How Long Does Pressure Treated Wood Last?

Are you planning to build a deck and wondering, “How long does pressure treated wood last?” Building a deck is an expensive investment and choosing the material that will last longest is a wise decision. Whether for framing only or the decking too, pressure treated lumber lasts a long time.

Pressure treated lumber exposed to freeze-thaw and wet-dry cycles without care or maintenance will last about 9-years. With proper maintenance and treatment, the same deck will last 40 plus years. Many manufacturers warranty their treated wood for 30 years against rot and insects.

In this article, we’ll explore how long pressure treated wood will last, what the treatment is, different grades and types of treated wood, and which is best for different uses. We’ll review what causes treated wood to rot and if it needs to be sealed, and finally, compare it with how long an untreated deck might last. By the end of the article, you’ll have a solid understanding of the benefits of pressure treated wood and how it can extend the life of your deck.

How Long Does Pressure Treated Wood Last

How Long Does Pressure Treated Wood Last?

The life span of pressure treated wood ranges from 9 to 40-years or more depending on where and how it is being used. The type of exposure, treatment, and whether it’s incised or not are other factors that affect treated wood. Additionally, the frequency and type of care or maintenance the wood receives also affects its longevity too.

Pressure treated wood is used for above ground, near-ground, on-ground, or in-ground construction. Where and how it is used often determines the type of treatment required. Wood standing upright or on edge commonly sheds moisture easier and lasts longer than horizontal face-up boards. Posts sunk into dirt won’t last as long as those set in concrete or gravel filled holes, so how it is used determines its lasting power.

Wood exposed to blistering heat, moist damp areas, snow, or near, on, or in the ground, weathers differently. Pressure treatment protects wood from insects and rot, not from prolonged moisture. Incised lumber allows for deeper penetration of the pressure treatment, and is common in lumber not used for decking or railings. The deeper the chemicals penetrate, the longer the wood will resist rot and insects.

There are three pressure treatment categories for wood. Waterborne chemical infusions are commonly used for commercial, residential, and industrial construction. Creosote treatment protects railway ties, marine timber structures, and many guardrail posts. Oil treatment is used less frequently today but is often used for protecting utility posts. The type of treatment plays a large role in how long the wood will last.

Regular maintenance is also important for protecting wood, even pressure treated wood. Removing snow and moisture-retaining leaves will help prevent rot, mold, and mildew. Trimming grass and keeping bushes away is helpful to allow the wood to dry out, as is improving drainage away from posts and structure.

What Is Pressure Treated Lumber?

Pressure treatment is a way to protect against rot and insects to preserve wood so it lasts longer. The ancient Greeks used olive oil, the Romans used tar, and throughout history, different oils and compounds have been used to preserve wood, including lye. In the last half of the 19th century, railway companies began large scale pressure treatment using creosote to protect ties and bridge timbers.

Pressure treating means the wood has been immersed in a liquid preservative and then subjected to high pressure. The pressure forces the liquid into the wood for greater protection from insects and rot. Oil-based preservatives and creosote leach out over time, and are considered acceptable for industrial use but too messy or toxic for residential use.

Modern pressure treatment involves immersing lumber in a water-based chemical bath and then subjecting it to high pressure in a cylindrical chamber to force the liquid deeper into the cell structure of the wood. The water evaporates leaving the protective chemicals behind. Incising is a process of mechanically making small cuts into the surfaces of wood for deeper penetration by the preservative solution.

For the past 15 or so years, most wood products used for residential applications include one of the following preservatives: ACQ (alkaline copper quaternary), Copper Azole, Copper Naphthenate, Copper-HDO, Borate, or Polymeric Betaine. Prior to that, chromated arsenicals and PCPs were used to preserve wood for residential uses.

Pressure Treated Lumber Grades

Pressure treated lumber is commonly available in #1 grade and #2 or construction grade. The better grade lumber has fewer knots or blemishes and the cut is less likely to warp, cup, or twist. It is used for decking and railings where its quality is visible. Construction grade has more knots and can be rougher looking. It’s used for framing where looks don’t matter.

Premium and select grades have the best grain, strength, and appearance. They are also pressure treated but are commonly reserved for special orders. Most 5/4 decking is standard to mid-range grade – somewhere between #1 and #2 grade lumber, or a combination of the two.

Many lumber yards only stock #2 construction grade pressure treated lumber, so you’ll have to pick and choose. If the lumber is still wet from the chemical treatment bath it will shrink more as it dries, which may cause cupping, twisting, warping, and bowing. Look for KDAT (kiln-dried after treatment) on the end-tag when purchasing treated lumber as it has less moisture in it.

Types of Pressure Treated Wood

Pressure treated lumber is commonly whatever type of softwood that is locally available. In the eastern States, southern yellow pine is often the choice. In much of the northern US and eastern-central Canada it is a mix of spruce, pine, or fir – normally referred to as SPF. Western Canada and the US use Douglas fir, fir, or hemlock. Chemically treating the wood doesn’t weaken or strengthen the lumber, it just helps it last longer.

