Unprotected woods in direct contact with soil will rot over time and attract pests like termites. Assuming you don?t want that behavior, here are several ways to protect yourself or at least the structure.
Pressure Treated Woods
The most common material one typically considers here is pressure treated woods. Preservatives are forced into the wood to create a chemical barrier from the soil, thus protecting the underlying wood. The actual preservatives used and their retention (lbs/cu ft) help determine effectiveness and where one can use the wood. The more preservatives retained in the wood, the longer term it could be used with direct contact to the soil. Think top decking wood vs. fence posts vs. foundation wood here for instance. While you don?t want to replace the top decking wood, you don?t need the same retention levels as a fence post which will be buried in the soil. And, foundation wood you never want to replace, if you can avoid it.
If you go the pressure treated wood route, which chemicals to use for the treatment can produce varying levels of success. As previously mentioned theretention of the chemicals determine their effectiveness. But what about the actual chemicals? For about ten years now Chromated Copper Arsenate (CCA) treated wood has been banned from residential use. For the longest time, it was the go-to chemical. If you?re dealing with older construction though, you may still run across it. The A there is for the arsenic in the treatment. Since that is a carcinogen, it is to be avoided. Sealing CCA-treated wood annually will keep the danger at bay but for new construction you?ll want to use a different treatment. Some options to consider today are Alkaline Copper Quaternary (ACQ), Copper Boron Azole (CBA), Cyproconazole, and Propiconazole, where the latter two are only for above ground applications.
Plastics and Composite Materials
Termites like wood not plastic. Polyethylene (HDPE or LDPE) plastic or PVC (polyvinyl chloride) survive well on the plastics side where material needs to come in contact with soil. On the composite materials side, you?ll find recycled and reclaimed materials like wood fiber (aka sawdust), rice hulls, and reclaimed plastics (like old grocery bags). Rubber timber, or old tires and plastic, can be considered here, too. Materials of this nature will survive like 10-50 years and are 100% insect resistant without the need for resealing every year. A common way to better preserve wood is to wrap it with plastic, where the wood might be in a cement footing and have a plastic coating wrapped around it. The only thing left touching the soil in the end is the plastic. Check with local building codes for what can be used structurally though. Typically, wood is a required element.
Naturally Durable Woods
You?ll find some wood species are naturally resistant to termites, or at least their heartwood is. The outer sapwood layers help conduct water to keep the tree alive but the inner heartwood of the tree can be used successfully where termite resistance is desired. In fact, some heartwoods are not just resistant to termites, but downright toxic. Consider construction-grade Alaska cedar, redwoods, or Tallowood, where the latter is a common Australian construction wood. Exotic hardwoods like Mahogany also work but are cost prohibitive and low in supply.
Metal Framing
Like plastics and composites, if you avoid the wood, you avoid the termites. Steel-framed structures are considered by many as an alternative to wood-framed homes. Steel however is only termite resistant, not termite proof. The feces from termites can corrode the steel. If they happen to get to the gypsum and eat the paper backing, what they leave behind could cause considerable damage. Also, while they might not eat the metal frame, you still have to worry about window and door frames. Use with care and make sure to spray the slab and protect any other areas that a termite could penetrate.
For structural needs, wood-soil contact is hard to avoid. Use an appropriately treated wood material with the retention level needed to best protect yourself from termite damage. If all else fails, you can avoid all risks of destruction by moving into a cave.
Unprotected woods in direct contact with soil will rot over time and attract pests like termites. Assuming you don?t want that behavior, here are several ways to protect yourself or at least the structure.
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1. Photo credit: cbb4104 / Foter / Creative Commons Attribution 2.0 Generic (CC BY 2.0)
2. Photo credit: US Fish and Wildlife Service - Recovery Act Team / Foter / Creative Commons Attribution 2.0 Generic (CC BY 2.0)
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