Let’s say that the rafters of a house and its attic have no plasterwork on them and they were aligned one beneath the other. If there’s ritual impurity under one of the rafters, everything beneath that one is rendered unclean. If the impurity is between a lower and an upper rafter, then what’s between them is rendered unclean. If the impurity is above an upper rafter, then what’s directly above it is rendered unclean all the way to the sky. If the upper rafters were over the gaps between the lower rafters, then if there’s ritual impurity under one of them, what’s beneath all of them is rendered unclean. If the ritual impurity is above them, then what’s above them is rendered unclean all the way to the sky.
Let’s say that a beam was laid from one wall to another and there was ritual impurity under it. If the beam has a space of a handbreadth, it conveys impurity to everything under it; if not, then the impurity extends both upwards and downwards. In order to have a space of a handbreadth, the circumference of a round beam must be three handbreadths, and a square beam must be four handbreadths. This is because a square is one-quarter larger than a circle.