Pressure-treated wood is designed for outdoor applications. It protects wood against wind, sun, debris and moisture. Treated wood lasts a lifetime when properly maintained. But treated wood is also associated with health issues. If you'd rather not deal with it at all, there are options to protect exterior wood.
Keep it Dry
Decay and rot are directly related to fungus, and these microorganisms require moisture to survive. Deprive fungus of the moisture it needs to survive, and it helps to prevent decay.
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Keep it Shaded
Sunlight can rob wood of natural oils, and cause bleaching and degradation. Natural chemicals, such as lignin, become depleted in wood. Take precautions to keep wood out of direct sunlight whenever possible.
Fungus needs a reliable source of oxygen, moisture and temperatures between 32 and 90 degrees to multiply and thrive. When these conditions are met, fungus turns on the wood as food source, causing rot and decay. Control the temperature to control decay.
It's difficult to keep exterior wood dry, out of the sun, and for the most part, impossible to control the temperature, but you can bring wooden furniture inside when it's not in use. Wet springtime or fall weather does the most damage to wood.
The durability of untreated lumber is directly related to species. Even the best sealants won't protect certain woods as well as others. Of the commonly used woods, teak, cedar, redwood, cypress, white oak and fir are some of the toughest, most weather-resistant species to look for. Alder, pine, hemlock and poplar are among the weakest and least weather-resistant.
Redwood, Pine and Cedar
Untreated redwood, depending on it's age, has a projected life span of 50 years or more when exposed to the elements. Pine varieties have a projected life span of only 5 to 10 years. Cedar has a life-span of between 15 and 20 years, and is priced mid-range, making it a good alternative to pine or fir, with better weathering qualities, at a fraction the cost of redwood.
Even though natural weather-resistant wood is the best choice for outdoor exposure, at some point it becomes vulnerable to decay. The only way to properly use untreated wood of any type outside is with the addition of water-repellent preservatives, sealer or paint that contain UV protection. Over-the-counter wood preservatives are available in clear versions, or with stain containing pigment or dye to color the wood.
The Borate Option
If you're dead-set against pressure-treated wood because it possibly contains arsenic, there is another option. Borate-pressure-treated wood can even be used for interior applications. Borates are naturally occurring minerals in rocks, water and living organisms. Wood treated with borate is considered one of the safest alternatives to standard pressure treatments.
You can apply borate to wood yourself. It's a thick liquid, that when mixed with hot water, can be sprayed through an ordinary hand/tank sprayer.
Take precautions to cover plants and wear gloves, and breathing and eye protection is also recommended when working with borate. Even though borate will preserve wood, it can leach out if the wood if it is subjected to sustained rainfall.