Types of support | Flexible supports | Rigid framed supports | Fixtures and fastenings | Metal brackets | Cables
Knee braces | Dangerous things to avoid | Non-flat surfaces | Improving stability | Building without trees
Fixing to non-flat surfaces
For any tree surface that is not flat there are various methods which allow a piece of wood to be fixed at a better angle to the trunk. Anything fixed to the tree should be plumb (ie faces run vertically up and down) to make attaching joists, braces or other structural parts easy. Don't cut into the tree to provide a vertical surface as this opens the bark to infections and compartmentalisation.
Small variations (up to 5°) in the angle of beams and other supports will not significantly affect their strength and usually don't need to be corrected. Larger variations (up to 30°) can be corrected using either a block of wood under one edge of the support or by using a wedge which is shaped to fit against the support and the tree. A wedge will put less stress on the support than a block of wood, when the bolt is tightened up. If the tree is more than 30° from vertical, use a simple metal bracket or suspend the beam with steel cables.
For large angles, or where flexible joints are required to allow tree movement, a more complicated solution is needed. There are two main ways to do this; bolting the support to a custom made metal bracket or suspending it with cable or chain. Brackets are more solid, but are costly. Cables are very adaptable but need care to fit securely and can allow the treehouse to move more than expected. See the separate brackets and cables pages for a detailed comparison of these construction methods.