Key wrods: Canvas succah, walls, s'chach, mat, construction
The Gemara (Succah 23a) states in the name of Abaye that any wall that will not remain standing when a normal wind is blowing has no halachic validity. It is evident from the context of the Gemara that this refers to a case where the wall will totally collapse as a result of the wind. This statement is repeated in the Gemara (24b) by Rav Acha Bar Yaakov but evidently has a different connotation. As an illustration of this requirement the Gemara describes a succah which makes use of trees for its walls. If the branches of the tree sway in the wind, the wall is invalid. The tree branches must be securely tied in a way that the wind will not cause them to move. Presumably, the wind will not actually knock over the unsecured branches. They will merely sway to and fro in the wind. Nonetheless, the Gemara deems this an invalid wall. Thus, whereas Abaye invalidates a wall if it will collapse in the wind, Rav Acha Bar Yaakov goes further and invalidates a wall that will merely move in the wind. Shulchan Aruch (630:10) cites the ruling of Rav Acha Bar Yaakov, and the Mishnah Berurah (no. 48) confirms the above explanation in the following comment:
If the wind causes the trees to sway, even if the wind is not strong enough to cause them to fall down entirely but merely causes the wall to move back and forth, the wall is not considered a halachically valid wall.Sha' ar HaTziun (no. 45) attributes this explanation to Rashi, Ran, Ritva and Or Zarua. The reason for this requirement is that the halachic definition of a wall is that which is positioned and fixed in one place. If the wall lacks stability and thus is capable of moving under normal circumstances, it does not qualify as a halachic wall.3, 4
Given this requirement, the status of a canvas wall tied in place needs to be examined. On the one hand, the middle area of the canvas tends to sway in the wind even when it is tied securely. Perhaps this movement, albeit slight, renders the wall unstable and thus halachically invalid. On the other hand, the wall never actually shifts position, by virtue of the edges of the canvas being tied to the frame. It could thus be argued that the wall is considered to be fixed and therefore constitutes a halachic wall. Interestingly, this very case is addressed by the early authorities. Although the above issue is resolved favorably, their actual ruling is effectively a restrictive one. Tur (630), quoting Rabbeinu Peretz, states the following:
It is improper to make the walls from sheets of flax... even if he tied them securely [because] sometimes they become unfastened, without his being aware, and the result is a wall that does not stand in the wind.It is clear from this ruling that in and of itself a canvas wall is acceptable. Apparently the minor movement caused by the wind does not affect its essential status of being fixed in place. There is concern, however, that at some point the wall may become untied, causing it to move in the wind and thus rendering it invalid. A person may not realize this and will continue to make use of this succah. Based on this concern, Rabbeinu Peretz rules that one should not make use of a succah of canvas walls. This concern is shared by Maharil and Kolbo and is codified in the Shulchan Aruch (630:10). It should be noted that this concern is not voiced by the Gemara5 and thus does not have the binding force of a rabbinical decree (gezerah) nor the consequence of invalidating the succah.6 Nonetheless, as this stringency is adopted by the Shulchan Aruch, it is to be treated as normative halacha and demands full compliance.
The widespread acceptance of the canvas succah is puzzling in light of this explicit directive that it be avoided. In such cases, when common practice seems to be at odds with halachic norms, the authorities often attempt to discover a basis to justify such a practice. In this particular instance, Rabbi Moshe Shternbuch suggests a possible justification.7 The ruling of the Shulchan Aruch is referring to sheets that are not secured on all sides. Thus the possibility of their becoming disconnected is a real one. However, when the canvas is secured on all sides as is the common practice, the possibility of becoming disconnected is remote. As such, even the Shulchan Aruch would concede that such a succah is entirely acceptable. Rabbi Shternbuch notes, however, that there is no indication in the Shulchan Aruch or in any other source to support such a distinction.8 At best then, this argument may serve as a justification for a de facto practice (limud zechut). Ideally though, Rabbi Shternbuch advocates that one abide by the ruling of the Shulchan Aruch and avoid using a canvas succah.
