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Pressure-Treated Wood: Types, Grading Standards & More

Wood can be impregnated with a variety of preservative chemicals to prevent rot and insect infestation in exterior applications like decks. Several species of wood can be treated, but a regionally available species usually predominates in any particular locale.  Pressure-treated lumber is ideal for outdoor construction as it has a long, useful life span and is much less expensive than alternatives. Treated wood can last more than 40 years. The treatment process involves placing a load of lumber in a huge cylindrical chamber called a retort with a door on one end that can be sealed airtight, and then forcing waterborne chemicals into the wood under pressure.

The strength and properties of lumber from different species of wood varies considerably. Species like southern pine and douglas fir are often used for deck framing. There are a handful of species commonly used for structural framing, and while relatively similar, their properties affect how far they can span as joists. Span tables will list species separately for this reason.

Grade is a description of the quality of a given species. As lumber moves through a mill, grading inspectors review each piece and separate them onto different paths. Knots, decay, grain angle, wane, damage, warp and other physical characteristics of an individual piece of lumber distinguish it into a specific grade. The grades provide a tighter presumption of the structural performance of each piece. A beat-up, decayed 2x8 covered in knots won’t be as strong as a clean, straight-grained one. Common grades, from worst to best, are utility and structural select. Span tables will also list grades in separate columns to further fine-tune the maximum allowable span.

Pressure-Treated Wood Grading Standards

 Many lumberyards stock treated wood in the broad category of > or better. If this is the case, you may want to sort through the lumber piece by piece to find clear, straight boards. Because treated wood is often still wet when it is delivered to the site, it will shrink slightly in width and thickness as it dries out. This can cause significant twisting, cupping, bowing and warping especially in lower-grade boards where knots and uneven grains are already present. It is possible that some boards will become so deformed that they will be unusable. You may want to store lumber by fastening boards together using a method called stickering to allow boards to dry evenly. You may also purchase kiln-dried after treatment (KDAT), which is much more stable, but also more expensive.

Premium - Highest grade for ¾ radius edge decking.

Select - The highest grade available, contains very few detects. It must meet a minimum 1/12-grain slope and have all knots sound encased. It has a high consistency and the appearance is very good.

Number 1- Will contain no splits larger than the width of the board. Knots may not be larger than 2 ¾". One hole is permitted every 3 ft.

Standard - Mid-range grade for 5/4 decking.

Number 2 - Grain slope must meet minimum 1/8 grain slope. Boards will contain no splits larger than 1.5 times width of board. Knots may not be larger than 3 ½". One hole is permitted every 2 ft. Wane corners.

Number 3 - The lowest-quality grade. Not suitable for deck construction.

Species & Sizes Of Pressure-Treated Wood

Treated wood is generally available as dimensional stock in 2x4s, 2x6s, 4x4s and 2x2s for rail components. 5/4x6s and 2x6s for decking. 2x8s, 2x10s and 2x12s for joists, stair stringers and beams. 6x6s for support posts and plywood.

The predominant species of treated wood is a regionally available softwood.  Most common are:

Southern Yellow Pine - Southern pine is the most common deck framing material in the eastern United States. It is strong and stiff. SYP logs yield a high proportion of sapwood, which works well to absorb preservative.

Red and Ponderosa Pine - Less strong than SYP, found in northern U.S. and Canada. 

Douglas Fir – It is very strong and is less prone to warping and spitting than SYP. Predominant material found in western U.S. and Canada.

Hem-Fir – It is weaker and more prone to warping and splitting than Douglas fir, but more receptive to preservation. It encompasses a group of western species.

Retention Levels For Pressure-Treated Wood

Different applications impose different hazards on wood, and require different amounts of preservative for protection. These amounts are called “retention levels,” referring to the amount of preservative retained in the wood after treatment. Furthermore, each preservative has its individual retention level for these applications. Be sure to obtain wood treated for the application you have in mind. The intended use is identified on the tag stapled to each piece of lumber.

 AWPA Standards 

 Code Preservative Name     UC1, 2 UC3B UC4A UC4B
 ACC Acid Copper Chromate  0.25 0.25 0.50 
 ACQ Alkaline Copper Quaternary (Type B or C)  0.25 0.25 0.40 0.60 
 ACQ Alkaline Copper Quaternary (Type A or D)  0.15 0.15 0.40 0.60
 ACZA Ammoniacal Copper Zinc Arsenate    0.25 0.25 0.40 0.60
 CA-B Copper Azole, Type B  0.10 0.10 0.21 0.31 
 CA-C Copper Azole, Type C   0.060 0.060 0.15 0.31
 CuN-W Waterborne Copper Naphthenate  0.070 0.070 0.11
 CX-A Copper HDO  0.206 0.206
 EL2 DCOI-Imidicloprid-Stabilizer  0.019 0.019
 PTI Propiconazole-Tebuconazole-Imidicloprid  0.013 0.018
 PTI PTI plus Stabilizer   0.013 0.013
 SBX Inorganic Boron (Formosan termites)    0.28
 SBX Inorganic Boron (non-Formosan termites)  0.17

Use Category Brief Description

UC1 Interior Dry
UC2 Interior Damp
UC3A Exterior Above Ground, Coated with Rapid Water Runoff
UC3B Exterior Above Ground, Uncoated or Poor Water Runoff
UC4A Ground Contact, General Use
UC4B Ground Contact, Heavy Duty
UC4C Ground Contact, Extreme Duty
UC5A Marine Use, Northern Waters (Salt or Brackish Water)
UC5B Marine Use, Central Waters (Salt or Brackish Water)
UC5C Marine Use, Southern Waters (Salt or Brackish Water)
UCFA Interior Above Ground Fire Protection
UCFB Exterior Above Ground Fire Protection