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Single Tree Octagon Design
Looking for feedback on Support Structure and material sources
(51) #1
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Subject: Single Tree Octagon Design
I am planning a single tree octagonal design for my son’s tree house and want a second opinion (or more) on the support design.  It will be basically an Octagon deck, 12” X 10”, we are not planning any full walls or enclosure but obviously railings and maybe some seating and we are going to have tarp panels that stretch up into the tree for shade/partial cover

My plan is to use 1" Lag screws either directly thru the beam into the tree or holding J brackets to hold two 4"X8"X12' beams on opposite sides of the tree.  The 2 beams will have knee braces down to the tree with 1” lag screws holding them there.  I will then have 2”X6” floor joists on approx 30” centers of varying lengths,  mirrored on each side of the trunck, 10’ long at the trunk down to 3.5” at the ends of the beams.  I plan to have knee braces down to the tree on the long floor joists at the tree, trying to decide how to brace the middle pair of joists. 
         -- How far can I safely cantilever the joists beyond the support beams or where the knee brace connects to the
             beams or joists?  

          -- Any other ideas on the support structure for a simple single tree octagonal design would be appreciated.  
          -- Are 1” lag screws sufficient as single connect points for this size structure? 
          -- Is there someone online where I can buy “J” brackets and HGD 1” lag screws?  (I did find one place with 1” lag screws in
              stock from an overrun, but they are not galvanized).  To have the screws made on such a small lot they are $50/piece.

Thanks for your input.
(38) #2
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Check out for your bracket's.

This company has 3/4 lag screws in stock, maybe they can special order.  Check the site they might have a store near you.
(1) #3
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Bolting beams to both sides of the tree is not recommended as it unnecessarily restricts the growth of the tree. A better solution for an octagonal treehouse would be to use radial supports to each corner, consisting of a beam and brace or beam and cable.

Cantilevering joists can be a bit tricky in a treehouse but are often used effectively to add floorspace. When weight is applied to the cantilevered end, the other end will be pulling upwards. This means all the supports underneath need to be able to withstand upward forces as well as downward. Assuming a strong enough joist (6x2 is too small) you should keep the cantilvered length of the piece of timber less than the distance between its two support points. For a 4-5' cantilever you should aim for 8x2 as a minimum. However, it is unlikely that your two support beams will have sufficient spacing to support this sort of cantilever effectively.

In order to help the cantilevered joists, you can tie them together with an outside facing of the same material as the joists. Then use cables or knee braces to hold this facing up. Running this facing around the outside of all the joists will help support the others too. The corners will be hard to connect without using brackets, but you should be able to find flat steel joining plates in large hardware stores which can be bent to suit the angle.

1" lags would be good for this size of treehouse, with at least 6" able to penetrate the tree. 3/4" could be used as an alternative but it is always good to overbuild treehouse parts. 3/4" galvanised lag bolts are available at McMaster-Carr or Bolt Depot; I don't have any web sources for 1" bolts. Brackets for treehouses have to be custom made but you might like to ask Out 'n' About (as already suggested) or Treehouse Workshop for a quote.
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