Another possible justification is based on the method of securing the sheets. In the case of the Shulchan Aruch, the sheets in question may not have been designed to be fastened or tied in place. Therefore the means used to secure them may not have been reliable. Canvas walls which are fitted with metal rings and are designed especially to be tied in place are more reliable and arguably not subject to this ruling.9 The reservations expressed by Rabbi Shternbuch would presumably apply to this distinction as well. Therefore, in compliance with the stringent ruling of the Shulchan Aruch, a canvas succah is best avoided.10
With some modification in design, it is possible to construct a canvas succah that would satisfy this stringency. The basis of this solution, which is actually recorded in Shulchan Aruch itself and has its source in the original comment of Rabbeinu Peretz, is the concept of lavud11 Lavud is an allowance that provides that a halachic wall need not be solid. Gaps are permitted as long as the width of each gap is less than three tefachim.12 From a halachic standpoint such gaps are considered to be filled. Conceivably, a wall consisting of a series of poles each being within three tefachim of the other would constitute a halachic wall. Installing such a configuration of horizontal poles in the frame of the canvas succah would create halachically valid walls entirely independent of the canvas sheets. As the height of the succah wall need not be more than ten tefachim.13(regardless of the actual height of the succah) it would only be necessary to add enough such poles as to achieve that height. A canvas succah incorporating this relatively simple modification would rate ideal in the eyes of halacha.14
The Gemara records two opinions to explain the view of Rabbi Yehudah. One opinion is that this succah is unacceptable because it is portable. Rabbi Yehudah requires that a succah be keva - a permanent, fixed structure. A succah that can be moved from place to place lacks keva and is invalid15. A second opinion in the Gemara explains Rabbi Yehudah's view entirely differently. According to this opinion Rabbi Yehudah accepts the principle known as ma'amid (literally, support). This principle states that the same restrictions which apply to the s'chach apply to the supports of the s'chach as well. Therefore, any material that may not be used for s'chach may not be used to support the s'chach either. The logic behind the ma'amid principle is that the rabbis were concerned that confusion may arise and people may mistakenly use the material of the s'chach supports for the actual s'chach. In order to safeguard against such an error, the rabbis demanded the same standards for the supports as required for the s'chach itself.16 This explains why Rabbi Yehudah does not allow supporting the s'chach on a bed. A bed is subject to ritual impurity (mekabel tumah) by virtue of being a utensil. As such it is unfit for use as s'chach. Accordingly it may not be used to support the s'chach because of the ma' amid principle.
Most authorities reject the ma'amid restriction. Some of these authorities rule in accordance with the view of the anonymous Tanna Kamma and allow supporting the s'chach on a bed.17
Others rule in accordance with Rabbi Yehudah18 but adopt the first explanation of the Gemara, that of keva.19 Accordingly, no restrictions are imposed on the type of materials allowed for the supports of the s'chach. There is a minority view, held by some, which rules in accordance with the second explanation of Rabbi Yehudah and adopts the restrictive ma'amid principle.20
This view, however, is not accepted by the Shulchan Aruch, as is evident from the statement of the Shulchan Aruch (630:13) that allows the above-mentioned case of using a bed to support s'chach. Mishnah Berurah (no. 59) explicitly elaborates that although the bed is subject to tumah this is of no consequence, as this restriction only applies to the s'chach itself and not to that which supports the s'chach.
However, Shulchan Aruch elsewhere (629:7) expresses reservation about the use of a ladder to support s'chach. A ladder, by virtue of being a utensil and thus subject to tumah, is unfit for s'chach. It may not be used to support s'chach because of the ma'amid principle. This would appear to be an endorsement of the minority view which adopts the ma' amid restriction. This is problematic in light of the Shulchan Aruch's lenient ruling in the case of the bed. Magen Avraham (629:9) suggests that essentially the Shulchan Aruch rejects the ma'amid restriction. Strictly speaking, a succah is valid even if the supports of the s'chach are of a material unfit to be used as s'chach. However, in deference to the minority opinion, the Shulchan Aruch rules that ideally the s'chach supports should conform with the ma'amid restriction. This is the conclusion of a number of other authorities as well.21 Thus the view of the Shulchan Aruch is that l'chatchiIa, s'chach supports should be of a material that is fit to be used for s'chach. B' dieved, the succah is valid even if this requirement is not satisfied. Mishnah Berurah (ibid.) adopts this ruling as well.22
The canvas succah is a case in point, since the s'chach is supported by a metal frame. Metal, as it does not grow in the ground, is unfit for use as s'chach. Likewise it may not be used to support the s'chach according to the ma' amid restriction. Strictly speaking, the succah is certainly valid and one fulfills the mitzvah of succah. Nonetheless, it fails to meet the ideal standard (l' chatchiIa) as outlined by the Shulchan Aruch.
