Let's assume that there is a lechatchila to not use metal object that are mekabel tumah (susceptible to tumah) for something that supports the s'chach ("maamid"), which can be avoided by using something that supports something that supports s'chach ("maamid d'maamid"); but if it was used there is no issue b'dieved with using a maamid that is mekabel tumah. See this article by Rabbi Mansour for more information.
A friend of mine uses wooden beams to support his s'chach (which rest on his wooden walls). One of those beams fell and split. Buying or making a new beam proved to be too difficult, so he just put it back together, using Elmer's wood glue and screws.
As mentioned above, this would never be a problem b'dieved, but might there be a problem lechatchila with
- wood glue holding together the beam
- metal screws holding together a beam
vis-à-vis using objects that are mekabel tumah as a maamid, or would we say that the metal elements in this beam are just maamid d'maamid?
