Ask the Rabbi: Hanging and Using Hammocks on Shabbat | Rabbi Daniel Mann | Beit Midrash

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Question: I want to buy a hammock and am considering two types. One is attached to two trees, and one is fastened to a frame that sits on the ground. Are there problems, for either type, with hanging them (ohel (canopy/tent-like structure)?) or using them on Shabbat?

Answer: The gemara (Shabbat 155a) says that one may not "use," including by leaning on, a tree, as well as something that is connected or supported by a tree (e.g., a nail, rope, ladder) on Shabbat. Therefore, it is forbidden to lie on a hammock if it is tied to the tree directly; it is permitted if the hammock is attached to something connected to the tree (Shulchan Aruch, Orach Chayim 336:13). (Many hammocks come with parts in a manner that it is not obvious if it is considered one apparatus connected to the tree, or that one piece is connected to the tree and the main part of the hammock is twice removed and permitted.) Also, the lying on the hammock must not make the tree(s) move (Mishna Berura 336:63).
Hanging a tree-attached hammock, even to something already attached to the tree, will be a violation. Lying in one hung before Shabbat will depend whether the hammock is tied to the tree, or to something attached to the tree, and on the strength of the tree and weight of the person. These issues are not a problem for a hammock that is attached to a frame.
Is hanging the hammock considered making an ohel? Since a hammock is mainly horizontal, it is a candidate for ohel, which can be forbidden to erect even if it does not connect to a vertical wall (Shulchan Aruch, Orach Chayim 315:1). However, it is permitted to erect a temporary horizontal structure if it is not done to protect that which is below it unless it connects to vertical walls/boards (ibid. 3; Rama ibid. 7). A hammock connected to mere trees/poles would not have that problem.
A net-style hammock may have another reason to not have a problem of ohel. It is permitted to hang a material that has more "air" than fabric (Tosafot, Eruvin 102a; see R. Akiva Eiger, OC 315:2). This may be the case with a net fabric for a hammock, while on the other hand, the fabric of some hammocks gathers together when no one is in the hammock. In the latter case, it might be more fabric than air.

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Is putting a body or a cloth on a net-style hammock, creating an ohel? Surprisingly, the gemara (Eruvin 102a) says that if the strands are within three tefachim of each other, it is considered, for the sake of leniency, a case where the ohel pre-exists and one can add on to it (see also Shulchan Aruch, OC 315:2). (It also does not appear that the body of a person lying down can be a forbidden ohel.) Swinging in the hammock will not be forbidden as creating an ohel in a new place, as it is permitted to move an existing ohel to a new place (Piskei Teshuvot 315:2). (Why, then, it is forbidden to use an open umbrella is a good question beyond our scope.) In any case, we saw that due to the lack of vertical walls or need to protect that which is below, a hammock is not a forbidden ohel.
Still, we cannot give broad permission to hang hammocks on Shabbat because the potential of other problems exists. In some models, there is a need or a likelihood of tying forbidden knots. Attaching the hammock to different types of frames can potentially be forbidden due to boneh or tikkun kli (different forms of forbidden building). Various parameters determine when an attachment is forbidden, and some of the major factors are: the connection’s strength, the expected or standard duration of the connection, and whether it is attached to something that is attached to the ground (see our Ask the Rabbi column for Va’ethchanan 5784). These can depend on the model and on how an individual makes and uses it. We would therefore expect someone (unless he is capable of working out the halachic calculations of his specific case) to put up the hammock before Shabbat if he will want to use it. It will then be permitted to use it in most cases (as we explained above).

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