Wood is a popular material for use in homes and construction alike. It’s important to consider the type of wood you use. All wood is not the same and as a woodworker or designer, you need to know about different types of wood. One of the most versatile and durable types of wood used today is pressure-treated wood.
Pressure-treated wood is wood that undergoes chemical and heat treatment processes that change the molecular structure of the wood to make it more durable. Although pressure-treated wood is useful for multiple outdoor applications, we use it for indoor applications also. It is useful to know about pressure-treated wood to get the best from it.
Types of Pressure-Treated Wood
Broadly speaking, there are three types of pressure-treated wood – alkaline copper quaternary, borate, and non-combustible. All these types of pressure-treated wood differ greatly from regular lumber. Let us take a closer look at each category:
Borate
Borate pressure-treated wood is wood that we treat with a water-based mineral salt solution. The salt used for treating the wood retains the color of the wood while providing adequate protection against fungi, mildew, mold, and insects. However, this wood is not indefinitely resistant to moisture and excessive humidity or moisture can damage the wood.
Alkaline Copper Quaternary (ACQ)
We treat this type of pressure-treated wood with ammonium alkyl and copper salts. Although it’s safe to use, it should not come into direct contact with animal feed or human food. Another characteristic of this type of wood treatment is that it results in the color of the wood changing.
Non-combustible Wood
Non-combustible pressure-treated wood is the third type of wood that also undergoes a chemical treatment. The unique feature of this category the process involves treating the wood with fire-retardant chemicals. The wood becomes resistant to combustion.
So, as you can see, pressure-treated wood falls under different categories according to the chemicals we use to treat them. The chemicals may vary in their exact nature from one manufacturer to another but the system of pressure treatment generally remains the same.
In the early days, from the 1930s until 2004, the process of pressure treating lumber was to add arsenic, but nowadays manufacturers do not use this element anymore.
Production of Pressure-Treated Wood
For manufacturing pressure-treated wood we first place the lumber in a horizontal airtight steel cylinder called a retort. The pressure chamber resembles a rail car. We then fasten the lumber in the pressure and apply a vacuum.
Next, in the presence of this vacuum, we add chemicals under high pressure, typically 160 pounds per square inch (PSI). The pressurized chemicals on entering the chamber penetrate the wood.
We then remove the lumber drain and dry it on a drip pad. We then ship it out to the consumers. The drying time for the treated wood depends on climatic conditions and the amount of sunlight that it receives.
The Chemistry of Pressure-Treated Wood
The process for manufacturing pressure-treated wood has more or less remained the same over the years. However, the chemicals that we use to protect the wood have undergone a considerable change. But the principle remains the same – adding chemicals to wood to make it more durable.
In the early days, that is, before 2003, lumber was treated with chromated copper arsenic (CCA). This chemical featured in pressure wood manufacture since the 1930s. The copper would kill microorganisms and the arsenic would kill insects.
The two compounds, arsenic acid, and copper oxide respectively dissolved in an aqueous solution of chromic acid served to bind the lignin and cellulose in the wood. Today, CCA-treated lumber does not exist due to its toxic nature. It was banned from 2004 onwards.
Nowadays, we get two types of pressure-treated lumber, ground contact, and non-ground contact lumber.
Ground Contact Pressure Treated Lumber
You can place this type of lumber on the ground or even bury it. Copper is the primary insecticide and fungicide. It also contains other complex organic compounds to fight copper-resistant fungi. Some older forms of ground contact lumber contain boric acid, but the use of boric acid is not in vogue anymore.
Non-ground Pressure Treated Lumber
This form of pressure-treated lumber does not contain copper. It contains alternative chemicals which kill bacteria as well. The omission of copper aims at preventing the wood from discoloring, which occurs with copper-treated lumber.
But this category of pressure-treated wood is not very durable if it comes into direct contact with leaves, dirt, bugs, fungi, or moisture.
The Search for Better Pressure-Treated Wood
Historically, the types of chemicals for pressure treating wood have never remained constant. The search continues to find more effective, environmental-friendly and safer chemicals to treat the wood.
Although chemicals have changed, the process remains the same. However, on a much larger scale where the lumber runs into several tons placed in huge tanks with a high level of mass production.
The search continues for new and safer chemicals. For example, non-biocidal wood protection methods are emerging slowly. According to pressure-treating experts, thermal modification changes the wood perhaps more effectively than chemicals. The higher the heat, the more durability it imparts.
