When there is a wall between the properties of Reuven and Shimon, and they share its ownership, both may use it. One may hollow out space from one side and insert his beams, regardless of their size, and the other may hollow out space from the other side and insert his beams.
If the wall belonged to Reuven alone, Shimon may not make use of it. If, however, Shimon hollowed out space in the wall and inserted one beam, and Reuven remained silent and did not protest, he established his right to the place of that beam. Even if his original beam was small and Shimon desired to change it to a big and thick beam, he may.
If the beam was part of a temporary sukkah, he does not establish the right to maintain it for 30 days. For Reuven will say: "I did not waive my right to protest. I allowed it merely because it was temporary." If the owner of the beam maintains the sukkah for 30 days, he establishes his right to it, for that is no longer considered temporary.
Different rules apply if he constructed a sukkah for the holiday. Within all the seven days of the holiday, he does not establish his right to maintain it. After the seven days are completed, he does establish his right.
If he joins the end of the beam to the wall with mortar, he establishes his right immediately, provided he brings proof that Reuven helped him in the construction or saw and did not protest.