Retzuos start out at 15mm in order that if they begin to tear you will still have some time to get them replaced before they become Possul.
The preferable (Kosher Lechatchilah) width is 11mm according to the Chazon Ish and 10mm according to Rav Chaim Naeh.
The minimal (Bedieved) width for making a Brachah is 9mm. If a Retzuah started tearing or if it is missing paint at one spot making the connected or painted Retzuah less than the minimal width it is Possul and must be replaced.
From time to time it is important to look over your Retzuos to ensure they are Kosher if you have any doubts be sure to bring them in right away.
A few years ago a client of mine gifted a Mezuzah to their next door neighbor as a moving in welcome gift. Just recently their neighbor just moved away and they left the Mezuzah he gave them on their door. My client took back the Mezuzah and brought it in for checking. We were both shocked to see how his neighbor had inserted the Mezuzah into the cover.
This is problematic for several reasons:
1) The top portion of the Mezuzah was hanging upside down. A Mezuzah hung upside down is not Kosher.
2) Many letters were cracked on the bend.
If you have a Mezuzah that does not fit in the case either buy a new smaller Mezuzah or a new larger case.
If you are gifting someone a Mezuzah be sure to give it in a appropriate cover.
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Many things can render Tefillin or Mezuzah not Kosher or at best minimally Kosher.
Some of these happen over time with aging and some unfortunately are Posul from the get go.
Letters, words and or Tagim that are missing, extra substituted, touching, Broken, Improperly Spaced, misshapen faded, cracked and weather damaged.
We document the problems we find and periodically upload them to the website.
We hope that this will educate and encourage the public to purchase only Tefillin and Mezuzas that are high quality from a reputable source, to take care of them properly and have them checked regularly.
Rabbi Kass was ordained by Rabbi Yisrael Meir Lau, former Ashkenazik Chief Rabbi of Israel. He is certified as a Sofer for both Kesivah and Hagoah by one of the leading experts in Safrus, Rabbi Avrohom Tzvi Vosner, Rav of the Vad Mishmeres Sta”m.