Now that all of the tools and proper hardware are gathered. It is now time to design the treehouse structure. I will not cover this process in this Instructable but will give you a few links to get started.

Treehouse Design

http://www.awc.org/calculators/

http://treehouseguides.com/faq.html

http://builtbykids.com/inspiring-treehouse-designs...

Brief on the Engineering

The tree connections need to be able to support the worst case loading scenario that could take place in the treehouse. I usually describe this as fully loaded to the Live Load Capacity, in an earth quake wind storm in the winter during a dance party.

This means that the total capacity of your connection points (if load and center of mass is distributed evenly) should be have a 5X safety factor. This means the capacity of your connections is 5 times greater than the total weight of the structure + the total weight of the people in the structure. Many of the tree bolts come with ratings. Since your installing this structure in a tree it is best to over engineer. My rule of thumb when rigging a structure with cabling is to base the capability of the structure on 3 rigging points although there are always more than 3 points distributing the load. This is to ensure that in a windy scenario should the structure swing onto only 3 points it is capable.

Wooooooood! Streeength!

Here are a few websites that will guide your engineering process if you will be performing any load simulations. Select your wood type and discover its properties so you can discover accurate results in your simulation.

The best one is www.matweb.com.

Enter the type of wood in the Text Search option in the middle of the page - try Oak and you'll see a bunch of options. Here are two links to a couple of the Oaks for example.. but generally pretty good cause they list for different moisture content, grain direction, etc.

http://www.matweb.com/search/DataSheet.aspx?MatGUI...

http://www.matweb.com/search/DataSheet.aspx?MatGUI...

Here's another one. A good long list of species, though not quite as much data on each one as MatWeb.

http://www.csudh.edu/oliver/chemdata/woods.htm