How to Pick the Best 4×4 Pressure Treated Fence Post


There are many different types of wood lumber and grades available at your local home improvement store so it is important to know the important items when purchasing the materials for your repair.

  • Pressure treated posts should be rated for ground contact – AWPA grade UC4A for general purpose or UC4B for heavy duty. Avoid peeler cores when selecting 4” x 4” x 8’ fence posts.
  • Pressure treated fence rails should be AWPA grade UC3B if left unpainted, grade UC3A if painted or finished with an opaque stain.  2×4 rails in 8-foot lengths are commonly used.
  • Nails, screws and other fasteners used with pressure treated wood should be double hot dipped galvanized or stainless steel; ASTM A 153 Class D

When a storm blows through the wind loads on your fence can be incredibly high and all of that load is transferred onto the post. You can work through the calculations of ASCE/SEI 7-10 yourself, or just know that it is important to select the strongest building materials available.

Pressure Treated Lumber

Pressure treated lumber is chemically treated to inhibit fungal growth and insect activity.  The three common types of pressure treatment are ACQ (alkaline copper quaternary), CA (copper azole), and the newest type is MCQ (micronized copper quaternary).  There are different grades of pressure treatment and proper selection depends on the severity of the location and how the wood will be exposed to the elements.

Fence Posts

When we consider the privacy fence for most homeowners, the fence post is the most critical element.  The post bears all of the stress from wind gusts and is exposed to moisture and insects underground that can cause the post to deteriorate and weaken.  Wood fence posts should be pressure treated and rated for ground contact. 

What about cedar posts?  The quality of wood coming out of the lumber mills has declined significantly over the past 20 years – it used to be that you could get old growth heart wood (from the center of the tree) that would last for decades as a post, but today the quality is simply to variable to recommend using cedar or other naturally rot-resistant wood.  Most of the trees brought into the mill are from 10-year re-growth and the posts will contain very wide rings from the sapwood layer that simply will not last.

How Long Will a Treated 4×4 Post Last in the Ground?

Not all wood is the same; even 4x4s that come from the same mill and have the same treatment can have dramatically different life spans when put into a demanding application such as holding up your privacy fence.

There are three main factors that will determine the life of your post: the strength of the wood, how well it accepts the pressure treatment chemical, and how well the post is protected from moisture.

The Strength of the Wood

First, eliminate any posts that have excessive knots – especially if they have included bark. The knots lower the lateral strength of the wood and the included bark diminish the integrity by allowing easy access for moisture and insects to the interior of the post. Most importantly check the end grain of the post. You want to select rings that are close together. The denser the rings, the stronger the post and the longer it will last.

Of course, an oak post will last longer than a pine post, but we are not talking about the species of wood here. Most pressure treated lumber is either pine or fir so when you walk up to the stack of lumber, which post do you pick out?

You want to avoid the center of the tree – it contains the pith, which is the youngest wood in the tree. The pith is weak wood because its job is to grow vertically very quickly and be flexible. Posts made from pith are more likely to warp, twist, bow, and splinter. You want to select posts with end grain that are tightly spaced from the middle part of the trunk because it is the heartwood that provides strength.

Heavier Does not Equal Stronger

What about weight? Won’t the heaviest post be the strongest because it will be denser wood? This is true for kiln dried dimensional lumber – a heavier piece typically means a tighter grain structure and stronger wood. But kiln dried wood has a moisture content between 5-10% and it doesn’t affect the weight much from piece to piece.

The pattern is for pressure treated lumber to be sold very wet, almost to the point of dripping. A significant amount of the weight is actually water weight. If you buy a wet post and then let it dry for a year, you will find that it will be many pounds lighter than when you bought it.

How Well the Wood Accepts Pressure Treatment Chemical

Different manufacturers use different pressure treatment chemicals but the common ones are alkaline copper quaternary (ACQ), copper azole (CA) or micronized copper azole (MCA). These treatments cause copper to bond with the wood fibers to slow decay. They also resist other factors that promote rot and structural degradation like termites, fungus and moisture.

Chromated copper arsenic (CCA) was used to pressure treat lumber until 2004, when CCA was phased out by an agreement between the EPA and lumber industry. Later studies showed that while arsenic leached in higher amounts from CCA treated wood, the ACQ treated wood leached a greater amount of copper. The takeaway for homeowners is to use a dust mask when cutting treated wood and take steps to mitigate any leaching into your vegetable garden.

Whenever you are buying pressure treated posts make sure that you get wood that is rated for ground contact. Ground-contact pressure treated wood has twice the preservative and is required for applications within 6 inches of the ground, with poor ventilation, or where it will be difficult to maintain or replace – these are all true of a fence post. This is commonly listed as AWPA grade UC4A; the preservative retention requirements are listed below:

ACQ retention of 0.40 lb/ft3 (PCF)
CA-B retention of 0.21 lb/ft3 (PCF)
CA-C retention of 0.15 lb/ft3 (PCF) under the Wolmanized brand
Note that the MCQ pressure treatment method has not been rated by the AWPA standard.

