We know that moisture is the enemy of wood, and have mentioned that numerous times in past articles. Moisture such that it would damage wood is not a problem inside your home absent a flood or serious water leak. But, moisture is certainly a danger to wood outside the home.
Outdoor furniture, fence posts, and deck supports immediately come to mind. Today we want to talk about wood in the ground. It will be exposed to moisture, for sure, and precautions need to be taken when installing fence posts, deck supports, or any other installation where wood comes in contact with the ground.
Untreated wood, wood that has neither been pressure-treated nor coated with some form of a wood preservative, will assuredly rot. In fact, it will begin rotting as soon as it is exposed to moisture.
Pressure-treated woods may be an option, just as coating wood with a sealant or wood preservative is another. Is there a best? Will it prevent rot forever? And if not, how much time will you have before rot sets in and the wood needs to be replaced?
Let’s dig in (pun intended) and see what we can learn.
In This Article
Untreated Wood In the Ground
Generally speaking, untreated wood in the ground or in contact with the ground will rot quickly. In a year or two, it will need to be replaced, whether it’s a fence post or deck wood that rests on the ground. You are wasting your money, time, and effort if you are using a pine, for instance, for your fence posts and apply no treatment.
But, there are some woods that have great staying power and are considered to be rot-resistant. These woods include:
Black locust. This was favored by farmers in the northeast to mark their lands and provide fencing to keep their animals inside their property lines. It is naturally rot-resistant and long-lasting in the ground. Today it’s common to see lines of black locust as hedgerows in New England.
Osage orange. In the midwest, farmers chose osage orange for those same purposes and reasons. The thorny tree was rot-resistant, also, and kept a good fence. Living stakes were used to create a living fence-hedge to keep animals contained on their property, and it proved an effective barrier.
In addition to these two rot-resistant, long-used kinds of wood for fencing, the list also includes:
- California redwood
- Bald cypress
These woods have the highest degree of natural protection from rot and are suitable for ground contact uses, including fencing.
These woods tend to be more expensive than others, though. Their appeal is their natural resistance to rot. But, there are ways to protect less expensive woods that will be put in the ground or in contact with it.
Heartwood will tend to resist rot much longer than sapwood. Heartwood, which is also called duramen, is the dead center of the tree. It forms as trees age and its inner cells lose the capacity to transmit water.
It’s generally darker than the sapwood that surrounds it, since it will also contain natural oils, dyes, and sugars. Heartwood is mechanically strong, providing adequate strength as a central column to support the tree, and resists decay.
Untreated heartwood will have a longer life expectancy in the ground than will sapwood from the same tree because of these characteristics. Still, though, it will tend to rot sooner than treated woods.
Wood Treatments to Prevent Rot
It’s not necessary to only use naturally rot-resistant wood for fence posts or wood that will come in contact with the ground. There are ways to treat wood to give it protection from moisture in the ground.
First, let’s understand what causes wood rot, and it is not old age. In a word, it’s fungus. Fungus needs moisture to thrive. It breaks down the fibers of wood and weakens its integrity. Eventually, the fungus overcomes the wood fibers, and rot sets in. The wood becomes weak and unstable, and eventually, that fence post will need to be replaced.
If you deny fungus the moisture it needs to survive, it can not thrive, and the wood remains safe from its destructive power over time. So, treatments that prevent moisture from entering the wood effectively deny fungus what it needs to grow and do its damage.
Some treatment methods can be done for you before you purchase the wood. A case in point is pressure-treated wood.
What is Pressure-Treated Wood and When Should It Be Used?
As the name implies, there is pressure involved in the treatment process. High pressure is used to inject a preservative into the wood. It extends the life expectancy of wood, whether it is in contact with the ground or simply exposed to the elements.
The preservative used in this pressure treatment is alkaline copper quat (ACQ), a water-based wood preservative. It takes a few days for the preservative to cure in the wood, and during this curing process, the wood can warp as the ACQ dries.
ACQ is a safe product, approved by the United States EPA as being nontoxic and safe for use in the garden. So, decks, fences, and raised beds are safe projects for ACQ pressure-treated woods.
