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Glossary of treehouse terms

Arborist

Professional with working knowledge and training in the health of trees. An arborist usually works in situations where trees are removed or pruned, in which case they may also be known as a tree surgeon.

Beam

Horizontal support which forms the base of the support system. Although both terms are sometimes interchanged, beams usually differ from joists in that they carry loads at multiple points along their length. Typical treehouse beams are made from 2x6, 2x8 and 2x10 lumber. For larger loads the beams can be doubled, eg two 2x10s placed together.

Box bracket

Metal bracket which provides a flexible joint. The support beam runs through a box section and is free to move back and forth within certain limits.

Brace

A diagonal frame member used to form a triangle in the support system, which reinforces the angle between components. Often used between the end of a cantilevered beam and a point lower down in the tree in single tree designs. Braces are also used between components when platforms are built on posts, as this improves the rigidity of the system.

Cambium

Living tissue beneath the bark of a tree. Typically forms a thin layer around the trunk or branch which expands during the growing season, forming new layers (or growth rings) around the tree. The seasonal difference in the appearance of these rings can be used to count the years that the tree has lived. Treehouse construction which cuts through the cambium can severely damage a tree.

Cantilever

Horizontal support which is supported at only one end. Purely cantilvered beams are not practical for treehouse construction due to the large leverage they produce at the supported end, but often joists will be allowed to cantilever a portion of their length over the underlying beams to provide extra floor area. Deep sectioned timber is better at sustaining the loads in a cantilevered component.

Circular saw

Electric saw with a circular blade used extensively in the building industry. Circular saws are very fast at cutting timber to length and with experience can be at least as accurate as using a handsaw. Popular brands include Skil (who produce the Skilsaw), Bosch and DeWalt. Battery-powered models can be used for thinner materials, such as plywood sheeting or shingles, but can be underpowered for cutting larger materials.

Compartmentalisation/compartmentalization

Protective mechanism in trees in which a damaged area is isolated in a 'compartment' to stop the spread of infections through the rest of the structure. This happens when holes are drilled into a tree to fit bolts for treehouses. If bolts are placed too close together, a number of holes can be isolated as one large area of damage which can lead to structural failure of the joint. This is why only one bolt should be used at each fixing point to the tree.

Cable

Steel cables can be used to support beams in situations where a brace is undesirable or impractical, or to allow a beam some movement as the trees move in the wind. They can also be used as a loosely fitted backup for a support in case the main joint fails. Cable should never be in direct contact with the bark as it will cause severe damage.

Deck washer

Heavy duty wide washer that can be used between a beam and the tree to help transmit the load more effectively. This can reduce bending the bolt within the beam or tree. Deck washers can also be used under the head of the bolt to spread the pressure from the head over a larger area of the beam.

Eye bolt

Heavy duty bolt with a loop at one end which is usually fully enclosed. Eye bolts may also have a shoulder which greatly increases the strength of angular loads. Steel cables are threaded through eye bolts fixed in the tree and the support beam. The forces on eye bolts act to try and pull the bolt out of the tree, whereas with bolts used for beams directly into the tree, the force is at 90° to the line of the bolt. For this reason, eye bolts are strongest when drilled completely through the trunk and fitted with a nut at the other side. The strength of an eye bolt is usually marked on the side of the eye. This strength guide is for loads along the line of the bolt, so they should be fitted in this arrangement wherever possible. Loads at 45° will reduce the working limit of the eye bolt to as little as 20% of the rated limit.

Fixed joint

Support fixing between a beam and the tree which does not allow movement. This is the simplest support joint and can be made with a lag bolt or through bolt. Nails and screws are sometimes used but can not support as much load as a bolt, and can cause much more tree damage.

Flexible joint

Bracket or cable support which allows the beam to move slightly as the trees move in the wind. Brackets are usually restricted to movement in the region of 2"-12"; anything larger can be supported by cable. This type of support is almost unique to treehouse building, because the house must remain level on a moving foundation. Flexibility is only required where more than one trunk or branch will be used to support the treehouse, and a flexible joint is only required at one end of the beam. If a beam between two trees does not have a flexible joint, large forces built up in strong wids can snap even large bolts.

Galvanisation/galvanization

Industrial process used to protect steel from rusting when used outside. A zinc coating is fixed to the bolt, nail or other fixing either by hot-dipping in molten zinc or by applying a thinner layer using electrolysis. Galvanisation produces a durable finish that does not easily chip. Unlike more expensive stainless steel options, it will still rust if the part is cut though, so bolts that are too long should not be trimmed. It is recommended that all external metal parts used in treehouse construction are galvanised. Internal metal parts such as screws and nails should also be protected but cheaper finishes can be used. Screws are not generally galvanised in the same way, as alternative coatings produce a smoother screw tht is easier to drive into the wood. Choose decking screws or stainless screws. Yellow/chipboard screws give poor water resistance and should only be used inside the treehouse.

