How long does pressure-treated wood last and how to make it last longer?

Pressure-treated wood is used a lot for building decks and fences. You might have some old constructions made from pressure-treated wood in your residence. Or you want to build a deck, so you’re looking for a material that would last as long as possible.

And you might be asking: how long does pressure-treated wood last? Does it have a lifespan?

In this article, we will give you the answers to these questions. We will explain how to treat wood, describe various grades and kinds of treated wood, tell which type is suitable for each purpose, and many other things.

How long does pressure-treated wood last?

In general, pressure-treated wood can stand strong enough for 9 to 40 years, or even more. It depends on where and how the wood is used.

The factors that affect the lifespan are the type of exposure, treatment, whether the wood is incised or not, etc. The kind of care and its frequency also affects the wood’s longevity.

Type of exposure

The type of exposure denotes where the pressure-treated wood is used: for above-, near-, on-, or in-ground construction.

Where and how the material is used determines the kind of treatment it should get.

Wood that stands vertically or on the edge usually sheds water and moisture easier and is more durable than horizontally placed boards. Posts placed into the dirt will last shorter than those put in concrete or gravel, etc.

Treated wood that is exposed to direct sunlight and lots of heat, moist damp areas, or snow, will weather differently. Remember that pressure treating protects wood from insects and rot, but not from long-lasting moisture.

Wood incision

The incised wood surface is penetrated by the treating agents more deeply. This type of wood is rarely used for decks or railings, though. Deeper penetration of chemicals will make the wood more durable.

Types of treatment

There are three main categories of pressure treatment.

The first one – waterborne chemical infusions – finds its application in residential, commercial, and industrial construction purposes.

Creosote treatment is used for railway ties, marine timber constructions, and wooden guardrail posts.

Oil treatment is less widespread nowadays, but is still used to protect utility posts.

The treatment greatly affects the wood’s longevity.

Maintenance

Even pressure-treated wood requires some good regular maintenance. It involves removing moisture-holding leaves, snow, and other residues, which prevents rot, mold, and mildew.

Trimming grass and keeping bushes at bay is also important since it lets the wood dry.

How long does pressure treated wood last? — Summary

Pressure-treated deck exposed to large temperature fluctuations and wet-dry cycles without maintenance will last for only about 9 years. With appropriate care and treatment, however, the same pressure-treated deck has a lifespan of 40 years or even more.

You will find that many warranties for treated wood are 30 years against rot and insects.

We will cover different causes of wood rot later in the article.

What does ‘pressure treated’ mean exactly?

Pressure treatment is done to protect wood against insects and rot, so the wood’s longevity increases. Wood treatment has a long history that goes all the way back to antiquity.

The ancient Greeks used olive oil and the Romans used tar to preserve their wood. In fact, many different compounds and materials have been used for that purpose throughout the whole history.

In the second half of the 19th century, railway manufacturers started large-scale pressure treatment using creosote to preserve bridge timbers and ties.

So, how is the treatment done?

The wood is placed into a liquid preservative and then exposed to high pressure. The pressure forces the liquid particles into the wood grain pores, so the outer wood layer becomes saturated with it.

Preservatives like creosote or oil-based liquids are washed out over time, so they are usually used for industrial structures. They are usually too toxic and inconvenient for residential use.

The modern pressure treatment method is described as this: lumber is dipped into a water-based chemical bath, then subjected to high pressure in a cylindrical chamber. The liquid penetrates the wood grain. The water evaporates, while the preservative chemicals are left in the wood.

The process can also involve incising to achieve better penetration by the chemicals.

The following preservatives have been used the most in recent years: ACQ (alkaline copper quaternary), Copper azole, Copper naphthenate, Copper-HDO, Chromated copper arsenate (CCA), Polymeric betaine, and Borate.

Pressure-treated lumber grades

There are two common pressure-treated lumber grades: #1 and #2. The #2 is also called ‘construction grade’.

The #1 grade has higher quality than the construction grade. #1 grade has fewer knots or blemishes, it is less likely that the cut will warp or twist. The better grade is used for purposes where appearance matters: for decking and railings.

The construction grade is rougher looking. It has more knots, blemishes, etc. The #2 grade is used for framing, where the wood is concealed and not open to the view.

There are also premium and select grades. They have the best grain, appearance, and sturdiness. They are pressure treated too but are usually reserved for special orders.

Most of the time, 5/4 decking is made of a standard to mid-range pressure-treated lumber grades. That is somewhere between grades #1 and #2.

