The Torah says that we celebrate Sukkos when we gather “from your threshing floor and your wine cellar.” Chazal derive from this
possuk that the
sukka itself (i.e. the
schach) must be made with the likeness of wheat stubbles and vines that, (a) grow from the ground, (b) are no longer attached to the ground,
[1] (c) cannot be fit to become impure (not food
[2] or a vessel).
[3]
Although wooden boards conform to these rules, Chazal disqualified boards which are wider than four
tefachim (12.5 inches, 32 cm) since they are similar to standard roofing panels,
[4] and a wooden roof is
possul min haTorah since it is made for protection and not for shade.
[5] Since these beams were disqualified, they are ineligible even if positioned sideways where they are narrower than four
tefachim.
[6]
Modern roofs use much narrower beams, and so
halacha extends this prohibition to any plank which is commonly used in roofing.
[7]
Regarding treated wood the halocho is that min hatorah one can use them for schach yet as mentioned depending on its width it might prohibited as a rabbinic prohibition.
[8]
Walls of the sukkah
the halacha is that although the schach is to only be made of certain things yet the walls of the sukkah can be made of almost anything which is strong and sturdy.
in addition to the many halachos of what constitutes kosher schach, there is a rabbinic decree that it should not be placed to lay upon something which is invalid to use for schach (for example davar hamikael tumah – something which is susceptible to become impure) and the reason is so that one will not mistakenly come to use that object for schach as well.
therefore, we do not lay the schach on metal bars or any other such things which are invalid for schach. Yet bideved – after the fact after one lay the schach on such things, one is allowed to lchatchila sit under such a sukkah.
[9]
In addition, one is allowed to lay the schach directly on the building (stones brick or even a cement wall) as there is no concern that one will take stones or bricks and use them for schach. (as everyone knows that stones and bricks are invalid for schach being that they create a permanent dwelling place, and a sukkah is to be a temporary abode).
Regarding metal slabs which are connected to a wall poskim write that that doesn’t have the halachic status of a wall and the above-mentioned concern exists there as well.
[10]
Regarding things which they are prohibited for mdrabonon a rabbinic prohibition the halacha is that one is allowed to place schach on them lchatchila.
[11] Therefore one can lay schach on beams which are 4 tefachim wide.
Mamid dmamid
Although some are machmir the halocho is that one is allowed to place the schach on kosher slabs although they are dependent or lying on bars or nails which are invalid for schach.
[12] And therefore although one does not lay the schach on metal brackets however if one places wooden slabs on the metal brackets one can lay the schach on them.
in any case if the schach is only touching the slabs but not resting on them there is no problem at all.
In practice
One can lay the schach on the pressured wood.
[1] סוכה י”א ע”בוהלאה. שוע”ר סי’ תרכ”ט ס”א.
[2] וראה שוע”ר שם סי”ט שלירקות (בגלל שסופן להתייבש) יש דין אויר ופוסלין בג’ טפחים.
[3] סוכה ט”ז ע”א. שוע”ר שם ס”ב.
[4] סוכה י”ד ע”א, שוע”ר שם סכ”ט.
[5] שוע”ר סי’ תרכ”ו ס”א.
[6] גמ’ שם ע”ב, שוע”ר שם ס”ל.
[7] הגהות מיימונית הל’ סוכה פ”ה אות א’, מג”א או”ח סי’ תרכ”ט סקכ”ב, שוע”ר שם סל”ב.
[8] ראה שוע”ר סי’ תרכ”ט סל”א.
[9] שוע”ר או”ח סי’ תרכ”ט סי”ג.
[10] ספר הסוכה ח”א פי”ח ס”י.
[11] בכו”י סי’ תרכ”ט סוס”ק י”ד. להעיר שפרמ”ג סי’ תרכ”ט א”א סקי”א ואמרי יושר ח”א סי’ מ”ג מחמירים אבל בנדון השאלה בוודאי יש להקל שגם עץ הוא חומרא.
[12] שוע”ר שם משנ”ב סי’ תרכ”ט סקכ”ו..
