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Why is bamboo kosher for schach? I know everyone uses them, but one should not use anything like planed wood which is used for building or straw for thatching roofs. I understand that certain bamboo canes are used for building (even lion cages!) in the countries where they grow, but they are not used in developed countries.
Source: Aruch hashulchan 629:32
http://www.star-k.org/kashrus/kk-mitzvos-schach.htm
the criteria for schach is three fold: 1) items that grow from the ground, 2) items that are no longer attached to the ground, and 3) items that cannot become tamei.
Bamboo meets these requirements and is therefore Kosher for S'chach
See also: http://www.moreshet.co.il/web/shut/shut2.asp?id=78220 which quotes the Teshuvas HaRashba (Part One, Chapter 58) as permitting bamboo mats used to make walls for use as Schach.
One of the requirements for schach is that it be something which cannot become tamei (impure), and therefore it cannot be a kli. In other words, it can't have been fashioned by a human with the intention of being used for something else. Therefore bamboo poles which were previously used for building a lion cage could not be used for schach. Every so often I hear rabbis reminding people that bamboo mats from the local big box store are not kosher for schach because they were manufactured for a non-schach purpose. (at least around here those are more common than bamboo lion cages). Most of the bamboo mats you see people using are (hopefully) ones that were made for schach and nothing else. Many also come with a certification from a rabbinic organization stating that clearly.
Basically, just because some bamboo is mekable tumah (susceptible to impurity), doesn't mean all the bamboo in the world is.
Mishna Succa1, 4:
זה הכלל, כל שהוא מקבל טמאה ואין גדולו מן הארץ, אין מסככין בו. וכל דבר שאינו מקבל טמאה וגדולו מן הארץ, מסככין בו.
That is the rule, everything which can become uncleaned and didn't growing from earth, we don't cover with him; everything which doesn't can become uncleaned and did growing from earth, we do cover with him.
Bartenura:
כל דבר שאינו כו' - לאפוקי כלי עץ ובגדי פשתן ומחצלאות, שאע"פ שגדוליהם מן הארץ אין מסככין בהן הואיל ומקבלין טומאה:
וגידוליו מן הארץ וכו' - דכתיב חג הסוכות [תעשה לך באספך] מגרנך ומיקבך, בפסולת גרן ויקב הכתוב מדבר, כלומר מן הנשאר אחר שאספת הגורן והיקב כגון קשין וזמורות מהם עשה סוכה :
Wood and linen skills despite of their come from earth,are not regular because they can become uncleaned.
We learn the criterion from the verse which says that you need to make a succa at the season of wheat and grape harvest and. This is considered as an allusion that you need to build the succa with the residues of the harvest, as strew and twigs
Practically Bamboo is very strong an will not be deteriorated by sun and rain until the end of the week of Succot.
Genuine Bamboo is not kli because nothing become kli until her owner decided this.
--concerning the core of the question, Gzerat tikra.
Mishna 6:
מסככין בנסרים, דברי רבי יהודה. ורבי מאיר אוסר.
Bartenura:
פחות משלשה דברי הכל כשרה דחשיבי כקנים
כי פליגי משלשה ועד ארבעה, רבי יהודה סבר כיון דלית בהו שעור מקום חשוב לא גזרינן שמא ישב תחת תקרת הבית. ורבי מאיר סבר כיון דנפקי מתורת לבוד גזרינן שמא יאמר מה לי לסכך באלו מה לי לישב תחת תקרת ביתי.
The prohibition is with width wood pieces which seems as a a roof of the house.
Wen the width of each bamboo piece is less than 4 tefachim, approximately 36 cm. Nobody enact gzerat tikra.
Difference with a thatching roof: (thanks to @Yishai). we need to ordering problems. Gzerat tikra is for boards (planed wood) which are width more than 3 tfachim for RM, and for RI at least 4 tfachim. Straws have nothing to with this gzera. The problem of a thatching roof is or one of its problem is that rain cannot passing through it
