Q. Kvod Harav, is one allowed to repair during the three weeks wall paper that fell of a wall, by just gluing it back to the wall
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A. Kovetz Halochos (5: 19) quotes Igros Moshe (O.H. 3: 82) regarding placing new wall paper during the Nine Days.
However, just gluing back existing paper that fell, Poskim permit.(See Nitei Gavriel 29: 3)
Rabbi A. Bartfeld as revised by Horav Dovid Pam, Horav Aharon Miller and Horav Chanoch Ehrentreu Shlit’a.