Rambam - 1 Chapter a Day

When a corpse is located in a closed structure or in one which had an opening, but its doorframe was destroyed and it was closed up, it imparts impurity to all its surroundings. Anyone who touches the back or the roof of the structure contracts the impurity that lasts seven days, because it is like a closed grave.

If an entrance had been opened in it, even if it was closed up, as long as the doorframe was not destroyed, one who touches the back of the structure or its roof, is pure. Only the area in front of the door is impure.

How large is the measure of a doorway? If an entire corpse is in the building, the entrance must be four handbreadths. If there is only an olive-sized portion of a corpse, the entrance must be a handbreadth. Any portion larger than an olive-sized portion is considered as an entire corpse and requires an entrance of four handbreadths

The following rules apply when there is a corpse in a building that has many entrances. When they are all closed, they all impart ritual impurity. One who sits near any of the entrances under the roof that protrudes over the entrance becomes impure.

If one of them was opened or one intended to remove the corpse through one of them even though he had this thought after the person died and even if he intended to remove it through a window that is four handbreadths by four handbreadths, this saves all the other entrances. Only the entrance that is open or about which he thought imparts impurity. The others are pure, because they are closed and the building is not considered as a closed grave. Similarly, if one began hollowing out an opening through which to remove the corpse, when he has hollowed out an area four handbreadths by four handbreadths, this saves all the other entrances.

If there was a closed entrance and he intended to remove the corpse through it and began to open it, once he opens it, it saves all the other entrances. If the building had many windows, but they were all closed, they are all pure. If they are open, they all impart impurity, but they do not save the entrances.

When there is a small entrance inside a large entrance, anything that covers either of them imparts impurity. If one intended to remove the corpse through the smaller entrance, the smaller entrance purifies the larger one. If they are both of equal size, anything that covers either of them imparts impurity. If one intended to remove the corpse through either of them, it purifies the other.

If the building had one entrance in the north and one entrance in the south and one intended to remove the corpse through the northern entrance, that entrance imparts impurity. If, afterwards, the brother or the relatives of the deceased came and said: "We will remove it only from the southern entrance," the southern entrance replaces and purifies the northern entrance provided one does not say so as a ruse. If he says so as a ruse, they both impart impurity.

The following rules apply when several houses open to an exedra and a corpse is located in one of the houses. If the usual practice was to remove the corpse through the exedra, the gateway and the houses become impure. If not, the gateway is impure and the houses are pure.

If there is a closed room in a house and a corpse was brought into the room through an open window, the outer portion of the house is pure. The rationale is that the impurity goes back and departs through the same window that it entered.

A grave does not impart impurity to the area around it unless there is empty space a handbreadth long, a handbreadth wide and a handbreadth high above the corpse. Should that condition be met, even if one built a structure over the space of one cubic handbreadth until the heavens, it imparts impurity in its entirety, because the entire structure is considered as a grave.

If the impurity was flush against the covering and there was not an empty space of a cubic handbreadth, the impurity pierces through and ascends and pierces through and descends. The only ones who are impure are those who touch the space above the impurity, stand over it from above, touch the space below it, or stand below it. If, however, one touches the side of the building, he is pure.

This is a great general principle with regard to the impurity stemming from a corpse: If the impurity is flush with its covering, the impurity pierces through and ascends until the heavens and pierces through and descends until the depths, but does not impart impurity to those on the sides.

What is implied? There is a heap of grain or a pile of stones with an olive-sized portion of a corpse in its midst. There are keilim next to the source of impurity, but they are not touching it. Those keilim are pure, but any k'li in the pile that is directly above or below the impurity is impure, because the impurity pierces through and ascends and pierces through and descends. If there was empty space, a handbreadth long, a handbreadth wide and a handbreadth high above the corpse, it is considered as a closed grave and it imparts impurity from all sides.

When a building was filled with earth or pebbles, it is as if the house no longer exists. It is like a pile of earth or pebbles. If there was a source of impurity in the midst of the earth, the impurity pierces through and ascends and pierces through and descends, but the keilim in the earth at its sides are pure.

When impurity that was flush against its covering was in a wall and one rested a sukkah against the wall, the sukkah is impure. The rationale is that the wall became one of the sides of a structure containing a corpse. Although the sides of the structure would be pure when touched by a person if it did not have a roof, once a roof was placed over them, everything in the structure is impure, because the impurity is contained in the structure.