As we have seen (section 1), the minimum height of the walls is 10 tefaḥim (c. 80 cm). They must be close to the ground; if there is a gap of 3 tefaḥim (c. 22 cm) between the ground and the walls, the wall is invalid. However, there is no maximum gap between the top of the wall and the sekhakh, as we view the wall as if it continues up to the sekhakh (SA 630:9).
One can erect walls by setting up poles or stretching lengths of rope within 3 tefaḥim of one another, because in such a case the law of lavud applies: since there is less than 3 tefaḥim (c. 22 cm) between the components, we treat the entire area between them as being solid. Even though the sun and wind enter through the gaps in the wall, the poles or strings are still considered a wall. It makes no difference whether the string or poles are arranged horizontally or vertically; as long as there is less than 3 tefaḥim between one pole or string and the next, lavud applies. However, some maintain that since lavud walls are inferior, they must surround the sukka on all four sides (MA; of course, a doorway does not invalidate this sukka). If the wall is made from crisscrossing components, like from mesh netting or chain link fence material, it is not considered inferior, and two walls plus a tefaḥ suffice (as explained in the previous section). In any case, the sukka must be fit for eating and sleeping in painlessly, as explained below (section 14).
Lavud, lit. “joined,” discussed above, means that we view gaps of less than 3 tefaḥim to be solid and filled in.
Gud asik meḥitzta, lit. “extend the wall upward,” means that once a wall reaches the minimum height of 10 tefaḥim we treat it as though it extends upward in a straight line, ad infinitum. Thus, it is not necessary for the wall to reach the height of the sekhakh; rather, as long as the wall is 10 tefaḥim high, it is kosher, and we view it as if it continues straight up to the sekhakh. The straight line is measured from the top of the wall. That is, even if the wall is at an angle, the principle of gud asik meḥitzta means that we view it as continuing perfectly vertically. (See Ḥazon Ish, Eruvin 71:6.)
Dofen akuma, lit. “bent wall,” means that if less than 4 amot of sekhakh at the side of the sukka (next to the wall) is invalid, we view the invalid sekhakh as a horizontal continuation of the adjacent wall – i.e., we view it as part of a “bent wall.” If the invalid sekhakh extends 4 amot (c. 1.8 meters) or more away from the wall, that wall is invalid, as dofen akuma does not apply to sekhakh that is 4 amot or more from the wall. In this case, the invalid sekhakh constitutes a barrier between the wall and the kosher sekhakh. Thus, for instance, in a home where the roof caved in and sekhakh was placed over the resulting hole, if there is less than 4 amot of residual roofing between the hole and the walls of the house, the walls of the house can be considered walls of a sukka. But if 4 amot or more of roof and ceiling remains around the hole, they cannot be considered dofen akuma, and it is necessary to erect walls directly under the hole in order to validate the sukka (Sukka 17a; SA 632:1).
Can these three principles be applied in combination? When it comes to gud asik meḥitzta and dofen akuma, that is, if the wall does not reach all the way up to the sekhakh, and the sekhakh directly over the top of the wall is invalid, the poskim disagree (MB 632:4).
There is agreement, however, that gud asik meḥitzta and lavud can be applied in combination. If the sekhakh does not go directly over the top of the wall but reaches within 3 tefaḥim of the wall’s vertical extension, it is valid. In this case, gud asik meḥitzta is applied to treat the wall as though it extends vertically to the level of the sekhakh, and lavud is then applied to close the horizontal gap between the sekhakh and the “wall” – the plane extending vertically from the top of the physical wall. However, if there are more than 3 tefaḥim (c. 22 cm) between the sekhakh and the plane extending vertically from the top of the wall, the sukka is invalid, because the sekhakh and the walls are disconnected (SA 630:9).