The end-tag on treated lumber is a font of information. It identifies if the wood was kiln-dried after treatment (KDAT), and how much chemical preservative the lumber retains, or its ‘retention level’. The amount of preservative determines where the wood is suitable to be used. Codes printed on the end-tags identify how much chemical is retained in the lumber and is measured in pounds of chemical or active ingredients per cubic foot of wood (pcf).

Different codes identify which to use for interior dry (UC1), interior wet (UC2), exterior above ground (UC3A-good drainage or UC3B-poor drainage), ground contact (general UC4A, heavy-duty UC4B, or extreme duty UC4C), marine use (UC5A-northern, UC5B-central, or UC5C-southern ocean waters), or pressure treated for fire protection (UCFA interior and UCFB exterior). Select the type best suited for the location and use.

The type of preservative is another factor to consider when selecting pressure treated wood for a project. The most common for residential use presently is ACQ or Alkaline Copper Quaternary. However, there are more than a dozen different preservative formulations used for pressure treating wood. The AWPA (American Wood Protection Association) sets the standards for the chemical retention levels for all the different preservative formulas, so the end-tag category identifies the best use regardless of the preservative used.

Best Pressure Treated Wood for Ground Contact

How long does ground contact wood last

Ground contact is when wood touches the ground or vegetation, is within 6-inches of the ground, or situated where it sits in, on, or over freshwater. Using the correct type of treated wood will extend the lifetime of the project and better protect the investment. The best pressure treated wood for ground contact is UC4A, B, or C, depending on the type of contact.

The amount of chemical per cubic foot of wood in UC4A is greater than UC1, 2, or 3 categories, and UC4B has more than UC4A, while UC4C has even more.

UC4A uses:

  • Normal exposure to decay
  • Use within 6-inches of the ground
  • Light ground contact
  • Damp areas, exposure to frequent moisture, or tropical climates
  • Extended contact with damp leaves or vegetation
  • Where there is poor airflow or circulation
  • For ledger boards, joist, beams, and posts which are difficult to repair, replace, or maintain
  • Docks, decking, stringers, and boardwalks

UC4B uses:

  • Posts that support permanent structures, docks, or piers
  • High exposure to decay potential
  • Wood foundations
  • Contact with freshwater or saltwater spray such as marine structures
  • Heavy duty functions or difficult to maintain or replace framing members
  • Garden posts, utility poles, or cross-ties
  • Exposure to tropical climates or all weather cycles

UC4C uses:

  • Direct contact or installation in concrete, gravel, or earth
  • Ground or freshwater contact of structural components
  • Severe risk of exposure to decay
  • Exposure to extreme weather cycles
  • Freshwater, land, and building pilings
  • Utility poles and cross-ties in areas of severe decay potential

What Causes Pressure Treated Wood to Rot?

Preventing pressure treated wood from rotting begins at the construction stage. Cutting boards to length or trimming off rough ends exposes inner wood that pressure treatment didn’t reach. All cuts and holes should be sealed with end cut preservative to prevent rot and insect damage.

The use of galvanized fasteners, brackets, and hangers can result in ‘galvanic rot’ as the chemical preservatives react to the galvanized material. The initial oxidation causes corrosion at surface contact and within the wood, leading to structural failure. Using coated or stainless steel fasteners along with stainless steel hangers and brackets will prevent ‘galvanic rot’. The use of a bituminous membrane or tape between wood and metal brackets or hangers is another way to prevent rot.

Painting, staining, or sealing pressure treated wood before it is dry can trap moisture and chemicals in the wood. The coalescing agents, binders, and chemicals penetrate and seal pores in the wood, preventing moisture from entering the wood. However, it also prevents moisture from being expelled from the wood, which leads to rot. Another issue of painting, sealing, or staining the surfaces before they have dried is the reaction with chemical agents in the preservative. It is best to wait for pressure treated wood to dry before applying anything to its surfaces.

Moisture can lead to rot in treated wood. It can damage deck boards as they expand and contract due to seasonal changes, and cause boards to cup or split. Cracks allow moisture to permeate deeper into unprotected wood causing rot, and cupping pools water and prevents the wood drying, thus causing rot.

Dirt and vegetation left on the deck or structural members can retain moisture and seeds. They can accelerate moisture damage by preventing the wood from drying, causing decking, joists, and beams to rot. The seeds can even send taproots into the pores and accelerate rot. Sweeping or washing dirt and dead vegetation off the structure seasonally will extend its life span.

Fungal infestations can lead to rot in pressure treated wood. Some pressure treatments include fungicide which initially protects the wood, unfortunately, it doesn’t last. Microscopic fungi organisms can enter pores and tiny cracks in the wood. Once inside, they begin to eat on the wood and multiply. As they eat, they cause decay and weaken or soften the wood from within, leading to rot. Annual or bi-annual treatment with a fungicide helps protect your pressure treated structures.

Lack of air circulation can cause even the best cared for structure to rot. Pressure treated wood needs to dry to prevent rot – not only surface or exposed wood, but the underside needs to dry too. Close to ground, ground contact, and skirted structures are especially susceptible as airflow is restricted. Structures surrounded by vegetation, set into hollows, or over or near water often also have difficulty drying out. Improving airflow and using UC4B treated wood will help prevent rot.

