שולחן ערוך יורה דעה · סימן רפ״ז

English

What doorway needs a mezuzah? A doorway does not require a mezuzah unless it has two doorposts and a lintel.

English

A house which has a doorpost here and a doorpost there, and on top there is a bow-shaped covering in the place of the lintel, if the doorposts are 10 tefachim or more high, then it requires a mezuzah, and if it is not 10 tefachim high, it is exempt [from requiring a mezuzah] since it does not have a lintel. Rema: And all the more so a door that is not 10 tefachim high is exempt from the requirement of mezuzah. The practice which is widespread in these areas is that most of the world relies on a single mezuzah that they place on the door to their houses and this is incorrect, and they have nothing [no legal source] to rely on for this. Therefore, every G-d fearing person should fix his house according to the law [so that there will be mezuzot] on all of the doors that require [mezuzot], as was explained.