However, with a slight modification the problem of ma' amid in a canvas succah can be resolved. This is based on the notion that the ma' amid restriction applies only to that which is directly supporting the s'chach. That which indirectly supports the s'chach (ma'amid d'ma'amid) is not subject to the restriction.23 Therefore, simply placing wooden slats to act as supports across the succah from one side of the metal frame to the other and then placing the s'chach on those wooden supports would solve the ma'amid issue.24 Since the metal frame is merely providing indirect support for the s'chach, it satisfies the l' chatchila standard as well.25
Based on this, the Ramo (635:1) cites the ruling of Hagahat Maimani that at the time that the s'chach is set in place the walls of the succah must be erect. To first set the s'chach in place and only afterwards to erect the walls would violate the Ta' aseh v'la min ha' asui rule. In the case of the conventional succah this presents no problem, as the s'chach is supported by the succah walls and thus cannot be set in place until the walls are first constructed. However, this is not the case regarding the canvas succah. Once the metal frame is constructed, one has the option of either first proceeding with the construction of the walls (i.e. tying the canvas to the frame) or with the placing of the s'chach. Mindful of the ruling of the Ramo, one should make sure first to complete the walls and only then place the s'chach.
In the event that one failed to do so, it is questionable what the status of the succah is. Mishnah Berurah (ibid. no. 10) records the view of the Bach who maintains that b'dieved the succah is acceptable. Aruch HaShulchan (ibid. no. 5) argues strongly in favor of this position. However, Mishnah Berurah notes that many authorities disagree and hold that the succah is invalid.26 It appears that Mishnah Berurah concurs with this view. As such, it is necessary to reset the s'chach. In addition, what emerges is that it is questionable whether one may use a canvas succah built by someone else unless it can be ascertained that the proper sequence was followed. Failure to do so may involve transgressing the Torah violation of eating in a non-valid succah. Furthermore, reciting the succah bracha in such a case may well be a bracha l'vatalah (a bracha in vain), a serious violation in its own right.
It would appear then that the conclusion of the Gemara is that a mat made specifically for the purpose of s'chach is acceptable. However, there are two considerations that potentially could disqualify even such a mat. The first consideration is that of Mar'it Ayin and the second is that of Gezerat Tikrah. Each of these will now be examined in detail.
This is an application of the well-known halachic principle of Mar'it Ayin27People may draw incorrect conclusions based on the situation as it appears, without investigating and ascertaining the facts.
The Rosh cites an opposing view held by R. Yeshaya of Trani (author of Piskei Rid) who is not concerned about Mar'it Ayin. According to R. Yeshaya, as long as the particular mat in question was made28 specifically for s'chach it is acceptable, regardless of local practice. This is the position held by Rambam as well, who rules that a mat made specifically for s'chach is acceptable (Hilchot Sukkah V:6). Rambam makes no mention of local practice. Clearly, Rambam rejects the provision which the Rosh adopts and holds there is no concern of Mar'it Ayin.29
The halacha regarding this dispute is recorded in Shulchan Aruch (D.C. 629:6). Shulchan Aruch rules in accordance with the view of R. Yeshaya and allows a mat made specifically for s'chach. Ramo, however, adopts the stringency of the Rosh and disallows such mats if the local practice is to use mats predominantly as cushions.30 Thus, the Ashkenazic community must take into account local practice out of a concern for Mar'it Ayin, whereas the Sephardic community need not.
