Kelim, Chapter Twenty, Mishnah Five
1) A wood block that was fixed to a row on a wall, whether he fixed it and did not built upon it or built upon it but did not fix it, it is susceptible to uncleanness.
a) If he fixed it and also built upon, it is clean.
2) Matting that was spread over the roof-beams, whether he fixed it but did not put on the plasterwork or whether he put on the plasterwork but did not fix it, it is susceptible to uncleanness.
a) If it was fixed and he laid plasterwork over it, it is clean.
3) A dish that was fixed to a chest, box or cupboard:
a) If in such a manner as to hold its contents in the usual way, it is susceptible to uncleanness,
b) But if it was in a manner that it cannot hold its contents in the usual way it is clean.
Section one: The Rambam explains that the “row on a wall” referred to here is the scaffolding in between which builders make their wall. The “wood block” is a thick piece of wood that the builders attach to the wall and to the building adjacent to the wall in order to keep the wall close to the building. If he fixes it to the wall but does not build upon it, or if he builds upon it but does not fix it to the wall, then the wood block is not considered part of the building and it is still susceptible to impurity. But if he fixes it to the wall and builds upon it, then it is part of the building and it is not susceptible to impurity.
Section two: The matting is used here as a base for the plasterwork which will serve as the ceiling to the house. If the matting has been fixed to the beams but the plasterwork has not yet been put on, or if the plasterwork has been put on, but it has not yet been fixed to the beams, it is not yet part of the building, and it is still susceptible to impurity. The matting is part of the building and therefore clean, only if it has the plasterwork and has been fixed to the building.
Section three: The chest, box or cupboard referred to here are large enough so that they are not susceptible to impurity. If one attaches a dish to one of these things in such a way that the dish can still hold things, that is the mouth of the dish faces up, the dish is still susceptible to impurity. But if he attaches it face down (not sure why one would do such a thing) then it is considered nullified vis a vis the chest, box or cupboard and it is clean.