paint the אמה by אמה area with black paint (some even inscribe the words "זכר לחורבן" on the black). Even though some may justify this custom, it would appear that this is not the best manner in which to fulfill the din (4). Placing a painting or picture of Yerushalayim or the Kosel Hamaaravi on the spot is definitely not in accordance with the halachah, since it is less a זכר and more a decoration (5). One should strive to use this as a reminder of what Klal Yisroel has lost so that we may increase our tefillos for the geulah (6).
Where to place the זכר לחורבן
The most ideal place for this reminder is on the wall facing the home's main entrance. If this is not feasible, one should use the nearest wall to the front door, or directly over the entranceway, where it will be readily seen from the inside or as one leaves (7). (פסקי תשובות)
There are three general opinions as to the exact size of an Amah by Amah: 1) 18" (inches) x 18". 2) 22" x 22". 3) 24" x 24". The largest size is preferable and one who uses this size performs the halachah in the most complete manner. However, one who uses a smaller measurement has upon to whom to rely.
One who purchases a house from a gentile does not need to scrape away any area of the house. If he does renovations or paints over the walls, he should then leave a זכר לחורבן. If he bought a house from a Jew who didn't leave a reminder, he should go ahead and make one (8).
(1) או''ח תקס:א (2) פרי מגדים תקס:א (3) משנה ברורה תקס:ב (4) שער הציון תקס:ח, אגרות משה ג:פד (5) נחמת ישראל דף רה (6) חיי אדם קלז:א (7) תקס:ב (8) או''ח שם