Consequently, for Ashkenazic Jews it is necessary to ascertain local practice in order to determine the permissibility of s'chach mats. Referring to his own times, the Mishnah Berurah (ibid. no. 18) comments that "in these places [i.e. Eastern Europe] all mats are made for cushions [literally, lying] and thereby even if he made it for s'chach it may not be used as s'chach." Accordingly, Rav E. Y. Waldenberg (Tzitz Eliezer X:29) disapproves of any type of mat. Rav Binyomin Zilber (Az Nidberu II:66) however notes that nowadays the observation of the Mishnah Berurah is rarely the case. It can no longer be said that the predominant use of mats is for cushions, and therefore there is no issue of Mar'it Ayin.31 Many other contemporary authorities32 share this view as well and accordingly allow s'chach mats.33
Since ceilings of homes are constructed out of wooden boards, the Rabbis expressed concern that using these boards as s'chach would create a misconception. People may be tempted to simply sit in their home instead of a succah. Therefore the Rabbis banned the use of wooden boards as s'chach. The width of the boards which are subject to this gezerah is debated in the Mishnah and Gemara (Succah 14a). The halacha follows the view that only boards which are four tefachim wide are prohibited (see Shulchan Aruch 629:18).34
The significance of four tefachim is subject to a dispute among Rishonim. The Ritva and others suggest that since boards of that width are sturdy, they are seldom used for s'chach. Thus, they resemble more closely the ceiling of a home. Those which are less than four tefachim wide are commonly used for s'chach and less likely to be confused with home ceilings. Therefore, these are not included in the gezerah.
Rashi, however, offers a different explanation.35 Rashi maintains that the width of four tefachim is significant because the majority of home ceilings are constructed from these size boards. Being that this is the case, the rabbis banned their use. Boards of less than four tefachim are not usually found in ceilings and are permitted. This observation, that the majority of home ceilings are constructed from boards not less than four tefachim, is challenged by the Ritva and other Rishonim.36 The Gemara (Bava Metzia 117a) states explicitly that it is quite common to construct ceilings using boards narrower than four tefachim. Accordingly, these boards should have been banned as well. Therefore, these Rishonim reject Rashi's contention that Gezerat Tikrah is on account of the majority of home ceilings.
The Ran, however, defends the view of Rashi. The Ran explains that while it is true that narrow boards are used in home ceilings, prior to their being installed they are joined together to form a wider board. Since at the time of installation the narrow boards form one wider board of four tefachim, Rashi is correct in stating that the majority of ceilings are constructed from boards of four tefachim. On account of this majority, the Rabbis banned this size board. Thus, according to Rashi, boards of four tefachim width which are formed by joining together two or more narrower boards are also included in the prohibition of Gezerat Tikrah. The Ritva and others disagree and hold that Gezerat Tikrah does not apply since the actual width of the individual boards is less than four tefachim.37
Rashi's view is shared by an opinion recorded in Teshuvot HaRashba (vol. 1 no. 213) and may in fact be the view of the Rashba as well. The question was raised as to whether it is permissible to nail together narrow slats of wood to form a lattice frame upon which leaves of s'chach would be placed. The Rashba responds by quoting "one of our rabbis" who prohibited this because once nailed together, the slats of wood are considered as one large board of four tefachim. The Rashba cites other authorities who allow such a frame and he himself is inclined to agree with this position in light of the fact that this is the common practice. Clearly, the position of "one of our rabbis" is that of Rashi, as explained by the Ran. The ban of Gezerat Tikrah includes narrow boards that, when combined together, form what is considered one wide board.
