Choosing a tree
You may already have a tree in mind for your treehouse. But if not, or you're trying to decide which of a selection to use, this may help you confirm suitability.
The higher you go in a tree the greater the sense of freedom and the more amazing the views you get will be. However, you must also think practically in terms of safety in the case of a fall, effect of the wind and quality of support. Children's treehouses are usually more suited near the ground up to 3 metres (10 feet) to reduce the danger from a fall.
Trees can move a lot in the wind and adding a treehouse, which is essentially a sail, will only help to increase the wind catchment area. In particular, this effect will be greater in a storm. Usually trees can deal with excessive wind speed by losing parts of their structure in the order of leaves, small branches and large branches. As each of these is lost, the wind catchment area of a tree decreases, helping to reduce the pressure which may lead to the entire tree blowing over. A properly constructed treehouse may not disintegrate until wind speeds are great, so all the pressure acting on it is helping your tree get pushed over. Treehouses in high wind areas should be in the lower third of the tree, where wind speeds are lower and the leverage of the force on the tree is reduced. If the wind poses a serious danger, keep size to a minimum and try to build a more curving, or circular, house to reduce the sail effect.
The points where you fix supports will need to be strong enough to hold the weight of the part of the house they are supporting. It is simpler building with a few long supports than lots of smaller ones. This will require several attachment points (four is good) across different trees. Although branch strength varies between species of trees, these are some guidelines. Excellent trees are oak, beech, maple, fir and hemlock. For a one storey treehouse with no overhanging parts a minimum thickness for four attachment points (one at each corner) is about eight inches. If you have more than one storey and/or the extra leverage and weight of overhanging sections, then you may need twelve inches or more. If your branches don't allow for this, use more attachment points so the weight is spread out better.
Building between different trees or trunks
If you are planning to use two or more branches, trunks or trees, you must be careful when fitting supports. When there is strong wind, a tree will twist and sway with a lot of energy. This is especially obvious when building across two very long branches because they catch the wind easily and can swing around a lot. You must not seriously restrict this movement because it could destroy your house. Different trees and different branches will move differently, so the supports must be able to cope with any tension or compression. The options are having a strong rigid framework or a weaker flexible framework.
Before you start work on your treehouse, read through the tree damage page for the common treehouse related injuries and how to avoid them.