American Wood Protection Association

To ensure your fence will stand for 10 years or more without worry the best method is to use pressure treated 4” x 4” pine posts that have the proper AWPA grade of UC4A.  For severe locations that hold moisture, such a low-lying areas or where there is heavy clay soil with you may choose to go with UC4B grade posts for extra rot resistance at a higher cost.

Most stores will carry #2 grade pressure treated posts rated for ground contact. The lumber grade is different than the pressure treatment rating, #2 lumber is used when appearance is not too important. Make sure that green post is really ground-contact rated!

We previously covered how the pith is weak wood, another reason to avoid the pith is that it does not accept the pressure treatment chemical well. Because the chemical doesn’t absorb the pith wood will rot earlier.

When the lumber mill makes plywood veneer they spin the log on a lathe and cut off the thin veneer. Just like unrolling wrapping paper, at a certain point they can’t peel any more and are left with the core. It is common for many of the 4×4 posts to be made from ‘peeler’ cores that you don’t want to use fence posts.

What is a Peeler Core?

The peeler core is a byproduct of plywood production but the mill won’t let anything go to waste so they turn them into 4x4x8 posts. The peeler cores contain the pith, which we just learned is the weakest part of the wood that doesn’t accept the pressure treatment chemical.

Pressure Treated 4x4 Posts End Grain
Pressure Treated 4×4 Posts End Grain

Examine the picture – both of the top posts are peeler cores and should be avoided. The bottom right post is a little better, but you can tell that is still close to the center of the tree. The bottom left post is the best of the group, it is from farther out from the center of the tree away from the pith. Generally, the flatter rings that are spaced close together will give the strongest post and the best value for your money. Ideally you want to find posts with end grain similar to the photo below.

Avoid the Bulls-eye

Now you know how to pick the best 4×4 fence posts at the lumber yard. Avoid posts with excessive bark or knots, and check the end grain to avoid peeler cores. Ideally you are looking for posts with tightly spaced grain from the heartwood of the tree.

But what do you do when the available stock looks like the image above? Every single one of these posts appears to be a peeler core. In this situation you are better off trying a different location, but you may have to get creative. Because peeler cores come from plywood production they are usually 8 foot lengths.

If all the 8 foot posts are peeler cores, another option is to buy a 10 foot post and then cut the excess. The 10 foot posts are usually not that much more expensive than 8 footers. If you use this option, be sure to put the uncut end into the ground and use a quality preservative on the cut end.

Protecting Your Posts from Moisture

Wood posts typically fail right at ground level – the post above the ground is still strong and so is the the wood further in the ground, but the section right at the ground line fails first. This ‘collar rot’ and is caused by moisture. To keep your fence standing for a long time the best thing you can do is keep anything that traps moisture away from your post.

We struggled with replacing posts that had broken off at ground level until we figured out a way to remove the concrete footing easily and without digging. Here’s how we slide the concrete out using water pressure.

Keep the ground clear around the post and remove any mulch or vegetation. If you use concrete footings to set your posts, make sure the concrete is domed and tapered away from the post so it doesn’t hold water. One of the big problems with concrete is that the wood can swell and crack the concrete or the wood will shrink and pull away from the concrete. Both of these situations hold water against the post and accelerate rot.

Pro Tip: One of the best things to preserve your fence and extend the life of your posts is to do annual maintenance. Reapply a quality wood preservative (here is our favorite for good price on Amazon) to cover any damaged areas from weed eating and recoat the post at ground level.

How Much Does a Treated Post Weigh?

With so much water in the wood grain from the treatment process the weight of a wood post can vary a lot. A typical 4x4x8 treated wood post will weight around 38 pounds. Remember that tight grain structure and avoiding peeler cores are the most important factors – a good post will be heavy.

How Much Does a Treated Post Cost?

Cost vary by geographical location and where you purchase your lumber. The table below has typical prices for what you can expect at a big box store for #2 pressure treated lumber rated for ground contact.

Nominal Post DimensionsLengthApproximate WeightCost
4×4 in8 ft34~42 lbs$8
4×4 in10 ft42~54 lbs$12
4×4 in12 ft50~66 lbs$15
4×4 in16 ft67~87 lbs$20
6×6 in8 ft67~77 lbs$23
6×6 in10 ft83~97 lbs$27
6×6 in12 ft99~117 lbs$37
6×6 in16 ft133~155 lbs$46

Remember that a 4×4 post is not actually 4 inches by 4 inches, each side will measure between 3.5 to 3.75 inches. Because pressure treated wood is sold very wet, the typical shrinkage from drying has not finished yet so pressure treated wood is always larger than kiln dried lumber. Length is not affected so an 8 ft post should measure 8 feet long.

