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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 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.
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. 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.
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.
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.
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” – RESIST that thought. 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 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.
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).
- Pressure treated posts should be rated for ground contact; AWPA grade UC4A. 4” x 4” x 8’ posts are suitable for most homeowner situations.
- 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
Reference: American Wood Protection Association