The position of the Rashba, however, is unclear, as the Rashba does not offer any explanation as to why such a frame is allowed. Presumably, the Rashba adopts the view of the Ritva and others who disagree with Rashi. Wooden slats, even when nailed together, are to be viewed and measured individually. They do not constitute one board measuring four tefachim wide and thus Gezerat Tikrah does not apply. However, it is conceivable that in principle the Rashba agrees with the view of Rashi and, once nailed together, the narrow slats are indeed considered as one wide board. The Rashba holds though that this is only the case when the slats are adjacent to one another. In this particular case of the lattice frame there are gaps between the slats where the s'chach leaves are placed. Therefore it cannot be viewed as one wide board. Otherwise, where one continuous board is formed, Gezerat Tikrah would apply. Both Magen Avraham (632:1) and Chaye Adam (146:31) adopt this interpretation of the Rashba when recording this ruling.38
Thus, what emerges is that the status of narrow boards joined together to form a wide board of four tefachim is subject to a debate among the Rishonim. One school of thought, namely Rashi, "one of our rabbis" cited by the Teshuvot HaRashba, and apparently the Rashba as well, views narrow boards nailed together as constituting one wide board and therefore prohibits their use because of Gezerat Tikrah. The other school of thought, held by the Ritva and others, allows their use as the narrow slats are viewed individually.
While no clear ruling appears in Shulchan Aruch regarding this issue, it appears that the halacha follows the stricter view. This is evident from the above-mentioned ruling of Magen Avraham and Chaye Adam. Slats joined to form a lattice are permitted on the grounds that there are gaps between them. This implies that slats joined together adjacently would be prohibited. Once joined they are viewed as one wide board and subject to the prohibition of Gezerat Tikrah.
It is this conclusion that has a bearing on the issue of s'chach mats. Many s'chach mats are made by joining together thin strips of wood. Based on the above halacha, such a mat is viewed as one unit constituting a wooden "board". Being that the width of the mat exceeds four tefachim, it should be prohibited because of Gezerat Tikrah. Indeed this is the position taken by Rav Shalom Y. Elyashiv on this issue.39 Other authorities, however, disagree and allow such mats. Rav Shlomo Zalman Auerbach argues that since a mat is thin and flexible, it does not constitute nor resemble a board which is thick and sturdy. As such Gezerat Tikrah does not apply.40 Teshuvot Avnei Nezer 41 offers a distinction based on the method of joining the wooden strips. In the above case they are nailed together and therefore the result is one large board. But a mat is made of wooden strips which are strung or woven together. Therefore, it does not constitute one board and is not subject to Gezerat Tikrah.
Even according to Rav Elyashiv it is only those mats made of wood which are problematic. A s'chach mat made of strips of bamboo or reed bears no resemblance to a board of wood. Gezerat Tikrah applies only to boards of wood and thus mats of these other materials are not included in the prohibition.42
1. Actually a succah made of sheets is already mentioned by Chazal. See P'sikta (Emor) cited in Moadim U'zmanim 1:84.
2. See Mishnah Succah 11a and 12a: Shulchan Aruch O.C. 629:1 and 630:1.
3. For a thorough analysis of this requirement, see Emek Bracha (R. Aryeh Pomeranzyck) Surrah no. 19. See also Iggerot Moshe O.C. V no. 40b.
4. Chazon Ish (Hil. Eruvin 77:6) disputes this interpretation and maintains that the rule of Rav Acha Bar Yaakov (and the Shulchan Aruch as well) only invalidates a wall moving in the wind when that movement causes the wall to lose the dimensions required for a halachic wall (e.g. the wind will cause the branches of the tree to separate from each other, thereby creating gaps of three tefachim, or the branches will bend below the minimum height of ten tefachim [see below]). But if the proper dimensions will be maintained, then movement in and of itself is not grounds to invalidate the walls. Chazon Ish acknowledges that the later authorities have not specified this. See, however, Kiryat Sefer (Succah 4:5) who adopts this interpretation. (See also Pri Megadim (Eshel Avraham) 363:4 and Tosafot Shabbat 363:9 in reference to eruv. Whether this has bearing on the halacha of a succah wall is debatable. See Iggerot Moshe ibid.)
5. See commentary of Bach (Tur ibid.) who finds support in the Gemara for the ruling of Rabbeinu Peretz. The Bach would agree, though, that this is not a formal gezerah. See also source quoted in note 1.
6. See Eliyahu Rabbah 630:19, Hilchot Chag B'Chag-Succah (R. Moshe M. Karp) Chap. 4 note 16.
7. Moadim U'zmanim, ibid.
8. See also Iggerot Moshe (ibid.) where Rav Moshe Feinstein dismisses a similar distinction on the grounds that there is no basis for it in any source "even in the works of the latest authorities such as Mishnah Berurah or Aruch HaShulchan."