When you are shopping for your fence posts you will probably find green pressure treated 8’ landscape timbers nearby and think to yourself “those timbers are a lot cheaper than these 4” x 4” pressure treated pine posts, I’ll just save some money and use the landscape timbers” – don’t do it!  The landscape timbers may last for 1-3 years but will certainly be weaker and fail sooner than the proper pressure treated pine post.

Fence Rails

Fence rails should also be pressure treated for the longevity of the fence, but the rails are not exposed to the same severe environment underground as the fence posts so AWPA grade UC3B is appropriate for unpainted fence rails.  If you plan to paint your fence rails or apply an opaque stain then AWPA grade UC3A may also be used.

Fence Pickets

The choice of fence pickets is more of a personal preference based on the appearance that you want and if you plan to finish the fence with a paint or stain.  Cedar pickets are the most common choice for their combination of pleasing appearance and natural insect resistance all for a reasonable price.  White cedar is a better choice if you will apply a translucent or semi-opaque stain as the color will be more predictable than red cedar – although many of the common stain manufactures have color samples for each of their stains on the common wood types.  Spruce or ‘pine’ pickets offer a more rustic appearance and are more commonly installed in 6inch widths.

Fasteners – Nails and Screws

Note the presence of copper in each of the pressure treatment methods – copper is necessary to provide the protection against decay and insects but it also has some negative effects.  Copper will corrode standard fasteners much faster than regular wood – fasteners should be hot-dipped galvanized or stainless steel materials (meeting or exceeding the requirements for ASTM A 153 Class D standard for durability).

What Species of Wood are used in Pressure Treated Lumber

The three softwoods below are commonly interchanged because their structural properties are very similar. In fact, the entire category is abbreviated SPF for spruce-pine-fir and may contain any of the varieties intermixed.

Pine and Yellow Pine

Pine is one of the most common woods used for privacy fences. The wood is affordable, yet durable. Pine is often treated with a water repellant stain to prevent rot and to make the wood last longer in wet climates. Pine can also be treated with CCA, or chromated copper arsenate, to make the wood resistant to insect and bug infestations.

Pine comes in all levels of quality, at different price points. The best Pine boards have no knots. This means that every part of board is usable when constructing your privacy fence. While it comes at a higher cost, it can often be more economical, since you don’t have to discard any material during construction. Pine boards with a knot-free surface also allow for easy treating with stains and coatings.

Fir and Douglas Fir

Fir is also a common, affordable and durable material used for privacy fences. Fir woods tend to have a more unique look than Pine. Fir wood can be extremely strong, while also quite easy to cut. Like Pinewoods, Fir can be treated with water repellant stain to prevent rot and extend the life of your fence. Fir can also be treated with CCA, or chromated copper arsenate, to have a similar insect resistance as more expensive varieties of wood.

Spruce

Spruce is most commonly used for prefabricated, stockade style picket fences. Like other standard quality woods, Spruce strikes a reasonable balance between affordability and durability. Spruce is not as prone to rot as Pine or Fir woods. To enhance the insect and bug resistance, Spruce can also be treated with CCA, or chromated copper arsenate.

Premium Wood Fence Posts

Whatever wood you choose, be sure to know how to treat and care for the wood to get the most out of your privacy fence. The right small touches can make any privacy fence look great for years. Here are three of the most popular woods with a higher level of quality that are also widely available:

White Cedar

Cedar is a solid wood with a middle-of-road price, high levels of rot resistance and unique coloring. Cedar is considered one of the most frequented fence materials because of its marble coloring and ability to absorb paint, stains, and other coatings very quickly and evenly. One of Cedar’s best qualities is that it contains natural oils to keep away insects. Keep in mind that Cedar needs to be treated with a clear stain to prevent turning grey over time.

While red cedar wood is slightly stronger than white cedar and also less prone to knotting, the real advantage is that white cedar has more oils so it is more resistant to insects and rot. White cedar fences outlast red cedar, sometimes by 10 years or more.

Cypress

Cypress mostly comes from the southern part of America so, depending on your location, it can be expensive to procure. Cypress is naturally rot resistant and will give your privacy fence a much longer lifespan than standard woods. Also, Cypress contains a natural chemical called cypretine that works great to naturally repel most types of insects. Like Cedar, Cypress needs to be treated with a clear stain to prevent turning grey in color.

Redwood

Once considered the gold standard for wood privacy fences, most people regard Redwood as the most beautiful wood to choose for a privacy fence. Redwood is extremely long lasting and holds up well under all weather conditions. Redwood is extremely resistant to bugs and rot, without needing any treatment. A clear coat is all you need to keep this beautiful wood in prime condition. Unfortunately the Redwood you could purchase 30 years ago is no longer available and was replaced with much lower quality wood.

Even so Redwood still holds its reputation and carries a price premium. Redwood is one of the most expensive woods available and not economical for large projects for most homeowners. However, Redwood is recyclable, so there may be old projects that you can salvage to obtain it cheaper. You can also save money by using redwood for your panels, and other standard quality woods for your posts.

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