Does Pressure Treated Wood Need to Be Sealed?

Pressure treated wood can last for decades if the correct preservative treatment is used for the location and climate. Treated wood, like all wood, is porous and will absorb moisture from rains and snow, and then dry out in the sun and wind. The wet-dry process can cause the wood to split, cup, or warp. To prevent damage and protect your investment, the use of a stain, sealant, or paint is recommended.

Regular care and maintenance of pressure treated structures should be no different from any other structure. Exposed wood surfaces take a beating from the weather and seasonal temperature changes, so extra protection is important. The product used often dictates the frequency of reapplication and ranges from one to five years. The wood should be clean and dry before any sealant is applied.

Ensure the wood is dry before applying a protective coating, otherwise, you risk sealing moisture into the wood and accelerating rot. The water test is a simple and effective way to check if the wood is dry. Sprinkle a small amount of water on different surface locations. If the water beads up, the wood isn’t dry, if the water absorbs into the wood, it is dry. Dry wood will absorb the sealant and protect the surfaces.

How Long Will a Pressure Treated Deck Last?

Pressure treated wood deck lifespan

There are many factors that affect the lifespan of a deck. The type of pressure treatment, construction methods, and the kind of fasteners, brackets, and hangers used can prevent or accelerate rot. Air circulation, climate, and the frequency of maintenance and the application of sealants also affect a deck’s longevity.

A properly maintained and constructed deck, made from preserved lumber of the appropriate classification, can last 50 years or more. Cleaning, sanding, and sealing wood surfaces will extend the lifespan of a deck. Maintenance may also include the occasional replacement of a deck board or two.

Pressure treated wood may discolor or appear patchy if left untreated. It also may become rough due to the effects of weathering. Regular maintenance and use of sealants will protect the wood so it will last longer. A little preventative maintenance can save the expense of replacing decking every ten to fifteen years due to aged and rough appearance.

How to Protect Pressure Treated Wood Underground?

Pressure treated wood is commonly preserved from rot and insect damage but is still susceptible to moisture and fungal damage. Additional protection before burying it underground is advisable. There are different ways to enhance the treatment to extend the lifespan of the wood depending on its purpose or use and the type of ground it will be set into.

Wood being set into the ground should be UC4B or UC4C treated. Two practices often employed to protect wood underground involve pouring either gravel or concrete around them. The concrete seals out moisture and all insects for full protection, while the gravel allows the wood to dry when the surrounding soil is damp. The gravel and concrete should extend above the surface several inches to prevent moisture pooling around the exposed wood.

Another method is to further protect pressure treated wood before it is set into the ground. The wood needs to be dry so it will absorb the extra protection. Drying may take several months, so forward planning is necessary. Use a brush, roller, or garden pump sprayer to generously apply one or two coats of fungicide enhanced sealer to the wood being placed underground. Let it soak in for several hours between coatings.

Some alternative ways to protect pressure treated wood that will be submerged in the ground include applying a coat or two of liquid termite preventative chemicals to the portion being buried (follow the directions and wear protective eye and respiratory gear). Painting several coats of thinned tar to the in-ground portion of the timber and allowing it to soak into the pores will enhance the protection. Another way is to coat the below-grade wood thoroughly with roofing tar and allowing it to air dry for 24 hours. Combining all three methods will provide even more protection.

Pro Note: Any additional protection should only be applied to pressure treated wood that has had time to dry. Not waiting for the wood to dry will prevent it from absorbing the extra protection, and risks locking moisture into the wood which can accelerate rot.

How Long Will an Untreated Deck Last?

Untreated spruce, pine, or fir (SPF) is commonly used today for framing or build structures that will be protected from the elements. For the better part of the last 50 or so years, many exterior residential projects have relied on pressure treated wood. Prior to that, woods like cedar, redwood, teak, and mahogany were used for decking by those affluent enough to afford them, and SPF was used by everyone else.

Untreated SPF decking left to the extremes of nature will gray, crack, roughen, cup, and warp within a year or two. It can maintain its structural strength for 10 to 30 years provided it can periodically dry out. However, properly maintained, painted, or sealed untreated wood has lasted centuries.

Prior to the development of pressure treated lumber, everyone used untreated wood for interior and exterior use. Applying paint, stain, sealer or some other protective coating was all the protection that was available. Untreated vertical barn boards have hung on barns for upwards of a century, whereas untreated boardwalks often need restoration every 3 to 5 years. A local bandstand built of untreated wood some 80 years ago is still going strong with some annual TLC and a fresh coat of paint every other year.


Pressure treated lumber can extend the lifespan of outdoor projects. It protects against rot and insect damage but requires regular maintenance and treatment to prevent moisture damage. With care, treated wood will last 40 or more years.

I hope you found this article informative and have a better understanding of what pressure treatment means, what type to use, and how long it will last. If you know someone who may benefit from this article, please share it. Your comments and suggestions are appreciated.

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