Construction and Construction Methods

The amount of sun required varies by species and past experience is catalogued by the American Bamboo Society. It is important to know that some bamboo species are native understory plants and need to be established in partial to full shade. In general, though, growth rates are significantly faster in full sun.

Harvest Mature Poles

Bamboo reaches maturity three to six years after the shoot first emerges. At this point the fibers are strongest and there is less moisture in the culm. The beauty of planting your own bamboo is that you will know the age of your culms, and therefore when it is time to harvest. Unfortunately this is often not the case, but there are a few tell-tale signs to look for in order to determine whether or not a culm is ready to be harvested. In tropical climates, the oldest culms are the ones with the most lichen and mosses. Clean smooth poles should be avoided as they are probabl new shoots and are lacking the strong structura qualities needed for building. Some species (i.e Genus Phyllostachys) have a characteristic white wa>, ring below the nodes that will gradually darken over time. Once it is no longer "white," the pole is in its third year. Other species acquire color slowly over time for example Phyllostachys nigra and PhyllostachyE nigra "bory" acquire their dark spots over several years. Also in the Phyllostachys genus, plants grow another sub-branch on the same lateral branch each year - therefore the age of the culm can be found by counting the branchlets.

Harvest When the Sugar Content is Lowest

It is best to harvest bamboo when the starch content of the plant is lower and therefore less susceptible to attacks by insects (though some argue that this is due to seasonal changes in insect populations and not the starch content of the plant). There is rich regional folklore and traditions that describe the optimal bamboo harvesting day and time. In Colombia, for example, it is widely believed that bamboo needs to be felled during the waning moon, just before sunrise. In parts of India, bamboo that is cut during the bright, new moon is believed to be less susceptible to insect attacks. The important point is to harvest when sugar is at a minimum: after the spring and fall growth spurts, and following the rainy season can make a difference.

When felling the culms, cut at the base, "over the first node located above the ground", using as narrow a cutting tool as possible to reach between culmn. A battery-powered Sawzall, chain saw, machete, pointed handsaw or a hatchet are all effective. make sure that a bowl isn't formed by rain water and rot the rhizome. While bamboo is relatively resistant to pests, the reason for the concern about sugars is related to attacks by powder post beetles, dinoderus minutus. Termites are generally uninterested, and other insects see the bamboc more as a home than food, the beetles will drill their 1/8" diameter holes and eat until very little structurE remains. The damage might not be obvious since thE beetles tunnel back and forth within the fibers and only visibly emerge when they're done - at which point thE thin-walled poles can be crushed with your bare hand The advantage of choosing the thicker walled poles is that the edible portion is the pithy interior fiber, not thE structural exterior material. In this case, the pole wil retain its structural qualities after an insect attack.

Use Less-Toxic Treatment Methods

There are two main ways that treating bamboo extends the life of the pole: it makes it distasteful to insects anc it changes the pH and keep the moisture levels low to keep out fungus. When thinking about treatment methods, consider how framing lumber is treated in your area, and do the same. Since Douglas Fir is never used exposed to weather, then keep bamboo inside in order to achieve maximum longevity. In general, untreated bamboo poles have a life of 1-2 years when exposed to the elements, 3-5 years when sheltered from rain, and up to hundreds of years when used indoors. The current less toxic treatment salts remain water soluble, but the task of treatment is to get it into the interior fibers, so it can, in theory, be exposed to the rain for awhile... it's just not great practice. In particular, do not expose the interior of bamboo to the outdoors. If bamboo is split open to reveal the interior, the sugar will cause little black dots of mildew to appear as soon as the rain starts. Surprisingly, one of the most effective treatments combines the ingredients found in eyewash and laundry soap. Boric acid -- found in Visine and Dr. Pepper, and most inexpensively sold as a fertilizer or mouse poison -- has little ill effect on mammals. Combine in equal parts with borax (sold as Twenty Mule Team laundry detergent) is not good for plants, but at about the 5% concentration that won't dissolve further, it is effective against both fungi and powder-post beetles. In Bali, the vertical soak diffusion method is used where the borax mixture is poured into the pole which has had the nodes punched out, leaving only the last one. The mixture is left in the pole for nearly a week where it diffuses through the pithier interior cell walls.

Another treatment method is the Boucherie system: a form of pressure injection that must be done within 24 hours of harvest or the capillaries close. This one can be a challenge. Various experiments have been done using smoke, but with mixed reviews. Japanese companies have perfected the method but the specific details have not reached other countries. At the moment, there are good things to say about hydrogen peroxide (but it's relatively expensive) and various plant-based traditional treatments practiced locally and worthy of intense study.

Work with the Bamboo once shrinkage has taken place

It is possible to work with fresh, green bamboo if adjustable joinery (a system that can be tightened over time as the poles shrink) is used, but it is recommended to work with dry bamboo. After harvesting the bamboo, leave the culm in the grove - upright, propped up off the ground and with the branches and leaves still attached   -  for a few weeks. It will continue to photosynthesize and use up the remaining sugar in the culm until the plant runs out of water. The powder-post beetles are unlikely to enter the culm during this time because they enter through the cuts (i.e. where the branch is cut off).