Keep in mind that many lumber yards only reserve the construction grade pressure-treated lumber. You will have to choose the lumber wisely. Lumber that is still wet since the chemical bath treatment will dry and shrink over time. That might cause warping, twisting, cupping, and bowing.

So, you should look for a kiln-dried after treatment (KDAT) label on the tag when choosing treated lumber. That type of treated wood has significantly less moisture in it and will not shrink or deform.

Pressure-treated wood types

Pressure-treated lumber is usually obtained from locally available types of softwood, most of the time conifers. Remember that chemical treatment of wood does not weaken or strengthen it, the treatment only helps it resist rot and insects and last longer.

Southern yellow pine is commonly used in the eastern States.

A mix of spruce, fir, and pine (SPF) is used in most of the northern States.

Douglas fir and hemlock are used in western Canada and the western US.

Pay attention to the end tag on treated lumber – it is a great source of information. It indicates whether the treated wood was kiln-dried after treatment (KDAT) and how much preservative chemical the wood holds, or its ‘retention level’. Codes put on the end tags tell you how much chemical the lumber contains. This value is measured in pounds of chemicals per cubic foot of wood (pcf).

The amount of preservative determines where and how the lumber can be used.

Here’s a short list of some codes that tell you what the purpose of the pressure-treated wood should be:

• UC1 – for interior, dry;
• UC2 – for interior, wet;
• UC3A – exterior, above ground with good drainage;
• UC3B – exterior, above ground with poor drainage;
• UC4A – ground contact in general;
• UC4B – ground contact, heavy-duty;
• UC4C – ground contact, extreme duty;
• UC5A – marine use, northern ocean waters;
• UC5B – marine use, central ocean waters;
• UC5C – marine use, southern ocean waters;
• UCFA – pressure treated for fire protection, interior;
• UCFB – pressure treated for fire protection, exterior.

Try to find the type that suits your purposes best.

Chemical preservative types

You should also consider the type of preservative when choosing pressure-treated wood for your outdoor projects. For residential use, Alkaline Copper Quaternary (ACQ) is the most common preservative.

However, there are many more preservative formulations that we mentioned earlier in the article. The American Wood Protection Association (AWPA) determines the standards for the chemical retention levels for all preservatives. So, when you look at an end tag on lumber, you see the best use and purpose of the treated wood regardless of the chemical used.

Best pressure-treated wood for ground contact

Ground contact is when lumber touches the ground surface or some vegetation; is within 6 inches from the ground; or is located where it sits in, on, or over freshwater.

As we mentioned in the previous paragraphs, you should use the correct type of treated wood to increase the lifetime of your outdoor projects.

The best pressure-treated wood for ground contact is UC4A, B, or C, depending on the exact type of exposure. The main difference between the types is the amount of preservative they have absorbed. The amount of chemicals per cubic foot of wood in UC4B is greater than that in UC4A. And UC4C contains even more preservatives.

UC4A purposes

• Normal exposure to decay;
• Can be used within 6-inches of the ground;
• Light ground contact;
• Damp environment, exposure to frequent moisture or tropical climate;
• Long contact with moist leaves or other vegetation;
• Exposure to an environment with poor airflow;
• For beams, posts, joists, and ledger boards that are difficult to maintain, repair, or replace;
• For docks, decking, stringers, and boardwalks.

UC4B purposes

• For posts supporting permanent constructions, docks, or piers;
• High exposure to decay;
• For wooden foundations;
• Direct contact with freshwater or saltwater spray (e.g. marine structures);
• For difficult-to-maintain or replace framing members;
• For heavy-duty functions;
• For utility poles, cross-ties, or garden posts;
• Exposure to all weather cycles or tropical climates.

UC4C purposes

• Direct contact with or installation in concrete, gravel, or earth;
• Ground contact of structural components;
• Freshwater contact;
• Severe risk of exposure to decay;
• Exposure to extreme weather cycles (harsh weather and humidity level fluctuations, etc.);
• Land, freshwater, and building pilings;
• Utility poles and cross-ties in areas with severe risk of exposure to decay;

What makes pressure-treated wood rot?

Preventing wood rot at the construction stage

You should prevent pressure-treated wood from rotting right at the construction stage.

When you cut boards to length or trim off rough ends, the inner layers of wood are exposed. Those layers haven’t been pressure treated as they are too far away from the lumber surface. That is why you should seal all cuts and gouges with the end-cut preservative. That will work as a pressure treatment preservative and prevent wood rot and insect damage.

