There are two main environmental factors to consider when choosing a lumber species for your deck.
Firstly if you are seeking a softwood species, it will generally require treatment with a chemical preservative to provide the durability required for a long a service life. From time to time concerns have been expressed about the potentially harmful nature of the products used, mainly the potential danger of the chemicals leaching out from the wood. Admittedly this is grey area with conflicting “evidence” being presented to back up claims one way or the other. One of the most common chemical preservatives used in the past has been CCA (copper-chrome–arsenic) and although such products have been used for decades, some countries or regions have now taken steps to ban the use of CCA, at least in public spaces.
In recent years a number of alternative wood preservation products have come on the market which are claimed to have little or no potential harmful consequences. However whether these products will provide the same long term protection of the timber as CCA, remains to be proven.
But the main environmental factor to be considered to is that of the destruction of forests and ensuring the any wood is sourced from forests which are managed responsibly on long term sustainable basis without affecting the surrounding environment and complying with all legal requirements of the country of origin. For all too long, tropical (and other) forests have been decimated to make quick, short term profits with devastating long term consequences. Uncontrolled or illegal logging of forests in the Amazon basin and in Sth East Asia have been of particular concern. Gradually however this situation is changing, although illegal logging is still rampant in many third world countries in particular.
In an attempt to ensure the responsible harvesting and sustainable forestry practices yet assure the ongoing viability of existing forest industries, various international organizations have been set up, the best known of which is probably the Forest Stewardship Council (FSC). This non profit organization aims to ensure responsible and sustainable management of forests and issues certificates of compliance for lumber or wood products that are harvested or produced from such forests. End products can also be certified by the FSC providing the entire chain from lumber harvesting through processing and production of finished products complies with the regulations and requirements of the FSC.
In recent years an increasing number of countries have set up their own standards and certification procedures. However in many cases the standards and procedures are nowhere near as strict as those of the FSC and to a large extent seem designed to give some quasi government backed assurance that wood is being harvested responsibly and sustainably. For list of forests and companies that are currently certified by the FSC see the web site http://www.fsc.org . Alternatively further information can be obtained from the organization SmartWood http://www.rainforest-alliance.org/ which is an independent non profit forestry certifier accredited by the FSC.
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