Moisture content varies greatly between culms (as well as between sections of the same culm) and as the bamboo dries it will shrink about 6-10% in diameter, but almost not at all along the length. A general indicator is that once the green color is gone - which might be between 6 weeks and 6 months - it may be checked to see if it is in the desired 10-15% moisture content. Air-dry the bamboo by storing it in a covered area, out of the sun, preferably vertical for good air circulation and to keep it from becoming more curved months or so after harvest.

Plan on Splitting

When it comes to bamboo splitting, do not ask "whether," ask "when." The bamboo you are working with will split, the key is to be prepared for this and know what to do when it happens. Outside of a few species of guadua, the fiber structure of bamboo runs in one direction, the length of the pole. The only exception is at the node where the fiber turns and runs perpendicular to the exterior to fill in the middle of the pole. Unfortunately, when the pole dries, the shrinkage of about 6-10% is entirely across the grain and the node shrinks differentially from the rest of the pole, causing the pole to split. This is especially true when a heat source, like the sun, hits the pole on just one side. Fence builders in Japan have learned that completely punching out the nodes can often prevent splitting. The stress can also be relieved by making a pre-emptive cut on the bandsaw the length of the pole, then gluing it back together (just as with a curly piece of wood). Joinery that wraps all of the way around the pole can also contribute to holding the pole together. Using banding or fiber binding at crucial joints will keep the fibers together, even if it never splits. With hollow bamboos, using something as simple as a radiator hose clamp can hold the fibers together. Joints can also be wrapped with twine for a different aesthetic.

Choosing the right bamboo specie for the job is extremely important. Some of the solid bamboos are much less prone to splitting: Phyllostachys heteroclada 'solid stem,' Dendrocalamus strictus, Otatea acuminate 'aztecorum', and several of the Chusquea genus. Of the hollow bamboos, those of the Guadua genus are generally thicker walled and have more of a helical fiber pattern. The lower portion of the culms especially will accept a nail or screw without a pilot hole, a characteristic not found in any other bamboo.

Don't let the Bamboo touch Soil or Concrete

Regardless of climate, a main objective of bamboo design is to prevent the wicking of moisture on the ground up into the bamboo. Building codes generally require eight inches between wood and soil, and at least several inches between wood and exterior concrete. The other simultaneous need in a post is to prevent it from pulling out of the ground. A good joint puts a separator like metal between the bamboo and the foundation, while at the same time holds onto that metal vigorously. The simplest way to achieve this is to make a bridge between the cellulose (bamboo) and the damaging water (in the ground or concrete). For example, use a piece of steel rebar or angle iron cast into the footing and mortar it several nodes into the bamboo to prevent withdrawal,

Give the Building a "Good Hat and Boots"

This is undoubtedly the most important lesson and one that is often overlooked. The first choice is to use all bamboo inside the structures. This will keep the sun and rain from affecting your building. Even small rafter tails peeking out beyond the overhang are guaranteed to split and provide insect habitat in a couple of years. In the United States, millions of board feet of our favorite framing lumber - Douglas fir - are used, but almost none on the exterior where it would quickly rot. The first thing that happens is the sun and rain dissolve the protective wax, the surface bleaches to silver, the heat from the sun on only one side of the pole causes it to expand unevenly then split, and the fibers become brittle and break.

Achieve Shear Strength with Triangles in your Truss Design or with Shear Walls

Use bamboo in tension and compression, not bending (imagine the flexing of a pole). It is important to recognize that bamboo wants to bend, but it is put to best use in pure tension or compression. Structural design principles for bamboo structures are the same as any other building system: for longer spans, design trusses. This will keep the poles from acting in bending. When designing, do not ask for the same curve in multiple poles. Tight curves especially will be more of a fight than you want. Recognize the ever-present curvature. Most bamboos naturally curve in two dimensions not three, in a direction that is related to the branches. Poles can be turned until the flat is found in order to put multiple poles in the same plane.

Build With Triangles

Designing a building with a series of triangles will provide stability by making the entire structure rigid. No shear walls will be required to carry the horizontal forces down to the ground. On the other hand, to rely on the strength of the joinery to this degree means that joinery should be optimized as much as possible. Additionally, longer spans have two or three bamboos stacked in the structural equivalent of a glu-laminated wood beam.

Use bolted and filled joinery systems

Here is where planning ahead helps the most. If you plan a structure that is quick to assemble and relies  on simple joinery that is hundreds of times stronger than traditional lashed or pinned joints, then a relatively small investment in hardware makes perfect sense.

In western cultures, we think of our favorite building materials, especially wood, as subtractive. We start with a large piece and keep making it smaller until it is right. The surface is usually made flat and of standardized shape. With bamboo, the thinking is more akin to masonry, with the shape predetermined and not easily changed. The key is to celebrate the difference and use bamboo where the shape is an asset and very visible in the finished surfaces. Design the structure in order to avoid time-consuming joinery, especially where materials must be coped and curved in order to meet the bamboo. This will allow more effort to be put into smart design rather than into laborious repetition.

Conceived by Simon Velez and Marcelo Villegas, the mortar-filled joint is a recent development in the history of bamboo buildings, and one that has made the largest impact.

The transcript of paper presented by Darrel DeBoer and Megan Groth during the VIII World Bamboo Congress held in Bangkok from 16th to 19th September 2009. The paper highlights some of the essentials of bamboo construction. It is reproduced here for the benefit of our readers.