Woodsense: Spotlight on Outdoor Projects

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This article is from Issue 17 of Woodcraft Magazine.

When it comes to outdoor projects, not all woods are created equal. Some resist decay and weathering better than others − and the right finish always helps.

By Udo Schmidt

CHOOSING WOOD FOR OUTDOOR PROJECTS 

The choice of wood you make for outdoor projects can mean years of enjoyment or potential disappointment. Fortunately, there are several species in North America very suitable for outdoor projects. Finding the right wood depends on the woodworker’s location and pocketbook. Some species are readily available in lumberyards – even in home improvement warehouses – while others are not available in commercial trade, but can be obtained from local sawmills. 

When buying wood that will be exposed to the elements, look for wood that is decay resistant, insect resistant and weather resistant. Approximately 10% of North American lumber production is used to replace damaged wood on structures or other element-exposed wooden objects. Regardless of the species, the sapwood part of any wood has no natural resistance to decay. The first thing to do is to understand the difference between heartwood and sapwood (see sidebar on opposite page).

Another important aspect to consider is the growth ring arrangement. A good example is western red cedar. If the growth rings are more than 1/8" apart, this wood is considered only moderately resistant to decay. With decay-resistant species, the closer the growth rings, the more decay-resistant the wood.

The main difference in wooden indoor or outdoor projects is the moisture content to which the wood will equalize. Indoors, wood’s moisture content fluctuates between 6% in winter and 12% in summer. However, outdoors wood seldom goes below a moisture content of 12%, except in dry desert conditions. It can also absorb moisture to over 20% (up to the species’ fiber saturation point) during a period of humid or wet conditions. In other words, moisture content usually fluctuates more in outdoor wood than indoor wood. This means that outdoor wood swells and shrinks in greater dimensions than wood used indoors. Using kiln-dried wood for outdoor projects is not necessary; however, lumber in home improvement stores is sometimes sold as kiln-dried. Most likely this wood is still well above its fiber saturation point and should be air-dried before using.

Regardless of the wood you choose, there are a few points to consider if you want to eliminate or retard decay.  

  • Apply several coats of finish to the end grain of the wood before assembling the project. The end grain is more susceptible to moisture absorption and will check and split long before the other parts of the wood. If this is not possible, apply several extra coats of finish on all exposed end grain after assembly. 
  • Keep your project out of direct sunlight and weather. This dramatically reduces maintenance. For instance, an Adirondack chair located under a covered porch might need only a coat of finish every two years. But if the chair is exposed to the elements, it needs two coats a year to keep it looking new.
  • Don’t bury bare wood in the ground (i.e., fence posts). Instead, build a concrete foundation and paint the buried part in heavy roof tar or creosote. Concrete traps moisture around wood much longer because it cannot drain naturally.
  • Keep water and snow away from your project. Don’t let a chair sit in a puddle of water or let snow lay on play equipment.

Tight growth rings indicate decay resistance in this old-growth cedar.

Even though most of the species mentioned here are highly resistant to decay, that doesn’t mean that they are also resistant to insects. For instance, teak is resistant to termites, but not to the marine borer. See the sidebar below for finish and preservation information. If you anticipate insect damage to your project, a product called Bug Juice can be added to the last coat of finish. A 1.66 oz. bottle is mixed to one gallon of paint, stains or oils. 

Weathering is the natural process of wood being exposed to sun and weather. UV rays create a chemical change in the wood’s lignin and carbohydrates. Then the heartwood’s extractives, which give wood its color and other characteristics, start to degrade. Weathered wood has a driftwood-gray patina. Sometimes, teak lawn and garden furniture are left to weather naturally.  

Highly decay-resistant wood can be classified into three categories: native hardwoods, native softwoods and exotics. Highly decay resistant hardwoods are not just scientifically classified as hardwoods, they are hard woods. These species include black locust, osage orange and red mulberry. They are very hard to work with, but are a good choice for projects that have ground contact (i.e., fence posts or play equipment). Osage orange is one of the densest of the native species. With a specific gravity of .85 at 12% moisture content, it weighs over 4½ lb per board foot. 