‘Galvanic rot’

Another problem is ‘galvanic rot’. It occurs where galvanized fasteners, brackets, and hangers are used. The reason is that the chemical preservatives are affected by the galvanized material. The initial oxidation leads to corrosion at the surface contact and inside the wood, which then can lead to structural failure.

The solution to the ‘galvanic rot’ is using coated or stainless-steel fasteners, brackets, and hangers. You can also take a bituminous membrane or tape and apply it between wood and metal parts, which will prevent rot, too.

Painting & staining

Never paint or stain any wood while it is wet! First, let it dry completely.

The binders, coalescing agents, and chemicals penetrate the wood and block the pores in the grain. That prevents moisture from getting into and out of the wood. The water that has not escaped the lumber will cause it to rot.

Another reason why you should not paint, stain, or seal the pressure-treated wood before it is dry is the reaction between chemicals in the preservative and the paint or stain. That can lead to a variety of consequences, so it is best to wait for the wood to properly dry.

Moisture damage

Moisture leads to rot and significant damage in pressure-treated wood. It can cause deck boards to cup and split as they expand and shrink due to seasonal cycles. The forming cracks will allow moisture to penetrate deeper into the untreated wood, causing even more rot.

Lots of dirt and vegetation residue left on the wood deck or structural parts can retain moisture and prevent the wood from drying. Thus, they increase moisture damage. Seeds from vegetation can even grow small roots into the wood pores, accelerating the rotting process.

That is why it is important to seasonally sweep and wash dirt and decaying vegetation off the wood structures. That will significantly extend the longevity of your constructions.

Fungi

Fungal infestations cause rot, too. Some pressure-treated wood does contain fungicides for protection. But while it protects the treated wood initially, it does not last very long.

Microscopic fungi enter tiny pores and cracks in wood grain, then they start feeding on the wood and multiplying. The lumber gets weaker and begins to decay, which again leads to rot.

You can treat your wood with a special fungicide annually. That will help you resist rot and protect pressure-treated structures.

Airflow

Lack of airflow causes wooden structures to rot. Pressure-treated wood is supposed to dry in order to avoid rotting, and good air circulation promotes that.

All sides of wood boards need to be exposed to good airflow. Those parts of wooden structures that are close to the ground, ground contact, or skirted, usually have problems with it, since the airflow is restricted.

Constructions that have contact with vegetation, water, etc. have problems drying out, too. To avoid rot in such cases, try to improve the air circulation and use UC4B treated wood.

Do you need to seal pressure-treated wood?

You can make your lumber last for decades if you pick the correct preservative treatment type for your particular location and climate. Regular maintenance should not be neglected and is not different from any other structure care.

Like any wood, treated wood is also porous and can absorb water and moisture from rain and snow. Then the wood dries out in the sunlight or wind. Such wet-dry processes lead to wood warping, cupping, and splitting.

But you can avoid that damage by using a stain, paint, or sealant. Paint is not used for appearance purposes only, it can serve as a protective coating as well!

Before applying any sealant or other covering, the wood should be clean and dry. You can run a simple water test to figure out whether the wood is dry or not. Just sprinkle a small amount of water on different surface locations. If the water drops stay on the surface, the wood is not dry. If the drops of water are absorbed, then the wood is dry.

Sealing the pressure-treated wood that is exposed to a high level of humidity is important, and we encourage you to do it.

How long does a deck made from pressure-treated wood last?

As we mentioned earlier, many factors affect the wood deck’s longevity. Those factors determine how fast the rotting process will be. Here’s a recap of the main factors we discussed earlier:

• Type of pressure treatment;
• Construction methods;
• Kind of fasteners, brackets, hangers;
• Airflow and wind strength;
• Climate and seasonal cycles;
• Quality and frequency of maintenance;
• Application of sealants.

If you choose the right materials and construct and maintain your project properly, your wood deck can last 40 years or more. From time to time, you might have to replace some boards as a part of the maintenance practice.

Pressure-treated wood may lose its original color or appear patchy if you leave it without treatment. It can also become rough because of the weathering. Using sealants and maintaining the treated wood regularly will help the wood last significantly longer and save you from replacing boards every ten to fifteen years.

Underground treated wood protection

Before burying your pressure-treated wood underground, we advise you to protect it additionally. There are multiple ways to improve the treatment. Your choice will depend on the treated wood’s purpose and the type of ground it will be placed into.

