Naturally Durable Wood Species

Many people have the mistaken belief that naturally durable wood species will not rot.   There are a handful of domestic wood species that are classified as naturally durable.   Some of the more common in the US South are sassafras, live oak, Eastern red cedar, catalpa and black locust.   In the West, redwood and Western red cedar offer natural decay resistance, particularly with heartwood lumber.  The heartwood is the darker wood in the center of most trees.  It is higher in chemical compounds that are responsible for decay resistance.  Some species have a heartwood that easily visually detectable and others do not.  

I purchased a swing set made of redwood about 15 years ago.  Over the years, I have replaced many of the parts of the swing set with pressure-treated southern pine.  My replacement pieces continue to perform fine.  However, each year more pieces of the redwood continue to fail due to wood degrading fungi.  Naturally durable wood species are NOT resistant to wood degrading insects and fungi.  They will provide better protection than untreated wood.  The US South has a very harsh environment for exterior wood in ground contact.  Untreated pine field stakes can show failure in as soon as 1 to 2 years.  My redwood deck began showing failure in 5 years for the pieces in ground contact and 7-8 for those above ground. 

There is no substitute for pressure treated wood for exterior applications.  This material is economical, safe, easy to work with and it will perform in excess of 50 years with minimal maintenance.  In the US South there are numerous utility poles that are well in excess of 50 to 80 years of age that are showing no signs of degradation.   For more information on naturally durable wood species please visit

This piece has suffered from internal wood degrading fungi due to its horizontal position in the swingset which allowed water to pool on the surface and elevate the moisture content to make it conducive for fungi attack.
This is a good example of surface weathering. The natural elements have degraded the surface of the board.

Fastener Corrosion

Fasteners consist of nails, screws, bolts, staples, etc and are widely used in the construction industry to connect lumber, sheathing, shingles, and other wood-based products.  Nail guns allow for nails to inserted at a rapid fire pace.  Chromated copper arsenate (CCA) was the primary preservative used for residential uses. 

CCA has been used for timber treatment since the mid-1930s. It is a mix of chromiumcopper and arsenic (as Copper(II) arsenate) formulated as oxides or salts, and is recognizable for the greenish tint it imparts to timber.

In February 2002, the U.S. Environmental Protection Agency (EPA) announced a voluntary phase-out by industry of most residential uses of this arsenic-based wood preservative. The agreement states that beginning in January 2004, CCA-treated wood can no longer be manufactured for decks and patios, picnic tables, playground equipment, walkways/boardwalks, landscaping timbers, or fencing.

Some of the water-borne alternatives to CCA for residential use include  alkaline copper quaternary (ACQ) compounds, copper azole (CuAz), copper citrate, inorganic boron (SBX), copper HDO (CuHDO) and others.  You will note that many of these biocides contain copper, which is an excellent fungicide and also a good conductor.  After the voluntary phase-out of CCA lumber for residential use a significant amount of treated lumber was produced that led to rapid corrosion of the fasteners.  A galvanic cell can occur and promote corrosion “rust” on the fastener.  Over time the integrity of the fastener can be compromised and the effective diameter of the fastener is decreased.  The solution is to always use galvanized fasteners and perform regular inspections of your fasteners for corrosion and your wood for early stages of decay.   Preservative treated lumber has been modified in recent years and the risk of fastener corrosion is now very minimal.  It is still advisable to use galvanized or stainless steel fasteners for any project that will be subject to moisture.  This is true for projects with untreated wood as well as treated wood.

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