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Fixing bolts into the tree

edited August 2014 in Supports
The general recommendation for fixing supports cheaply is to fit a single, large lag bolt into a cleanly drilled pilot hole. This reduces the number of puncture points to one, and eliminates dangerous problems of compartmentalisation.

Nails and screws are not strong enough for treehouse loads, especially when storms twist the structure.

Support beams should not be bolted directly to more than one point, because the tree will not be able to move freely in the wind. This will build up very large forces in the support and risks failure of the attachment points. Instead use a flexible support bracket at one end of the beam, or use steel cables to suspend one end.


Comments

  • I read somewhere that compartmentalisation is generally around 1ft in diameter. Is this correct? Would this mean that building in a tree with limbs smaller than 1ft would be unwise? And what of placing TABs on opposite sides of a 2f diameter tree? Would the 2 x 1ft compartmentalisations entirely close off the entire trunk of the tree?
  • Normally the compartment itself is limited to a small area (within an inch or so) around the damage to the tree and its purpose is to block access to a wound by pathogens (bacteria, fungi and insects). Keeping the contained area as small as possible is important for the tree so it can keep nutrient and water flowing to all areas.

    The problem comes if damage (eg, bolt holes) are too close - this can be interpreted by the tree as one large area of damage and a compartment forms around the whole area. So arborists have concluded that it is best practice to space bolts by 12" vertically to make sure that separate compartments can form. Bolts can be spaced a little less than this horizontally, but try to keep the spacing as large as practical.

    This means that a 1' limb could be used with a single bolt. You could also use smaller limbs but there will be limits to how much weight they can safely take. It depends on the tree type, loading at that point, and type of joint. Consult an arborist for recommendations about your tree.

    As the compartments are generally small, you can place several bolts/TABs around a 2' diameter tree. Often bolts can be spaced slightly less than 12" horizontally, so you could fit 6 around the trunk.

    More specific installation recommendations will be available from your TAB supplier.
  • Hi! i'm new in this forum and i'm very interested in making my own treehouse. I'm from south america and i have a problem with the translation of 'bolt'. Previously it was said that it's recommended to fix a single bolt and there is no need for go through the entire trunk. But from internet i get that the difference between a screw and a bolt is that the bolt takes a nut on the other side. So, what type of bolt should i use? i know this is a very simple question, and probably i'm translating wrong but it is something too important and i want to get it right. (some images may be useful) Great site and thanks for all the info you share.
  • edited December 2016
    These "bolts" can also be called "lag screws", which is probably more correct, as they are fixed more like screws. The "bolt" name is used in some locations, but they are interchangeable. Another similar product is a "coach screw". It's likely the name comes from the fact that they are fitted with a wrench or socket, like a machine bolt would be.

    They should be galvanized or stainless steel for use in trees, and a large washer should be used under the head to prevent the head sinking into the support.

    Here's an image from Wikipedia. These are zinc-plated, which is not protective enough for exterior use.


    Lag screws aka lag bolts 001
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