“וכתבתם על מזוזות ביתך ובשעריך”
The מחבר (יו”ד סימן רצ”א סעיף א’) says a מזוזת יחיד – the mezuzah of a private individual should be checked twice every seven years and a מזוזת רבים – a mezuzah which belongs to the public, should be checked twice every fifty years. The חוט שני says twice in seven years means that seven years should not go by without the mezuzos checked. Most poskim hold it means once every three and a half years which is the accepted practice. Even in our days when the mezuzos are well protected the halacha of the Gemara doesn’t change. Even mezuzos that are inside the house and not subject to the rain must be checked. The reason is because a mezuzah can become פסול if it remains in one place and is never used.
Someone who borrows mezuzos from a gemach does not have to check them. Since they are only temporary they are not included in the obligation to be checked. Someone who has a mezuzah gemach must have them checked twice every seven years. The reason is because לפני עור לא תתן מכשול. This is another reason why someone who borrows them does not have to check them. If one is not sure if three and a half years have passed since they were last checked, even though we say ספק דרבנן לקולא, it is still better to have them checked. Even a mezuzah that was put up as a חומרא to beיוצא כל חשש ספק should also be checked.
Rashi in Yoma says the case of מזוזת רבים is שערי חצירות ומדינות ועיירות. The Poskim say that the mezuzah of a בית הכנסת is also a מזוזת רבים. What about about the mezuzos of a dormitory? Is it considered של רבים since the הנהלת הישיבה have to make sure there are mezuzos, or do we say that since the dormitory rooms are loaned to the bochurim the obligation to check the mezuzos is on them and it is like a mezuzah of שותפין and must be checked every three and a half years? Reb Chaim Kanievsky shlit”a says מסתבר דהוי כיחיד וצע”ק.
The עמק ברכה asks since the בדיקה is only מדרבנן because מדאורייתא מוקמינן לה אחזקתה – since it was כשר before we assume that it is still כשר, one should have to put up a different mezuzah while it is being checked. Why it is permissible to be מבטל the mitzvah of mezuzah in order to have it checked? The עמק ברכה answers that the חכמים were only מחייב us a בדיקה. They were not מטריח us to put up a temporary one, ויש כח ביד חכמים לעקור דבר מן התורה בשב ואל תעשה. Others explain that this is not considered a ביטול of the mitzvah. The mitzvah is to have a mezuzah and to also have it checked periodically. Having it checked is part of the mitzvah. Since he is having them checked to make sure that he is still fulfilling the mitzvah it is not called being מבטל the mitzvah. It is still better to minimize the time that the mezuzos are not up.
The אחרונים bring the minhag of checking the mezuzos during Elul. Many people also check their mezuzos during an עת צרה. How is this permissible? If the תקנת חכמים is to check them only every three and half years, it would seem that doing it at any other time is being מבטל the mitzvah. The answer is that since he is doing it in order to make sure that the mezuzahs are כשר and he is fulfilling the mitzvah properly, it is not considered being מבטל the mitzvah.
The דעת קדושים holds that if the בדיקה will take longer than a day, other mezuzahs should be put up. This is also the psak of R’ Elyashiv zt”l. He learns that it is only considered a ביטול מצוה if they are not up for an entire day. However as mentioned earlier other poskim hold that even longer than a day is not considered ביטול מצוה since they were taken down in order to be checked. Still if one has extra mezuzos and the checking time will take longer than a day he should put them up. One can take down all of the mezuzos at the same time. He does not have to do it in a way where only one mezuzah is down at a time. Reb Dovid Feinstein shlit”a says that he remembers his father Reb Moshe zt”l doing it this way.
When checking the mezuzah, one is looking to make sure nothing was erased and there are no holes. One does not have to check for missing letters or extra letters. There is a machlokes haposkim if one must also check to make sure the שרטוט is still there. Anyone can check the mezuzos, even if he is not an expert in דיני סתם.
The ישועות חכמה quotes the ספר זכירה who writesהבודק מזוזות תדיר ומשגיח שיהיו כשרים, יאריך ימים, וניצול משדים ומרוחות, ויזכה לבנים כשרים.
Prepared by R’ Avrohom Yehoshua Ziskind
 Sources:ספר דעת נוטה וקונטרס מזוזות ביתך מהגר”ח קניבסקי שליט”א