Native softwoods most often used for outdoor projects include cedar, redwood and cypress. Except for redwood, these species are readily available and reasonably priced. They are all easy to work with and have low shrinkage properties. Western red cedar is the wood most frequently used for outside applications like house siding and decking. Of the exotics, teak is the typical choice for outdoor furniture and marine products. Among the highly decay-resistant species, it is also the most expensive. 

Teak has many unique characteristics which make it the preferred choice for projects exposed to the elements. With a specific gravity of about .5 to .6, it is as dense as native oak and has similar workability. Teak has one of the lowest shrinkage properties of all wood species, which makes it extremely stable. Its natural oil also makes it a preferred choice for an unfinished, weathered-look project. Unfortunately, depending on its growth location, teak can have a very high silica content. Silica has a dulling effect on all cutting and machining tools. 

By choosing the right wood, and with the proper finish and occasional care, your outdoor project should bring you years of enjoyment.

—Born and educated in Germany, Udo Schmidt came to the United States in 1979. After 12 years in the lumber export industry, he started his own cabinet shop. Schmidt has written numerous magazine articles and is the author of “Building Kitchen Cabinets” (Taunton Press, 2003). 

The reddish heartwood of eastern red cedar is very decay-resistant.

Heartwood & sapwood: the difference

In a living tree, the heartwood is for support and doesn’t conduct water or store food. In contrast, sapwood contains both living and dead cells and transports water and nutrients from the roots to the leaves. The thickness of the sapwood varies greatly among species, from only ½“ in black locust to up to 6“ in hickories and maples. After several years of growth, the cells become inactive and increase in extractive content like resins, gums, waxes, oils, polyphenols and tannins and become heartwood. Heartwood is usually darker in color than sapwood because of its higher level of extractives, which also makes wood from the heartwood of some species highly resistant to decay.  

Even though stains and mildew are not the same fungi as decay organisms, they indicate that conditions are right for decay to begin.

STAINING

Stains in wood are caused by minute parasitic organisms that need water, warm temperatures and oxygen to grow. Fungi feed on sugar in the sapwood of logs and lumber with a moisture content of more than 22% or when the relative humidity is more than 92%. The affected wood is then sap stained. 

Blue stain is the most common type of sap stain. The discoloration varies in color depending on the infecting organism, wood species and moisture situation. Blue stain shows as bluish to bluish-black or gray to brown. The affected areas can be spotty or streaky. In severe cases, the entire sapwood is discolored. 

Although sap stains seldom change the strength properties of wood, heavy sap stains reduce the toughness of the wood and its ability to withstand shock. Stained areas also have a higher water absorption capacity and therefore are more susceptible to other organisms such as decay fungi. Even though decay fungi are not a stain, they live and thrive under the same conditions as mold and sap stain. 

Organisms like fungi need food, water, oxygen and warm temperatures to live and grow. Temperatures of 75° to 85° are optimal. However, fungal growth slows to half the optimal rate at temperatures of 32° to 50° and the organisms die at temperatures over 130°. 

Another characteristic of fungi is that they can go dormant for long periods of time. Only high temperatures and chemical treatments kill them. Any fungal stain is permanent and cannot be removed or lightened by chemicals like bleach or oxalic acid.  

These cypress shutters were finished with Cabot’s Australian Timber Oil.

FINISHING FOR OUTDOORS

Choosing the right finish for an outdoor project can be more important than choosing the wood. Finishes for the outdoors are sometimes called preservatives; they are specially formulated and usually contain UV blockers. Siding or decking stains would be one choice for a project. Types of decking stains range from natural, to semi-transparent, to solid. As with indoor stains, the color variations seem endless. 

Marine Spar Varnish is based on an oil like tung or linseed. Other ingredients include resins, solvents, driers and UV blockers. This finish is both penetrating and film building. As the name implies, it is primarily used in the marine industry.

Tung oil is probably the most recommended finish for fine outdoor wood projects. Some tung oils are almost pure and some have driers, solvents and UV blockers added. Carefully read the labels and pick the right oil for your project.

If you would like your project to weather to a driftwood patina, maintain the surface with light coats of boiled linseed oil. This helps protect the wood, but allows it to age.

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