Pressure-treated wood is not waterproof, though. In fact, ACQ is a water-based product. But, pressure-treating will still add decades of protection to the life-span of the wood, and many consider 25-30 years as a reasonable expectation. Pressure-treated wood in a concrete setting can last for upwards of 70 years even.
Because it is not waterproof, and if you are not setting those PT posts in concrete, it is a good idea to apply some sort of sealant to the surface and the bottom of the posts. While the ACQ treatment will resist rot and insect infestation, it does not prevent the wood from absorbing water over time.
Dipping your PT post in something like Thompson’s WaterSeal WaterProofing Stain so that all sides and the bottom are coated will add to the level of protection your post will enjoy. The combined effect of the pressure treatment and the sealing coat will extend the life of the post by 7-10 years. Follow the instructions on the Thompson’s product for the number of coats to apply and method of application to ensure a tight and adequate seal.
Copper naphthenate is another sealant commonly used to treat woods that will go into the ground. Copper naphthenate in oil is a heavy duty wood preservative, and it is applied to such things as utility poles, railroad ties, and bridge timbers, as well as fence posts, pilings, and wooden guard rails along roads.
It is effective in fighting fungi that will cause wood damage, as well as wood-boring insects. It has a low toxicity to humans and animals but is harmful when ingested or the skin has been exposed to it. As a result, gloves should be worn when applying it to your fence posts.
For PT woods that will rest on the ground, rather than be embedded in it, sealants are still a highly recommended step. Penetrating stains with tint to protect the wood from both moisture and UV rays are a wise treatment.
So, to recap the use of pressure-treated wood, we have these additional measures of protection:
Concrete or foam. Setting a fence post in concrete gives the wood full protection from moisture and insects. Either one surrounds the post and offers protection all around the post. Concrete is easy, even the post set type that goes in dry and draws moisture from the surrounding soil. Foam serves the same purpose and is easy to mix and pour into the hole surrounding the post.
Fences aren’t the only yard project that will call for posts. Perhaps you have steps that you want to add a handrail to. We found a neat video with a very cool suggestion for additional stability in addition to the concrete setting.
Sealants. Dipping a fence post in a sealant such as a Thompson’s product so that all surfaces, including the bottom, are coated, will provide an additional measure of protection to the wood.
Paint. Although not specifically as effective as a sealant designed for the purpose, painting your fence post can also give an additional level of protection for the wood. Just remember to paint all surfaces, side and bottom, for the maximum protection. At least 2 coats should be applied.
Installation. Don’t simply dig a hole and put the post in it. Dig deeper and wider than you need; fill the bottom of the hole with stone to facilitate drainage so the post won’t be sitting in water; after placing the post in the hole, surround it with gravel dust to provide both greater stability and a porous casing that will also facilitate drainage. If the post isn’t ever going to sit in water, there’s less opportunity for water to seep into any exposed area of wood.
Copper Naphthenate. Any wood preservative containing copper naphthenate is a good choice, too. Check the American Wood Protection Association for recommendations for preserving wood for ground contact whether in or on. Consider a product like Tenino Copper Naphthenate Wood Preservative. It’s readily available at local hardware stores and the big DIT stores, as well as online from the big retailers.
Some suggest wrapping a fence post in plastic (carpenter’s wrap) as another method to increase protection. While this might give a very short-term protection to the wood, the plastic wrap eventually will fail. We consider this to be a waste of money and effort. Stick with a concrete setting or a sealant as far more reliable and long-lasting.
You can choose to use an untreated wood for your outdoor projects where it will have contact with the ground whether in or atop. You can use a preservative or a sealant on it, and set it in concrete. But, the untreated wood will eventually succumb to moisture and insects, and have a relatively short lifespan. So, that choice to use untreated wood is also a choice to replace the post sooner than would otherwise be necessary.
Or you could choose to use pressure-treated wood and start with a wood that already has a great deal of protection, to begin with, and the added measures that can be taken with it can transform a lifespan of 25-30 years into possibly as many as 70 years. That makes it a much better choice.
The additional measures you take will be up to you after that. If done properly and well, a fence post in the ground might very well outlive you.