Making an ‘ohel’ (tent) on Shabbos (abridged)MBY 315:6-13 [Based heavily upon Kitzur Shulchan Aruch: 80:77-82, “Metsudah” edition, with notes, translated and annotated by Rabbi Avrohom Davis]
77) An ‘ohel’ (tent, i.e. a roof that shelters) – even if it is made only to shield from the sun or rain - and even if it is only temporary, if it is (a minimum of) one ‘tefach’ (handbreadth = 3-4 inches) by one ‘tefach’ and one ‘tefach’ high, it is forbidden to be made on Shabbos. (Footnote #198: This is a rabbinic prohibition, lest one come to make a permanent tent/roof.) Therefore, with a baby’s crib that has wooden hoops on the top, upon which a sheet is spread (Ed. you know, like those covered wagons in the Western movies, which have those wooden hoops to hold up the canvas cover?), the sheet must not be spread on Shabbos or Yom Tov unless at least a tefach was spread before Shabbos, in which case the remainder is only an addition to a temporary tent, which is permitted. If the hoops were close to each other, i.e. less that three tefachim apart, they are in themselves considered a tent (for it is a law to Moshe from Sinai that any space less than three tefachim is considered ‘lavud’, i.e. connected and closed), and thus it is permissible to spread a sheet over them. (Footnote #199: It is permitted to open and close the hood of a carriage because it is on hinges and is like opening and closing a door (See Chazon Ish 62:6 and Rama 627:3).
78) The cover of a large box (e.g. trunk) which is not attached with hinges is forbidden to be removed from the box, because that is equivalent to demolishing a tent. Likewise, it is forbidden to cover the box with it, because that is like making a tent. When placing a board upon an open barrel to serve as a table, you must place the barrel with its open side to the ground, because otherwise you will be making a tent when you cover the open barrel with the board!
79) If a chimney has its opening on the side, so that closing it is like adding to a partition (Ed: i.e. vertical rather than horizontal), it is permissible to close it. But if the opening is on the top of the chimney, it is forbidden to close it on Shabbos and Yom Tov, because that is like making a tent. (Footnote #200: According to Tosfos Shabbos, this is forbidden only when the opening is three tefachim wide. But if it is less than that, ‘lavud’ applies, which means that it is considered closed and covered already, and thus you may cover it. Mishnah Berura 315:20 relies on this ruling.) If an iron door is installed there that swings on its hinges, it is permissible to close it, for then it is like any door that swings on hinges.)
80) If a garment was spread over the opening of a barrel to cover it: if the barrel is not entirely full, and there is an empty space of a tefach between the beverage and the cover, you should not cover the barrel completely, because that would be like making a tent. Rather, a little of the opening of the barrel should be left uncovered. (Footnote #201: This refers only to a garment specifically used for this purpose.)
81) Any partition or tent when you are forbidden to make on Shabbos, once it is made, you are forbidden to remove it, for it is like demolishing a tent.
82) It is forbidden to carry a covering that is made to protect from the sun or the rain. (Ed: Here is a related quote from The Thirty-Nine Melachos, by Rabbi Dovid Ribiat, p. 1079: “Umbrellas may not be used on Shabbos or Yom Tov even it previously left opened, and even when there is a proper Eruv, because an umbrella is considered a Halachic canopy, since it protects the area beneath it from rain or sun.”)