But the loss of strength of wood in such cases tends to be inversely proportional to the heat applied. Charcoal is a durable material that is practically impossible to rot but it lacks structural stability.
Today copper-based ground contact lumber serves as the most popular form of pressure-treated wood. In fact, a considerable quantity of non-ground contact pressure treated is gradually being phased out. But for now, copper is the element that rules the world of pressure-treated wood.
Wood Preservative Testing
A lot of the wood preservative testing occurs in the field rather than in simulated conditions in a test lab. For example, wood scientists travel to places with a high pest population. They bury treated and untreated wood samples in the ground to check for the efficacy of the chemicals used.
In the United States, Hawaii is a popular location for testing new chemicals on pressure-treated wood due to the high humidity and sub-tropical conditions. On the other hand, Australia, being arid is a prime location for testing the durability of pressure-treated wood against termites.
The timelines for testing these samples are long. Accidents will happen. For example, while testing for fungal decay, mass migrations of termites can appear and ruin the experiment.
Applications of Pressure-Treated Wood
Usually, pressure-treated wood is best used for outdoor applications. The obvious reasons for this are the toxic chemicals for treating this wood rendering it unsuitable for indoor applications. However, you will see many outdoor uses for pressure-treated wood. So, let’s take a look at some of the best uses of this durable form of wood:
Pressure Treated Wood for Decks
Pressure-treated wood makes an excellent deck. After painstakingly building a deck, and spending a lot of time and effort, you don’t want it to rot or fall prey to insects within the next couple of years. However, you need to select the type of pressure-treated wood for your deck carefully.
Different grades of pressure-treated wood exist and you might like to use a stronger variety of wood than required for making the foundation rather than you would use for the flooring and the rails.
Also, although your lumber might be pressure treated, sealing it with an oil-based stain will not only enhance its life but also reduce direct human contact with the harmful chemicals of the wood.
Pressure Treated Wood for Docks and Posts
For any application where the wood comes into direct contact with soil, pressure-treated wood is the ideal choice. One of the ideal examples is using for docks and posts. Particularly posts and foundational lumber intended for outdoors and driven into the ground are structures that where you need to use pressure-treated wood.
You may not need to use pressure-treated wood for all the parts of an outdoor structure. For example, you can use regular wood for railings. But you would still do well to make the foundation from pressure-treated wood.
In the case of a dock that will be in contact with water, pressure-treated lumber is the way to go. If you don’t use pressure-treated wood you will soon see your dock rot and deteriorate.
A point to note here is that pressure-treated wood holds well against rot and insects. But regular corrosion of lumber will still occur. Also, even pressure-treated wood will warp and split on drying. Therefore, you need to understand the limitations of this type of wood.
Pressure Treated Wood for Sheds
A shed in the garden is likely to contain all your gardening tools and implements. You don’t want your wooden shed to rot and allow rainwater to damage your tools. Making a shed from pressure-treated wood is a good decision. In the case of a semi-permanent shed, builders would possibly make the foundations of the shed using pressure-treated wood.
This sort of wooden foundation might not provide the solidity of concrete but it keeps insects and rot at bay. However, it is not a good idea to use pressure-treated wood for large, permanent structures.
When to Use Pressure-Treated Lumber
Although we use pressure treated wood for outdoor applications only, it is worthwhile to know the exact circumstances when pressure-treated wood is the best choice. The obvious reason why you should use pressure-treated wood is when your wooden surface will directly contact moisture.
You need to use this type of wood for instance in posts buried underground. It is also useful if the wood is going to be directly in contact with masonry or concrete. The reason here is that the porous concrete wicks water like a sponge.
Therefore, pressure-treated wood lasts longer than any other wood under such circumstances. Whenever moisture is likely to be in contact with wood it should be pressure-treated wood. In fact, the international building code calls for using pressure-treated wood for the last six inches above the ground siding.
You should also make sub-flooring in bathrooms and kitchens from pressure-treated wood because of the likelihood of water leakage from such rooms.
When NOT to Use Pressure Treated Wood
Wouldn’t it be advisable to use pressure-treated wood all over the house and keep your wood protected from moisture and insects? However, you shouldn’t do this because of the toxic chemicals involved.
Some of the older pressure-treated wood might contain arsenic while the newer materials contain copper which can also harm humans over a prolonged period. If you rub your eyes or eat food after touching such wood you could ingest some of these harmful chemicals.
