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Guide to Pressure Treated Wood: Types and Uses

Lauren Bongers

By Lauren Bongers
Updated April 15, 2025

Using pressure-treated lumber for projects like decks and fences will keep your outdoor spaces beautiful for years, resisting rot and decay. Pressure-treated lumber comes in two types with different properties. In this guide, we'll discuss the two types of pressure-treated wood and their uses.

What is Pressure-Treated Lumber?

Pressure-treated lumber is a type of wood treated with chemical agents that protect it from the elements. Wood is susceptible to fungal decay, insect rot and termite infestation but treating it can avoid these common issues from occuring. Pressure-treated wood, or lumber, is ideal for building decks, fences, gazebos and other exterior projects.

What are the Different Types of Treated Lumber?

Lumber is popular for construction use as it's durable and versatile. It can be used to build benches, sheds, decks, furniture and more. There are two primary types of lumber used: above-ground pressure-treated wood and ground-contact pressure-treated wood.

Above-Ground Pressure-Treated Wood

Above-ground pressure-treated timber, or wood, is treated to resist rot, fungal decay and termites. It's suitable for use where lumber will not directly contact the ground. Above-ground pressure-treated wood boards come in a range of sizes. Some uses of above-ground pressure-treated wood include:

How is Above-Ground Pressure-Treated Wood Made?

In above-ground pressure-treated wood, preservative chemicals are injected into the wooden material under immense pressure. This preserves the exterior and even the inner part of the board. Once it dries, the treated wood is ready for use and can last for years, depending on the quality.

Which Chemicals are Used in Above-Ground Pressure-Treated Lumber?

Two of the most popular chemical preservatives currently used are copper azole and alkaline copper quaternary. The copper makes them resistant to fungi and insects that could damage the wooden material. These preservative treatments are AWPA standard and building code compliant (IRC and IBC).

How Does Above-Ground Pressure-Treated Lumber Look After Treatment?

After treatment, the wood will either be greenish-brown or brownish-tan in appearance. However, it will return to its natural wood color after drying. It can also turn to an even lighter color when exposed to direct sunlight. Applying a UV protective finish is suggested to enhance long-term beauty.

Ground-Contact Pressure-Treated Wood

Ground-contact pressure-treated wood makes contact with soil, freshwater or concrete. Its treatment prevents wood decay and insect infestation upon ground contact. Ground-contact lumber ranges in size and style. Uses of ground-contact pressure-treated wood include:

How is Ground-Contact Pressure-Treated Lumber Made?

It's also designed with chemical preservatives, which protect it against decay caused by fungi and bacteria in damp environments. These preservatives also repel termites and carpenter ants, thereby protecting structures. Pressure is applied to press the preservatives inside the cell structure. These preservatives also meet AWPA standards and are building code compliant (IRC and IBC).

Which Chemicals Are Used in Ground-Contact Pressure-Treated Lumber?

Some regular preservative treatments are chromated copper arsenate and alkaline copper quaternary. Their formulations contain different proportions of copper, chromium and arsenic to protect wood exposed to ground contact.

How Does Ground-Contact Pressure-Treated Lumber Look After Treatment?

After treatment, the wood is greenish and turns brown after some use. This happens because cut ends and drilled holes reveal an untreated inner core of the wood. After exposing these parts, it's recommended that they be painted with a brush-on UV protective finish.

Which Pressure Treated Lumber to Choose — Above-Ground or Ground-Contact?

When it comes to picking the perfect lumber from different types of treated wood, there are some important factors to consider including:

  • Use case: If your structure is placed above ground, opt for above-ground pressure-treated lumber; otherwise, ground-contact wood is preferred.
  • Preservative type: Above-ground lumber uses less chemicals than ground-contact lumber and is suitable for areas with minimal influence from insects and mold.
  • Structural integrity: If you plan to use the lumber for heavy loads, select lumber with a greater overall thickness. This will allow you to use bigger bolts and create stronger structures.
  • Quality: A quality mark demonstrates that the wood complies with treatment standards provided by groups like the American Lumber Standards Committee. Opt for lumber that comes with a quality mark.

Tip

If approved by the manufacturer, paint the surface of pressure-treated lumber to increase the product's lifespan.