Remember that the type of wood set into the ground should be UC4B or UC4C treated wood.

Concrete & Gravel

The most common way to extend this lumber’s lifespan is to put gravel or concrete around it. The concrete seals out moisture and insects completely. The gravel allows the wood to dry better and is used if the soil around is damp. Both concrete and gravel should expand above the soil surface several inches to avoid moisture pooling around the bare lumber.

Applying extra protection

Another, more time-consuming method, is to apply extra protection onto the pressure-treated wood before it is put into the ground.

Use a brush, roller, or garden pump sprayer to apply one or two layers of fungicide-enhanced sealer onto the wood. After making the first coat, wait for several hours before applying the next one. Keep in mind that full drying can take up to several months, so you need to plan it.

Another sealer that you can use is a liquid termite-preventative chemical. It is supposed to be applied to the part of the wood that is being buried.

You can also use thinned tar to protect the in-ground part of the lumber. Paint several coats of thinned tar and allow it to soak into the pores of the wood grain. You can use roofing tar for this purpose.

The lifespan of an untreated deck

Untreated kinds of wood like spruce, pine, or fir (SPF) are usually used for framing or building structures that will not be exposed to rot-causing factors. For the past 50 years, many exterior residential projects have used pressure-treated wood. Before that, most people used SPF, while wealthy ones could afford cedar, redwood, teak, and mahogany.

Untreated SPF decking usually grays, cracks, roughens, cups, and warps within a couple of years. But it can maintain its strength for 10 to 30 years if it has the opportunity to dry out from time to time. Even untreated wood, if it is properly painted, sealed, and maintained, can last centuries – you can see examples of such decking in old houses.

Before the pressure treatment technology development, everyone used untreated wood for all purposes. All the protection they had was paint, stain, sealer, or other protective coating types. Some bandstands built using untreated wood about 80 years ago are still strong enough. They are kept that way due to annual maintenance and a fresh paint coat every other year applied to them.

FAQ

How long does painted pressure-treated wood last? (How long does stained pressure-treated wood last?)

If you decided to paint or stain pressure-treated wood, you’d like to know how long it will last. Painted pressure-treated wood lasts significantly longer if applied correctly. If you paint or stain pressure-treated wood when it is completely dry, it might add several decades to its lifespan.

How long does a pressure-treated wood fence last?

A common pressure-treated wood fence lasts about 15 years. Fences are made, so they can resist insects and rot. The fence longevity also depends on the same factors such as climate, maintenance, etc.

If your residence is located in a wet area and the fence is often hit by heavy rain and strong wind, the lifespan will be shorter. Strong UV rays are no good too: they can cause the pressure-treated wood to fade and split. The hot climate also reduces the longevity of your fence.

Can pressure-treated wood be dangerous?

In the past, there had been some concerns about the health risks that pressure-treated wood posed. CCA (chromated copper arsenate) is one of the preservatives used in treated wood that is potentially dangerous. But the CCA-treated wood is used for industrial purposes only. So, you cannot find it in your house or yard.

All of the pressure-treated wood used for residential purposes is totally safe. You don’t need to worry about any risks if you buy it from a trustworthy supplier.

Can I burn pressure-treated wood?

No, you must not burn any pressure-treated wood! In many areas, there are special regulations that prohibit burning it.

This is done because the chemical preservatives in the wood are released into the air when burnt. The fumes cause serious respiratory issues and might even increase the risk of lung cancer when inhaled.

You can find more information about the effects of these toxic chemicals and the consequences of burning pressure-treated wood on your own.

Can I sand pressure-treated wood?

Yes, you can sand pressure-treated wood. But you must do it carefully and take all the precautions needed.

When you sand pressure-treated wood, tiny wood particles with toxic chemicals inside them can enter your breathing system, causing the issues described above. Also, never sand CCA-treated wood since it is really toxic and not worth the risk.

You can sand-treated wood that contains less dangerous chemicals, like ACQ. To protect yourself, wear safety goggles and a good mask.

How long does pressure-treated wood last? — Conclusion

In general, pressure-treated lumber is stronger and more durable than untreated one.

The standard lifespan of pressure-treated wood is 40 years. But now you know it can last even longer if you give it some tender, loving care.

Applying an extra layer of protection or a sealant will extend the longevity of your projects. But first of all, you should pick the right type of treated wood for your particular purposes.

Good luck with woodworking!