What wood should you use for your outdoor projects?
Good question. And the answer to this question begins with other questions: what would you like the wood to do? Keep its color and appearance? Remain flat and straight? Maintain structural integrity for years and years?
|Note: While many softwoods (wood from conifers, such as western red cedar, southern yellow pine, etc.) work for outdoor projects, we are only discussing hardwoods here – wood that comes from deciduous, broad leaf-bearing trees.
Believe it or not, how you choose to finish the wood and how diligent you are with caring for the finish has more to do with the endurance of your project than the hardwood you choose.
When using hardwoods for exterior projects you have four items to consider:
- The final project’s exposure to weather and sunlight
- The wood’s resistance to decay
- The method of finishing, treating, or top coating the finished product
- Routine maintenance
These are the things that determine the longevity of your projects that reside outside. In general, any wood will work for an outdoor project if it’s kept dry and regularly maintained. However, some woods do have a higher resistance to decay than others, and a list of such woods is below. Regardless, using hardwoods outside comes with trade offs: the wood will change in color and the end product must undergo regular maintenance.
Weathering, Moisture, and Exposure to Sunlight
No wood will withstand weathering without chemical protection – left bare, hardwoods will turn gray in color, take on surface and end checking (cracking), and/or distort in shape (such as warping, twisting and crooking) as they absorb moisture and dry out. Different woods will react sooner than others in this department.
Sunlight is the demise of color and grain features in hardwoods that are left bare or not maintained regularly. To keep a wood looking like it did when you first finished the project, it’s imperative to use a UV blocking finish or top coat and keep it out of direct sunlight as much as possible. The general rule of thumb still remains: dark woods tend to get lighter with age, and light woods tend to get darker with age – regardless of the project being an interior or exterior one.
Increases in humidity cause the wood fibers to absorb the moisture in the air in an attempt to equalize with its environment. Wood is stable when it reaches the Equilibrium Moisture Content (EMC), but it takes time for wood to equalize. During this time, wood moves. It will swell (expand) along the rings of annual growth.
Worse, when the air dries out (or the local humidity decreases), the wood will again attempt to equalize. As water escapes from the wood fibers, the wood will shrink – this is what causes end- and surface-checking, twisting, cupping, and crooking. Of course, these defects are augmented with rapid moisture changes. But woodworkers should know how to plan their joinery, panels, and components to allow for fluctuations with climate changes so as to minimize problems with their projects.
Resistance to decay
Rule one: Keep your wood project dry, and it won’t decay.
Decay comes from fungus, and fungus needs certain conditions in order to live and cause damage. According to the Forest Products Society:
Eliminate any one of these and decay fungi cannot survive.
- An adequate supply of oxygen
- A favorable temperature (32° - 90°F)
- Moisture in excess of the fiber saturation point (> 25-30%)
- A suitable source of energy and nutrients (i.e. the wood)
For woodworkers making outdoor furniture (or homeowners using said furniture), the easiest condition to control is keeping the wood away from saturating moisture. An Adirondack chair, for instance, that’s left in the grass throughout Spring and Summer will probably soak in too much water at the legs, allowing fungus to grow and attack.
An afternoon rain storm isn’t going to pose a problem in respect to decay – after the rain has passed, the furniture will dry out. But remember the other problems, above, with wood as it dries.
You can improve your odds against fungus by using a hardwood that has a high resistance to decay; such as a dense or oily wood that soaks in water much slower than other woods.