Is height of 10 Tefach:[1] A wall is only defined as a wall if its height reaches ten Tefach [82 cm.[2]] from the ground. If the wall’s height reaches below ten Tefach from the ground it is invalid. If the wall reaches ten Tefach from the ground but is elevated from the ground, then if it is within three Tefach from the ground and reaches to ten Tefach from the ground, it is valid.[3] Thus ideally one can have a wall which is 7.1 Tefach tall and place it 2.9 Tefach from the ground and it is valid. Once the wall contains a ten Tefach height, it is valid even if it is a great vertical distance from the Sechach, [so long as the Sechach rests within 20 Amos from the ground].[4] It is valid even if it does not rest directly under the Sechach but is within three Tefachim from it.[5] If, however, it is past three Tefachim from the Sechach, then it is invalid.
[1] See Michaber 630:9; Admur 315:3; M”A 315:3; Michaber Y.D. 282:8 regarding Sefer Torah; Rama 240:11 regarding candle; Elya Raba 240:14; Olas Tamid 240:3; Kitzur SHU”A 150:6; Kaf Hachaim 240:44; Birur Halacha 240; Piskeiy Teshuvos 240:17-18; Sheyikadesh Atzmo 32:9; Piskeiy Teshuvos 630:8
The law if there is a plank of wood resting on the short wall: See Mikraeiy Kodesh Sukkah 1:7; Chazon Ish 104:6; Piskeiy Teshuvos 630:8
[2] See Shiureiy Torah p. 249 that an Ama Sochekes is 49 cm., and accordingly a Tefach Sochakos is 8.17 cm [i.e. 1/48 of a cm times 8.] and by the Shiur Sukkah of 7×7 one should measure with an Ama Sochakos, and the same would apply here Lechumra. Thus, its exact Shiur is 81.7 cm; See also M”B 633:2 and Shaar Hatziyon 633:2 that by Sukkah we follow the stringent approach in the dimensions, either Dochakos or Sochakos, and by the 7×7 dimension we follow Sochakos, and the same would apply here by the ten Tefach dimension; See also Midos Vishureiy Torah pp. 48-57 regarding Ama Sochakos and Dochakos; Piskeiy Teshuvos 633:1; Minchas Chinuch Mitzvah 325 who rules that by Tefachim we don’t apply the extra Dochakos or Sochakos, and this is only done by Amos
Other opinions-Chazon Ish: According to the Chazon Ish, the measurement is 98.2 cm following the 9.82 measurement of Tefach Sochakos. [See Piskeiy Teshuvos ibid]
[3] The reason: As we apply the rule of Lavud.
[4] Michaber 630:9
[5] Michaber 630:9; See Piskeiy Teshuvos 630:8
The reason: As we apply both the rule of Gud Asik and Lavud to the Sechach. [M”B 630:43] Unlike ruling of M”B 630:31 relating to Dofen Akuma, that we do not apply both Gud Asik and Lavud. See Mikraeiy Kodesh Sukkah 1:7 and Piskeiy Teshuvos ibid footnote 45