It is better to use non-pressure-treated wood inside homes and pressure-treated wood for outdoor surfaces. Further, an area inhabited by pets and kids should also use non-pressure-treated wood. You could use durable wood like cedar which lasts longer but lacks the harmful chemicals of pressure-treated wood.
How to Use Pressure-Treated Wood
So much for where to use and where not to use pressure-treated wood. But let’s consider the best way to use this versatile material. Building structures from pressure treatment is a good way to make things last a long time. If you use pressure-treated wood to make a deck it will last many more years than if you used untreated wood.
You can even apply an oil-based sealant which will prevent the risks of harmful chemicals with pressure-treated wood. Evidence exists that oil-based sealants prevent arsenic, copper, and other harmful chemicals from reaching the surface of the wood.
If you follow these procedures, you can get the best out of pressure-treated wood in a safe manner. If you cover this wood by other materials like if you use it in sub-flooring, then you don’t have to worry. Further, unlike engineered wood, pressure-treated wood will not release gases and volatile organic substances (VOCs) that have a polluting effect on the surroundings.
When you work with pressure-treated wood, ensure to wear protective gear like gloves and breathing apparatus and you will do just fine.
The lifespan of Pressure-Treated Wood
Decks made of pressure-treated wood can last up to 10 years but poles can last as much as 40 years. As we mentioned above, there are three types of pressure-treated lumber and the other classification is “above-ground contact” and “ground contact” wood.
The lifespan of Above-ground Contact Pressure Treated Wood
You can use this type of pressure treated wood for framing, railings, deck boards, and fence slats. We use it for outdoor structures that do not touch the ground. The pressure treatment on this type of wood would be less severe than the wood that goes into the ground. This type of wood can last for 10 years or more so long as you take care of it by sealing and finishing the wood adequately.
The lifespan of Ground-contact Lumber
With ground-contact lumber, we pull out all the stops while adding chemicals. This wood is heavily dosed with powerful chemicals rendering it much more durable and long-lasting but significantly more toxic than above-ground lumber. Ground-contact pressure-treated wood can last up to 40 years.
Harmful Effects of Pressure-Treated Wood
Understandably, pressure-treated wood can be harmful if it comes into direct contact with people, children, or pets. It contains chemicals that kill bugs. Because of the high level of pesticides that this wood contains the wood makes it extremely toxic.
Pressure-treated wood is sure to kill or repel wood-gnawing insects like carpenter bees, carpenter ants, and termites, particularly the wood falls in the category of ground-contact lumber. But with the presence of children like on playground structures, you should take care where you use it.
However, if you use arsenic-free wood, then you can consider it safe for children to be in contact with it. Of course, while constructing playground structures, children shouldn’t be exposed to the sawdust that gets generated during such construction work.
Also, you cannot use pressure-treated wood for marine applications due to the hazard of harmful chemicals leaching into the water and harming the environment. If you use wood to make dock pilings, rot-resistant hardwood is a better choice. For example, greenheart is wood contains no chemicals. You can also use marine-grade plywood used for constructing boats.
Safe Handling of Pressure-Treated Wood
Today, pressure-treated wood is quite safe – definitely not as hazardous as it used to be. However, to be on the safe side you should wear protective gear like a face mask, a respirator, and gloves when you work on it.
Since 2004 pressure-treated wood doesn’t contain arsenic, although it still contains powerful chemicals. Therefore, wearing protective gear is a good idea. Further, the best place to work on pressure-treated wood is outdoors in a well-ventilated area.
Disposal of scrap material from pressure-treated wood projects should be done properly in designated dumps. Do not dispose of pressure-treated lumber by burning it as you might release some hazardous volatile chemicals into the atmosphere.
Conclusion
Pressure-treated wood is a great option when you need to make outdoor wooden structures that need added durability. But although pressure-treated wood resists insects and rot it will ultimately deteriorate over time like all other forms of natural wood.
Moreover, there are some risks associated with the use of pressure-treated wood. If you are unsure about the safety of this wood in certain applications you can use natural extra-durable natural wood like redwood, ironwood, Ipe, and cedar. Although these alternatives might cost more, you have a safe source of lumber.
So, if you want to guard your wood against insects and rot for outdoor projects pressure treated wood is a great choice. But the bottom line here is that you need to realize that even pressure-treated wood has its limitations.
If you plan a woodworking project that involves outdoor structures, consider using pressure-treated wood. We hope that this information helps you get the best out of pressure-treated wood in your future woodworking projects.
Happy Woodworking!
