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One bolt for two beams?
John Steele 2009-03-28, 20:53 #1
since Mar 2009 · 1 post · Location: Sacramento, CA
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Subject: One bolt for two beams?
My treehouse design has a triangular foundation of three framing beams resting on three tree limbs, each approximately 9" in diameter. As I will have two framing beams connected at each branch, I was planning on using two lag screws per limb. But now I am concerned that two screws so close together will cause compartmentalization. Would I reduce the chance of damage to the tree if I used one bolt that goes straight through the limb, extending about 6" on either side of it?

If this isn't advisable, the alternative will have to be one lag screw per branch, each screw supporting the two framing beams.

Thanks.
vamture 2009-07-25, 00:27 #2
since Jul 2009 · 1 post
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I have pretty much the same question.  2-tree design with two 2x10 support beams spanning the two trunks.  I was planning to use a single lag bolt at each attachment (so 4 lag bolts total), and was thinking I would vertically offset the lag bolts on opposite sides of each trunk by a couple of inches.  But after reading the compartmentalization page it sounds like the bolts will still be much too close together.  How have other people handled this?  Thanks.
The Treehouse Guide Administrator 2009-07-27, 16:25 #3
User title: The Treehouse Guide
since Aug 2006 · 67 posts
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A through bolt would cause similar damage to having bolts on either side of the tree and is unlikely to cause a problem for the tree. Restricting the outward growth of the tree is more of a problem.

John Steele: My treehouse design has a triangular foundation of three framing beams resting on three tree limbs, each approximately 9" in diameter.
This design could be improved by having metal brackets at each corner of the triangle which are then bolted to the tree with one specialised tree bolt each. You would get much higher strength than with lag bolts and the bracket would cause much less restriction to the growth of the tree than bolting wood on either side of each branch. It would be worth considering using flexible joints at two of the corners to allow the tree to move in the wind without stressing the supports.

vamture: ...two 2x10 support beams spanning the two trunks.
To avoid restricting the tree's growth unnecessarily, use a single 4x10 or doubled 2x10s on one side of the trunk only. A single tree bolt could be used at each end. One end of the beam should be allowed to move in a flexible bracket or by suspending from steel cable, again fixed to one bolt. Another arrangement to span two trees with two beams is shown below. The plywood can be replaced with braces underneath for larger loads.

[Image: http://www.thetreehouseguide.com/constructiontutorials/images/twotreeplywoodbrace.png]
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