Ohalot, Chapter Fifteen, Mishnah Eight
1) [With regard to] the courtyard of a tomb:
a) A person standing in it remains clean as long as there is a space of four cubits square, according to the words of Bet Shammai.
b) Bet Hillel says: four handbreadths.
2) [With regard to] a roof beam which had been used as a covering stone for a tomb, whether it is standing upright or lying on its side, nothing becomes unclean except what [touches] opposite the opening of the grave.
3) If the end [of the beam] were made the covering stone of a grave, only [that part] up to four handbreadths [from the grave] becomes unclean.
4) [This applies] when [the beam] is going to be cut.
a) Rabbi Judah says: all the beam is connected.
Section one: The courtyard described here is at the entrance to a tomb. According to Bet Shammai, the courtyard must be at least four cubits square to be considered its own space. If it is this size, a person standing there will not be defiled by the tomb because he is not considered to be in the “tomb”. Bet Hillel provides a smaller measure. If the courtyard is at least four handbreadths square, it is its own space.
Section two: A roof beam was used to close up a burial grave. Typically, this was done with a stone. A person who touches the part of the beam that is opposite the stone is impure; but if he touches it elsewhere, he is pure.
Section three: In this case he puts the end of the beam up against the cave, with the beam extending out from the grave. The first four handbreadths of the beam are considered to be covering the grave and are impure and defile one who touches them. After that, the beam is pure.
Section four: The above is true if he intends to cut off the beam and leave only the four handbreadths that cover the grave. If he intends to leave the entire beam there, it all defiles.
Rabbi Judah rules more strictly. The whole beam is connected to the part that covers the grave and therefore, even if he intends to cut it off, it is still impure.