Garnier Limb (GL)

Engineered tree bolt designed by Michael Garnier of the Out 'n' About Treesort for use in some of his treehouses. The GL was produced to comply with engineering regulations relating to accommodation used by paying guests. Since then, variation of the design have been used with great success around the world. The original design was not patented and can be freely adapted. Due to the cost of metal fabrication, GL use is generally limited to treehouse building companies who are able to order economical quantities. Further details of the design and fitting of GLs are covered by Out 'N' About Treesort.

Also known as a Treehouse Attachment Bolt (TAB).

Girth

Width of the tree at a certain height (usually chest height) above the ground. For a roughly circular tree, the girth can be calculated by dividing the circumference by pi (3.14). The width of the tree at points higher up is referred to as the diameter, as girth has a specific meaning in tree terms.

Heat preservation

Method of treating wood (normally softwoods) using high temperatures. These change the structure of the wood and make it much more resistant to insect and fungal attack.

Joist

Horizontal member used to fill in a framework in either a floor or roof. Common sizes for treehouse use are 2x4 and 2x6.

Joist hanger

Commercially available metal bracket which makes joist installation faster, so long as they are installed within a rigid framework. Although these are used for traditional house flooring, treehouse floor joists are subject to movement in the wind and lag bolts can be a stronger option than joist hangers. Screws should not be used to fix joist hangers as they do not have enough shear strength. Check the manufacturer's recommendations for the correct size nails to use.

J bracket

Style of metal bracket used for flexible joints. The 'J' shape allows the beam to slide but not escape the bracket.

Lag bolt/lag screw

Large bolt with a coarse thread, commonly used for fixed joints on supports into the tree. They have a hexagonal head which is tightened by a spanner or wrench. Also useful for fixing into the end-grain of joists and attaching framework to support posts. A pilot hole should always be drilled for lag bolts, the same size as the central core of the threaded section of the bolt.

Nailgun

Tool powered by compressed air, gas cartridge or electricity that drives nails or staples of any size into wood. Commonly used by professional treehouse builders due to the large time savings when installing flooring, cladding or roofing materials. The cost of equipment is usually prohibitive for amateur use where a portable drill used as a screwgun is a cheaper alternative.

Post

Generally refers to a vertical piece of timber used to support a treehouse from the ground. These posts are usually 4x4 or larger embedded in concrete, but they can also be attached to steel brackets set in the concrete. This allows easy replacement in the future and protects the base of the post from dampness.

Screwgun

A portable drill used with a Philips or crosshead bit to drive screws. The cost of drills has fallen so low that they have overtaken the hammer as the preferred method of fixing lightweight materials. Professional screwguns have an automatically fed strip of screws for easier and quicker operation. Screws can be fitted in a matter of seconds—sometimes faster than nails. Screws have several advantages over using nails in construction.

Shingle

Roofing material made from western red cedar, chosen for its natural weather resistance and straight grain. Shingles are made by sawing the log into tapering slices which are then laid on a roof in overlapping rows. Shingles are a popular external finish on treehouses, and often the walls are also shingled. Installation must be carried out correctly to ensure a waterproof finish, using a combination of the correct overlap and roof pitch.

Span

Distance between two points of support, usually in relation to horizontal beams. The distance timbers can span is related to their depth and, to a lesser degree, their width. Large spans need extra support from braces, cables or posts to prevent sagging.

Timber frame

1. General term for the structural framework of the treehouse before any walls or roof are added.
2. A building discipline based on heavier timbers and joints made entirely from wooden parts.

Turnbuckle

Steel accessory installed as part of a cable support to allow the cable to be tightened after fitting. Usually consists of two eye bolts with opposing threads which are threaded into an elongated frame. Once in position, rotating the frame tightens or loosens the cable. This level of adjustment is important for setting an accurate level beam and for adjustments in the future when the cable and tree have settled.

Treehouse Attachment Bolt (TAB)

Any of a range of engineered bolts designed specifically for supporting high loads in living trees. The original example is known as the Garnier Limb. A design blueprint based on the GL is available here. Usual features of these bolts include:

Wood preservative

Chemical solution applied to wood to allow increased resistance to rot and insect attack. Preservative consists of a fungicide and an insecticide. Pressure treated wood has had preservative forced deep into the fibres under pressure. Treating wood yourself should be done after the wood has been cut to size. This allows the timbers to stand overnight for maximum absorption through the end grain. Certain woods such as cedar, oak and teak contain natural protective oils and don't require preservative.

Pressure treated wood used to contain arsenic compounds which are an effective preservative but can be transferred to skin on contact and in large quantities can be harmful to health. These chemicals have now been phased out from consumer supplies in the USA.

Wood can also be protected using heat treatment which involves no extra chemicals.

Zip line

A long steel cable stretched between two points with a pulley and harness device to allow the rider to travel safely from top to bottom. Also known as a death slide or flying fox.