וְגִיגִּית סְדוּקָה מוּנַּחַת עַל גַּבָּן. וּפָקְקוּ אֶת הַמָּאוֹר בְּטָפִיחַ וְקָשְׁרוּ אֶת הַמְּקֵידָה בְּגֶמִי, לֵידַע אִם יֵשׁ שָׁם בַּגִּיגִּית פּוֹתֵחַ טֶפַח אִם לָאו.

and there was a cracked roofing placed atop the two houses. If the roofing was intact it would have the legal status of a tent over a corpse, rendering everything in the alleyway, and, through the windows, everything in the houses, ritually impure. However, since the roofing was cracked and the corpse was directly beneath the opening, if the opening was the size of a handbreadth or more, the entire alleyway and the houses would not become impure. Only the area directly over the corpse extending through the opening is impure. And that is the reason that they sealed the window of the house with an earthenware vessel, so that the ritual impurity would not enter the houses, and they tied an earthenware shard with a long reed-grass inserted into the opening in the roofing in order to ascertain whether or not there is an opening there in the roofing the size of a handbreadth.

“וּמִדִּבְרֵיהֶם לָמַדְנוּ שֶׁפּוֹקְקִין וּמוֹדְדִין וְקוֹשְׁרִין בַּשַּׁבָּת״. עוּלָּא אִיקְלַע לְבֵי רֵישׁ גָּלוּתָא. חַזְיֵיהּ לְרַבָּה בַּר רַב הוּנָא דְּיָתֵיב בְּאַוְונָא דְּמַיָּא, וְקָא מָשַׁח לֵיהּ. אֲמַר לֵיהּ: אֵימַר דְּאָמְרִי רַבָּנַן מְדִידָה דְּמִצְוָה, דְּלָאו מִצְוָה מִי אַמוּר? אֲמַר לֵיהּ: מִתְעַסֵּק בְּעָלְמָא אֲנָא. הדרן עלך מי שהחשיך וסליקא לה מסכת שבת

The mishna concludes: And from their statements and their actions, we derived that one may seal a window, and measure, and tie a knot on Shabbat. The Gemara relates: Ulla happened to come to the house of the Exilarch. He saw Rabba bar Rav Huna sitting in a tub [ avna ] of water and measuring it. He said to Rabba bar Rav Huna: Say that the Sages said that it is permitted to measure on Shabbat only a measurement for a mitzva. However, with regard to a measurement like this one, which is not for a mitzva, did they say that it is permitted? Rabba bar Rav Huna said to him: I am merely acting unawares and am not at all interested in the measurements. Therefore, it is not prohibited.