Fixtures and fastenings
To attach your treehouse to a tree, you have to bear two things in mind: trees are alive, continually growing outwards, and joints between treehouse and tree will be subject to a lot of stress from people inside and strong winds. Both these effects put a lot of pressure on the fixtures holding the support beams to the tree.
Nails and bolts
Although nails are cheap, you should buy galvanised bolts to fix your supports as they are much stronger and you will only need a small number. You can drill a hole right through the tree and fit a long bolt, or you can use lag bolts (sometimes called coach screws). These are like giant screws with hexagonal heads that tighten into a pre-drilled hole. Lag bolts are generally the best thing to use because the pre-drilled hole in the tree only has to be the length of the bolt less the beam thickness, not the entire diameter of the tree as is necessary with full bolts. This means the holes can be drilled using a normal drill bit. A socket wrench or spanner can be used to tighten the bolts, but a wrench usually gives better leverage and will not slip as easily as a spanner.
Pre-drill using a bit the same diameter as the internal part of the threaded section
- in this case about two thirds the diameter of the non-threaded section
To increase the strength of the wood to tree joint when using bolts, add a large washer between the beam and the tree. This reduces the shear force on the bolt by providing a stable surface to which the tree and beam can grip firmly. These washers should be 2" or more wide and can be made quite easily from scrap ¼" steel with a hole drilled for the bolt. They are sometimes sold as deck washers or simply large square washers. Washers, like bolts, should be galvanised for rust protection. A standard washer should be used under the head of the bolt to spread the load into the beam.
Read through the page on tree damage to learn how to minimise the damage you cause when the supports are fixed to the tree.
The 'rising treehouse' effect
Although trees grow in height, this does not mean the treehouse will get lifted higher and higher over the years. See the trees FAQ.