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Tree damage caused by treehouse building

If a treehouse is designed poorly it is easy to cause damage to the tree, but by following some simple guidelines you can minimise damage as much as possible. It is impossible to cause no damage at all, but trees have evolved several techniques to tolerate damage and remain healthy. As trees are living organisms, they differ from familiar building materials in the following four ways.

Infections

Airborne or insect-borne bacteria and fungi can infect a tree, causing localised rot and death and in some cases gradual or sudden death of the tree, eg Dutch elm disease and sudden oak death. The bark of the tree helps to protect the living layers underneath from exposure to these attacks. If the bark is damaged, the living tissue is exposed which increases the chances of infection. For treehouse building, this means reducing the damage to the bark to an absolute minimum.

Cutting the trunk or branches

Never cut pieces out of the tree to allow supports to fit better as this exposes a lot of living tissue. See the non-flat surfaces page for alternatives. If you need to remove branches to make room for the treehouse, see the tree pruning page.

Nails and screws

These should not be used to fix supports as you can only achieve safe support with several fixtures, thus introducing lots of holes into the surface of the tree. Use nails and screws for the flooring, framing and panelling only.

Bolts

The general recommendation for fixing supports cheaply is to fit a single, large lag bolt into a cleanly drilled pilot hole. This reduces the number of puncture points to one, and eliminates dangerous problems of compartmentalisation (see below).

Slings, ropes and cable

Anything tied around the trunk or branches will damage a large area of bark as the tree moves, and if tied around the whole branch will gradually strangle the branch. You may have seen fence wire that was wrapped around the tree many years ago and has since been grown over completely. Trees can sometimes grow over obstacles, but it is much better to completely avoid the risk of killing them.

A lot of people use ropes to fix treehouses so that they "don't hurt the tree", but the damage the ropes actually do is much greater in the long run. Ropes should only ever be considered for very lightweight treehouses that will be taken down each year to relieve the pressure on the bark. Using wide webbing straps or slings will help to spread the load over a greater area.


Growth over time

As the years pass, the tree adds new growth layers (or rings) outside the previous ones, gradually getting wider and wider. The inner growth layers do not grow once the season has passed, which is why growth rings in felled trees can indicate how well the tree grew in any year throughout its life. The growth happens beneath the bark layer and occurs fastest when the tree is young. After the tree matures, the age of which depends on the species, tree growth reduces.

Treehouses are usually bolted to the tree, so the bolt is fixed to the heartwood. Since growth is added to the outer layers, the tree will expand around the supports. In itself this does not cause a great deal of damage initially, but it will disfigure the tree quite badly. After many years, the tree can be damaged if a large beam or panel is blocking its growth. A treehouse built at home may be taken down after 10-15 years, leaving any evidence of the treehouse visible in the trunk, so it is wise to reduce the amount of restriction your supports place on the tree.

Fixed supports

Spacers can be used between the beams and the tree to allow it to grow outwards for longer before reaching the supports. These can be made of 2-3" sections of galvanised steel pipe. You will need thick bolts (eg 1") as the weight of the beam will now be pressing down at a distance from the tree surface. A better overall solution is the use of a custom artificial limb, such as Michael Garnier's Garnier Limb. These tree bolts have a special collar, designed to mimic a branch collar, which allows very high loads to be applied up to 5" from the tree. The design of the original bolts is availablehere, so you can have several bolts made locally for your project. Such bolts can cost around $50 a piece, so it is a more expensive system but is well tested for heavy loads and large commercially built treehouses. Michael Garnier provides further information.

Brackets

Metal brackets fixed to the tree will be overgrown in the same way as beams. Again this is mainly a problem of disfiguring the tree rather than harming it, but certain brackets may stop working or get distorted by the growth. Brackets made to allow the beam to slide will gradually either seize up or start to wear away the new tree growth as it expands against the beam. A spacer can be used in the same way as described above, or an artficial limb can be used as a mount for the bracket.

Floor, walls and roof

If the tree passes through the floor and/or roof and walls, growth of the tree over time will mean it expands inside the hole cut for it. As these holes usually surround the tree completely, strangulation of the branch or trunk can occur. This must be avoided by keeping a 2" gap between the tree and edge of the wall, roof or floor. If movement of the tree is likely, even more space should be left so that a swaying branch or trunk will not be rubbed away as it hits the edge of the hole.


Compartmentalisation (or compartmentalization)

When a tree is damaged it attempts to reduce the spread of disease and rot by isolating the damaged section. A barrier layer grows around the damage over several years and nutrients no longer flow to the area. This is called a compartment. As the tree cannot regrow damaged tissue as an animal does, it seals it off and continues to grow around the damaged area. If a single bolts is used, a compartment will be formed immediately around the bolt, as the hole itself is a damaged area. If the compartment forms correctly, the tree will once again be safe from infection at that point and the bolt will be solid.

Certain situations will form an undesirable and potentially dangerous compartment. Fitting bolts, nails or screws close together in the tree is the most common problem. The tree may treat each hole as a separate injury and set up separate compartments, but it may treat the whole area as damaged and form one large compartment so that after a few years the area to which the support is fixed will die and lose a lot of its strength.

It isn't possible to tell where a compartment is forming from the outside of a tree, so it is very important to avoid risking this happening in the first place. The safe separation distance has not been scientifically determined, and will vary with tree species, but a rule of thumb is to use lag bolts at least 12" apart vertically and 12" apart horizontally. Usually this means each point of support will have one bolt only - depending on the weight of the treehouse 1" bolts or Garnier Limbs will need to be used to provide safe levels of strength.


Changed weight distribution

Trees are anchored in place with their root system. Just as a tree may not grow symmetrically above the ground, the roots can grow more in one direction than the other. A tree leaning over in the south direction can improve its chances of surviving by growing more roots on the north side to help balance the weight. Adding a large treehouse to a tree can affect its weight distribution, especially if the treehouse leans out to one side of the tree. The tree will gradually react to this, but it will take a few years to build up extra strength in the root system, during which time the tree can be more vulnerable to storm damage.

Splitting the treehouse over two or more trees will keep the applied weight acting straight down through the trunk. Building a circular treehouse around a tree will do the same thing. It is easy for a treehouse to get over 1000kg (2200lb) which is a lot of extra weight for the tree to take. If your treehouse will be this heavy, try to keep the centre of gravity over the trunk, or split it out over several trees.