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
Improving stability in treehouse supports
One of the side effects of building in a tree, especially when using flexible supports, is that the treehouse may move around excessively. This can happen as a result of strong winds or movement of people inside the treehouse. In the first case the wind is moving the foundation underneath the platform, which drags it around in an oscillating motion. When movement comes from people inside the treehouse, the changes in inertia and weight distribution generate forces pushing the structure around.
The importance of using flexible supports
In a normal ground-based structure, the normal way to improve rigidity is to add more and more material, in particular diagonal members, which brace the frame by forming triangular shapes within the supports. This is very effective, but in a tree the situation is different, because the foundation is not fixed. Unless you build on a single trunk, you will need to span supports between different branches or trunks, and it is very important to allow these different parts of the tree to move independently. If they are fixed together immovably, a storm will build up massive forces that will break the bolts or beams themselves. One or more points will need to be fitted with flexible supports.
The problem this causes is that it amplifies any movement in the treehouse, because if the building slides or swings in one direction there is little to resist the movement. Solutions need to reduce the movement (or amount of oscillation it causes), while still allowing the trunks or branches to flex in the wind. The positions of the fixed and flexible parts of the supports becomes important here.
Natural flexibility of tall trees
Trees are naturally flexible so that they can bend in storms, rather than snapping. The type of tree influences how flexible the trunk and branches are, but as a general rule the higher the treehouse, the more the structure can move around with the tree. This happens even if supports are bolted directly to the trunk or branches.
Building across two or more trees sometimes improves this situation, as there will be more trunks to resist movement. However, this is usually not possible due to the spacing of trees at height. Diagonal braces can help to improve rigidity, but sometimes the only way to overcome the movement is to build lower down.
Lateral movement caused by flexible joints
The excess movement caused by flexible joints usually causes the treehouse to shift from side to side, as the supports move back and forth over the flexible joint. Movement can also be rotational if the flexible joint slides perpendicularly to the direction of the support beam. The main way to reduce these effects is to choose the largest trunk/branch to have the main fixed joint. This anchors the treehouse to the least moving part of the tree, so the other support points are able to move around under the treehouse, rather than moving the building itself.
Rotational movement around single trunks
A treehouse built around a single trunk can experience disconcerting rotation when people move inside, with the effect worsening the further out from the trunk you go. When viewed from above, the only attachment points are in the middle of the platform, which may be a circular or square shape. The central attachments are not resistant to rotation because the beams are able to move slightly against their bolts. This small rocking effect gets magnified at the outer edges of the platform, and can result in movement of several inches. There are a few ways to improve the situation, but some need to be applied in the early planning stages.
Bracing the floor system so that it acts as one solid component is the first step. This helps to counteract the natural flexibility of the wood which can add to the movement of the platform. A simple way to do this is to use plywood for the floor and screw it down with screws every 6" into the support beams. If you are using joists floating on top of radiating support beams you will need to fix the joists firmly to the beams using metal brackets or strapping. If you can still see some flexing in the beams themselves after this step, you can add plywood or diagonal bracing underneath the beams as well.
As most of the movement originates at the attachment points where beams meet the tree, this is the main area to improve. Very large bolts or special treehouse attachment bolts (TABs) are the best way to control movement at this point, as they can hold the beam more firmly, allowing less flex within the joint itself.
The wider the tree is to begin with, the better the support beams can resist rotation. This is because rotation of one beam can be more easily resisted by other bolts that are further away from it on the trunk of the tree. A narrow triangle is formed within the supports, and triangles are good at improving rigidity. A small tree will have a very narrow 'triangle effect' and therefore is less able to brace against rotation.
There can still be excessive movement even with all these measures in place. To remove this you need to provide some form of anchor to resist the motion. One way to do this is with a post down to the ground, fixed in concrete. The post is fixed to the outer end of one support beam and then a diagonal brace is fitted between the end of another beam and the base of the post. This produces a wide triangle that fixes the support beams. If you only use one post and brace then the platform can still move enough to take account of wind movement. If a post is not suitable because of height or looks, steel cables can be used to anchor the end of one support beam to other trees or points on the ground. This works like the guy ropes on a tent, fixing the support beam and therefore holding the whole platform in one position.