Fastening a tree house to a tree with nails is the obvious approach, offering speed and strength. However, if the tree does not belong to you or you aim to build only a temporary structure, you may wish to avoid using nails. There are several ways of approaching the task. Whichever method you use, it is essential to make the fastening just as strong as if you were using nails.
Bolt the tree house to the tree if you do not wish to use nails. Use galvanised bolts to avoid rusting. Select heavy duty bolts where the tree house is of heavy construction. Alternatively, screw the tree house to the tree.
Lash the tree house to the tree with ropes if you do not wish to puncture the tree with bolts or screws. Drill holes in the main members of the tree house structure. Thread strong rope through them and tie robust stopper knots, such as man rope knots. Tie the other ends of the knots to strong branches. Aim to have at least half a dozen tie-points. The tree house may sway slightly in the breeze when lashed rather than nailed, screwed or bolted down.
Build the tree house on a secure platform if you do not wish to use any fastenings at all on the tree. Use robust timber -- such as the trunks of other trees -- to create supporting legs. Butt the inner faces of the tree house structure up against the trunk so it enjoys structural support. The swaying motion sometimes associated with some rope-held tree houses is thereby dampened.
Build a mid-trunk supporting ledge into the tree using timber stanchions if you do not wish it to be a platform-based construction. This will necessitate puncturing the trunk to provide anchor points. However, you will not need to use nails. Ensure you chisel out the anchor points to about half the thickness of the trunk, to provide adequate structural integrity.
Cover the tree house with a good strong roof, to ensure it is watertight.
Wear a safety harness when climbing trees and observe the further advice of The Health and Safety Executive. (See link in Resources).
Wear a dust mask and goggles when operating power tools.
Pruning, lopping or otherwise damaging a tree protected by a Tree Preservation Order (TPO) is illegal.