9. Hilchot Chag B'Chag (ibid.), Succah Kehilchata (Rabbis Y. Shvartz and S. Gelber) Chap. 4 note 2.
10. See ruling of Rav Moshe Feinstein (ibid.) that one should not use a canvas succah unless there is no alternative. The commercial availability of the canvas succah, cautions Rav Moshe, does not imply halachic endorsement.
11. The source for lavud is Halacha.L'Moshe MiSinai. (Shabbat 97a, Succah 6b).
12. See Mishnah Eruvin 16b and Succah 7a. For this purpose three tefachim is approximately nine inches.
13. Approximately 30-40 inches depending on the various opinions of the size of a tefach.
14. It should be noted that lavud is not always to be relied upon for the walls of a succah. See Mishnah Berurah 630:7. However, in this instance it is the tied canvas that strictly speaking forms the walls. The lavud wall merely serves as a backup in the event that the canvas becomes untied. See Sha'ar HaTziun 630:49.
15. Rashi (s.v. sh'ein lah keva). See, however, Ran quoting Ra'avad for a different explanation.
16. See Ran, Ritva, and others who identify this as a gezerah. Rashi (s.v. sh'mamida), however, explains that it is considered as if the s'chach itself is mekabel tumah. See commentary of Bach (Tur 629).
17. Among them are Rabbeinu Zerachia HaLevi, Rambam (Commentary to Mishnah), Rabbeinu Yeshaya (in Shibolei Haleket), and Maharil (no. 83) who cites this as the view of most Geonim. Rabbi Yehudah's view is rejected because it is a minority view. See Piskei Rid. (For an additional reason see Rambam ibid. and Meiri.)
18. The discussion in the Gemara devoted to explain the reasoning of Rabbi Yehudah is evidence that his view is accepted. (See Ran.)
19. Among them are Rosh, Tur (630), and Terumat Hadeshen (no. 91)
20. Ran and Ritva.
21. Pri Megadim (EsheI Avraham) 629:9, Chaye Adam 146:30, Bikurei Yaakov 629:13, Chazon Ish 143:2, and others.
22. See also Mishnah Berurah 629:22; Biur Halacha 630:1, and Shaar HaTziun 630:60.
23. See Mishnah Berurah 629:26. Shaar HaTziun (51) cites Magen Avraham, Gra, and the "other Acharonim" as the source of this halacha. Chazon Ish (143:2) disagrees with this view and holds that there is no difference between ma'amid and ma'amid d'ma'amid. Accordingly, l' chatchiIa, one should not make use of any material that is unfit for s'chach to support the s'chach even indirectly. (This includes nails which hold together the walls, which in turn support the s'chach Those who follow the rulings of Chazon Ish employ wooden pegs in place of nails.)
24. Succah Kehilchata Chap. 5 section 5:2. Placing wooden slats to lay on the metal poles themselves (so as to separate between the s'chach and the metal) is a questionable solution. Removing the wooden slats will not cause the s'chach to fall but rather to rest on the metal frame. Ultimately it is the metal frame that is supporting the s'chach. When the wooden slats are placed, across the metal frame, however, the s'chach would fall if not for the slats. Thus it is the slats which support the s'chach and not the metal frame (ibid. note 20.) In any event, wrapping the canvas around the top of the metal frame, in order that the s'chach not come in contact with metal, clearly accomplishes nothing.
25. See, however, Kehilot Yaakov (Succah - new edition no. 18) where the Steipler Gaon points out that the wooden slats which serve as support for the s'chach also partly cover the area of the succah. Thus, he argues, in addition to supporting other s'chach they themselves are s'chach. As such, the l'chatchila standard has not been satisfied, being that these wooden slats are s'chach supported by metal.
26. Shaar HaTziun (635:12) identifies these authorities as Levush, Taz, Eliyahu Rabbah, and Bigdei Yesha.
27. Neither the Rosh, Tur, nor Shulchan Arnch identify this concern as Mar'it Ayin. The term is found in later sources when discussing the view of the Rosh. See, for example, Bikkurei Yaakov 629 no. 6.
