S'chach — Beams with Recesses Carved to Receive Slats

Is an aluminum pergola for a sukkah, with wooden slats 4.5 cm wide placed inside U-shaped recesses in wooden beams attached to the aluminum, kosher lechatchilah? The slats are easily removable, and the recesses are not carved all the way through from side to side. Are the wooden beams on which the slats rest considered something that is susceptible to tumah, like peshutei klei etz?

Shalom u'vrachah 

The beams in such a pergola are not susceptible to tumah, and they are kosher for use as s'chach. 

Peshutei klei etz are not susceptible to tumah.

Mishnah Kelim 2:1

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