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Rabbi Daniel Mann

Can I extend a wire from trees in my property to slats attached to my house to form an eiruv?

Answer: The halachic name for the apparatus you are talking about is a tzurat hapetach (the form of a doorway), which consists of two vertical posts (lechis) and a horizontal wire from one to the other (kaneh [al gabeihen]). Regarding whether a tree can be used as a lechi, the gemara (Eruvin 11b) says clearly that it can. However, there are several conditions that you will have to meet, and it will take careful explaining on our part and clear perception of the halacha and the engineering on your part for you to fulfill them correctly.
The main reason that trees are rarely used as lechis is probably the halacha that the horizontal wire must be straight above the lechi (although there can be vertical space between them), not to its side. Therefore, if one were to tie a rope around the trunk of a tree or attach a string to it with a nail, it would be invalid. The string cannot be attached to branches for two reasons: almost all branches are horizontal, and it does not help that they extend from the vertical trunk (see Netivot Shabbat 19:(72)); the branches generally sway too much in the wind to be valid lechis (see Shulchan Aruch, Orach Chayim 362:1). What can be done (in some trees) is to attach a string (using a nail or by wrapping) on top of the trunk where it splits into main branches. See picture 1.
Another possibility is to connect the string to the tree in a manner that is not valid in relation to the tree but to attach a lechi to the tree in a manner that the string goes directly over the lechi. This must be checked with a plum line going down from the string to see that there is a point on the string that is directly above the lechi. This system will only succeed if the tree is very straight or leans in the correct direction in relation to the lechi. The lechi must be connected somewhat strongly so that it will not be uprooted or swayed by normal winds (Shulchan Aruch, OC 363:5). It must be at least ten tefachim tall and start within three tefachim of the ground (ibid. 362:11). See picture 2.
Regarding the side of the house, there is also what to be concerned about. A wall cannot serve as a lechi, which must be somewhat distinct as part of the doorway and not part of the rest of a structure (Magen Avraham 363:28). On the other hand, as a doorpost, the lechi can be connected to or protruding from the wall (ibid.). Therefore, if there is a thin protrusion from the main direction of the wall or you can attach a valid lechi to the side of a wall (with the same requirements as above), that suffices. If by slats, you mean that you want to attach the string to a horizontal or diagonal protrusion from the house, this would have the same problem that we discussed regarding attaching a string to branches.
Because of the complicated nature of some of these issues, we urge you to arrange an on-sight consultation with someone who knows the laws of eruvin.
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