28. In fact R. Yeshaya rules that a mat made for no particular purpose but purchased to be used as s'chach is also acceptable. On this point the Rosh is in agreement (provided there is no concern for Mar'it Ayin).
29. See Beit Meir (O.C. 629:6) who notes this and further demonstrates that Rashi and Ran are of this opinion as well.
30. See Mishnah Berurah ibid. no. 17 and Sha'ar HaTziun no. 26.
31. See also Aruch HaShulchan (629:13) who writes that mats are no longer used for cushions but rather for packing merchandise. Such usage also qualifies a mat to be a utensil and unfit for s'chach. As such, Aruch HaShulchan concludes that Mar'it Ayin remains an issue. Nowadays, however, mats are not used for packing either. (See also comments of Rav Tzvi Pesach Frank in Mikraei Kodesh Sukkot 1:16).
32. Note the authorities cited in the next section. Even Rav Shalom Y. Elyashiv, who objects to mats on grounds of Gezerat Tikrah (see later), concedes that there is no concern for Mar'it Ayin. (See Az Nidberu XII:35.)
33. See Hilchot Chag B'Chag 6:24 (based on Beit Meir as cited in Biur Halacha 629 s.v. omedet l'shchiva) for an additional reason why Mar'it Ayin does not apply to the type of s'chach mats available nowadays.
34. Four tefachim is equivalent to approximately 12-16 inches. It is debatable whether the prohibition nowadays applies to even narrower boards as these are used in ceiling construction. See Hilchot Chag B'Chag 6:31.
35. Ibid. s. v. machloket b'nesarim sheyesh bahem arba.
36. See Ramban and Re' ah.
37. The Ritva views the onlooker as the source of concern that prompted Gezerat Tikrah. Therefore, the method of installation is immaterial. Rather, it is the resulting ceiling that is the determining factor. As such, as long as the individual boards are less than four tefachim it is permitted. See Ritva 14a. Rashi, however, maintains that the source of concern is the one constructing the succah. Therefore, boards joined together to form one large board may lead to confusion in his mind. Thus a gezerah is necessary in this case as well. See Rashi 15a s. v. b'bitulei tikrah and Meiri thereon.
38. See also Bach (626) who implies the same. Beit Yosef, however, is inconclusive as he quotes the ruling of the Rashba but fails to provide a rationale.
39. See letter of Rav Elyashiv in Az Nidberu (Rav Binyomin Zilber) II:66. Rav Elyashiv raises an additional problem with s'chach mats based on the ma' amid principle. The strips of wood are joined together by string. Without the string the wooden strips would not stay in place. Thus, the string is serving to support the s'chach. As mentioned, the halacha requires that l' chatchila that which supports the s'chach be of a material fit for s'chach. If the string may not be used as s'chach then using it as a ma' amid does not satisfy the l' chatchila standard. See also Hilchot Chag B'Chag Chapter 6 note 33 and Succah Kehilchata Chapter 5 note 15.
40. See Sefer HaSuccah (R. Eliyahu Weisfish) p. 449 no. 7. Presumably this distinction would not be apparent to an onlooker viewing the s'chach in place but rather to the one constructing the succah. This is consistent with Rashi's view - see above note 37.
41. Orach Chaim 443:5.
42. Such s'chach mats, known as "canes" mats, are commercially available and reportedly are endorsed by Rav Elyashiv.
The debate concerning s'chach mats centers around the following question. According to the view of Rashi, which the halacha evidently follows, Gezerat Tikrah includes wide boards that are combined from narrower ones. Mats then should be prohibited as well. Yet the Gemara mentioned above explicitly allows those mats made specifically for the purpose of s'chach. Why are mats different? At least one of the following three distinctions must be acknowledged.
1) A board is sturdy while a mat is flexible.
2) Only when nailed or glued together are narrow boards viewed as one wide board. When tied or woven they continue to be viewed individually.
3) The mats which the Gemara permits are made of material other than wood.
Those who allow s'chach mats adopt one of the former distinctions. Rav Elyashiv adopts the last. (Hilchot Chag B'Chag ibid.)