Natural lumber is an inexpensive building material that works great in protected areas, like inside the walls and floors of a house, but it does not hold up well in areas that are moist or prone to insect infestations. It will eventually rot, decaying slowly until it is no longer structurally sound.
The pressure-treating process was developed to combat wood’s natural weaknesses. Wood is placed into a sealed pressure chamber, and one of several types of chemical preservatives are injected deep into the grain of the wood, using alternating pressure and vacuum cycles. These chemicals protect the wood from rot, water damage, fungal decay and infestations of wood boring insects, such as termites or carpenter ants. Some formulations also add fire resistance.
The chemicals most commonly used to protect wood today include alkaline copper quaternary (ACQ), copper azole (CA), micronized copper quaternary (MCQ), and sodium borate (SBX). Previously, chromated copper arsenate (CCA) was also widely used, but it has now been restricted to only a handful of special applications, including docks, telephone poles and piers, due to health concerns.
Compared to natural wood, pressure treated lumber is much more durable in exposed conditions, though it is also more expensive. It can be used in nearly any outdoor project, including decks, fences, posts, docks, piers or swing sets, and it is just as easy to work with as natural wood.
Here are a few of common scenarios where pressure treated lumber is recommended, instead of natural wood:
#1 Foundation Sill Plates
In a standard stick-framed house, sill plates are laid on top of the foundation or around the perimeter of a concrete slab, and bolted to the concrete. On a foundation, rim joists and floor joists are added to the sill plate to create the first floor, and on a slab, the sill plate forms the bottom of the wall. Often, when the sill plate is bolted in place, the foundation has not cured and still contains high levels of moisture, and even if it is dry, the sill plate is still in a position that makes it prone to moisture and insect infestation. If it were made of natural wood, it would quickly rot away, compromising the walls, floors and the entire structure of the house.
Today, sill plates are made from pressure treated lumber, which is less permeable to moisture, fungal growth and insect infestation. It also protects the rest of the structure from damage by providing a barrier between the moisture-absorbent concrete and the natural wood in the regular framing, plus it is strong enough to support the weight of the structure without being damaged itself.
#2 Wood With Constant Weather and Sun Exposure
Wood that is above ground, but not protected from rain, snow, or the harmful ultraviolet radiation of the sun can quickly rot, crack or split. For projects like decks, fencing, swing sets, railings, picnic tables and benches that are constantly outdoors, pressure treated lumber is the perfect material. It requires very little maintenance, only periodic resealing, and it will resist damage from precipitation, insects and the sun. For deck posts or fence posts that are in contact with the ground, slightly higher concentrations of chemical preservative are required.
#3 Ground-Contact Posts and Poles
For many projects like decks, light posts, fences, and pier and beam foundations, posts or poles must be driven into the ground and remain in contact with the moist earth. Poles and posts made of natural wood would quickly rot, or become infested with fungi or wood-boring insects, eventually causing the post or pole to break off at ground level. By using slightly higher concentrations of wood preservatives, pressure treated lumber can be used in situations where it is constantly in contact with the ground. The chemicals protect it from decaying as fast as natural wood, leading to a much longer lifespan.
#4 Docks and Piers
In situations where the structural components are constantly immersed in water, such as boardwalks, docks, piers, sea walls or other marine structures, natural wood is not viable at all, except for a few naturally water-resistant species. In most cases, pressure treated lumber is the best, most economical choice for building such structures. With the correct chemical formulations it can withstand constant salt water spray, freshwater immersion or saltwater immersion. Often, chromated copper arsenate is still used, due to its higher resistance to moisture than newer formulations. Once installed, pressure treated lumber can last for many years submerged in water, and unlike steel it is not vulnerable to corrosion or electrochemical reactions, and it can last as long or longer than naturally moisture-resistant wood species.
#5 Bulkheads and Retention Walls
Natural wood is relatively weak and brittle, and it is vulnerable to the elements, making it unsuitable for use in many bulkheads or retention walls that are designed to hold soil back from a body of water or from traveling down a slope. Thanks to the treatment process, pressure treated lumber is much stronger than natural wood, and it is resistant to the elements. This makes it ideal for use in bulkhead or retention wall systems, where there are large amounts of pressure behind the wall. Pressure treated poles are driven into the ground along the length of the wall, and used alone or in combination with sheathing to create the final barrier. These structures help prevent erosion and keep the area in front of the barrier safe from falling debris.
Working with Pressure Treated Lumber Safely
Because pressure treated lumber contains some harsh chemicals, it is best to take precautions when working with it. Use gloves to handle the wood, and wash your hands after you are done. When cutting or drilling pressure treated lumber, do it outside, in a well-ventilated area, and wear a dust mask and safety glasses to keep yourself protected.
These are just a few of the situations where pressure treated lumber comes in handy. It is much stronger than natural wood where moisture, fungus or insect exposure is prevalent, and it is much more affordable that alternative metal or composite materials. It is quite versatile and can be used to build hundreds of projects.