Design

Treated Wood

Many post-frame buildings use treated wood embedded directly in the ground to form a permanent wood foundation. Post-frame buildings that use the proper treated wood products suitable for post-frame foundations have a long history of success.

Many people think all treated wood is the same, and are not aware there are many types of wood commodities available that have been treated in various ways to meet specific purposes. Because decks and playground equipment are not generally considered crucial building structural elements, treated wood for those applications is not produced to the same specifications as for post-frame buildings. Properly treated and installed posts will ensure a post-frame building will last.

The USDA Forest Products Laboratory has conducted long-term tests in the harshest environments found within the continental U.S. Chromated Copper Arsenate (CCA) type II at retention level .26 pcf treated test stakes have showed no signs of failure since their installation in 1949. Modern post-frame buildings typically use the newer, improved formula CCA type III at retention level .60 pcf.

Specifying Treated Wood by Use

Wood treatment is governed by standards set by the American Wood Protection Association (AWPA). Treated wood for post-frame skirt boards should be treated to AWPA use classification standard UC4A; a larger amount of treatment chemical is forced more deeply into the wood for UC4A than is for deck, playground and other common uses. Laminated columns and solid-sawn posts should be treated to at least UC4B, or a minimum of .60 pcf CCA. If you feel an extra level of wood protection is needed, you may order wood treated to UC4C, or .80 pcf CCA. Kiln Drying After Treatment (KDAT) also improves the performance of treated wood products.

Employ a quality contractor who will use the proper materials – a minimum of .60 CCA or UC4B for columns, and a minimum of UC4A for skirt boards and other members above ground.

AWPA now organizes standards according to its Use Category (UC) system.

CCA-Treated Wood Still Allowed by Use

CCA-treated wood is still allowed in most types of post-frame construction in accordance with the 2001 AWPA Treated Wood Commodity Standards, which may only be applied to CCA. The table below describes where CCA may be used, according to EPA guidelines.

Solid-Sawn Timbers and Laminated Columns

Some builders prefer solid-sawn columns, and others prefer the performance of laminated columns.

Solid-sawn posts were key to the early development of the post-frame industry. With improved treatment techniques and new chemical preservatives, solid-sawn posts offer an economical and practical solution to quickly erect a post-frame building.

The species and grade of solid-sawn posts used may affect a building’s quality. Solid-sawn posts of poor quality may warp, twist, split or become otherwise dimensionally unstable. Quality builders who use solid-sawn posts select high-grade timbers to ensure greater dimensional stability and quality.

Southern Yellow Pine is one of the easiest species of lumber to treat because it contains more sapwood and less heartwood than many other species. Heartwood is more difficult to treat than sapwood. Lacerations may be mechanically etched into species with more heartwood, so treatment chemicals may penetrate more deeply into wood fibers. Treaters who produce wood commodities for post frame should conduct random core sampling to ensure their processes are providing the amount of chemical penetration needed to comply with wood treating standards.

Instead of solid-sawn posts, many post-frame builders today use laminated columns. Laminated columns typically consist of three or more plies of 2x dimension lumber fused into one large member using nails, glue, or both nails and glue. Laminated columns made specifically for post frame may have warranties that span several decades, or that offer a lifetime guarantee.

Mechanically-laminated columns are used in many post-frame buildings. Laminated columns may be fabricated to any length by splicing shorter pieces of wood together. They are fabricated to be straight and are inherently more stable because of the laminating process. Finger-jointing may increase the stiffness characteristics of a laminated column. Laminated columns are able to deal with increases in bending moment because weak areas in one layer of a laminated assembly are supported by adjacent layers.

Because laminated columns are composed of lumber that is usually less than 2” wide, there is a larger surface area for treatment chemicals to pass through compared to solid-sawn posts. A laminated column will generally have more chemical retained within it than a solid-sawn post of the same size. Also, smaller members are likely to have more sapwood, which is most amenable to treatment chemical penetration.

A quality builder will know manufacturer recommendations for fasteners and connectors to be used with treated wood and electrical barriers that may avoid corrosion of panels. Stainless steel fasteners and panels with a thick galvanizing layer provide the best corrosion resistance.