What Is Kiln-Dried Lumber?

What Is Kiln-Dried Lumber?

Landscape architects can specify several different types and grades of lumber for landscape construction. Two common forms construction-grade lumber are green lumber and kiln-dried lumber.

Green Lumber

Green lumber is higher in moisture than kiln-dried lumber

Green lumber has an average moisture content above 19 percent. As green lumber dries, it can shrink, warp or twist. Green lumber is usually less expensive than kiln-dried lumber and can be used where movement will not weaken a structure or wreak havoc on a design detail.

Kiln-Dried Lumber

Kiln-Dried Lumber leaving the kiln

Kiln-Dried lumber has a much lower moisture content than green lumber.

In the case of kiln dried lumber, the wood is dried in giant kilns with carefully controlled temperature and humidity levels. The goal is to get the boards to dry quickly and evenly, and to prevent warping that could render the wood unusable. The alternative is air drying, in which lumber is allowed to sit in a breezy area so that it loses moisture and contracts. Air drying tends to take longer, and it brings the moisture content closer to 15% than the desired 6% to 8%. —Source: Wise Geek

As a result, kiln-dried lumber will move less during and after construction.

Kiln-dried lumber is marked with a grade stamp that indicates the lumber has been dried before leaving the sawmill.

Lumber Grade Stamps

Lumber Grade Stamp

Kiln-dried lumber will be stamped with the mark “KD” or “SDRY”. If the lumber is imported, it will also be heat-treated to kill any pests. Imported wood is marked “KD-HT” to indicate that it is dried and heat treated. All kiln-dried wood will also have a certification mark which indicates which organization’s standards which the wood complies.

Some other abbreviations that you may notice on the lumber is the mill code and the species mark.

The following species are organized from strongest to weakest:

  1. Doug Fir, D Fir-L: Douglas fir (L=western larch)
  2. SYP: Southern pine
  3. Hem, Hem-fir, H-F: Hemlock or fir
  4. S-P-F: Spruce, pine, or fir

Kiln-dried wood moves less and is preferred for structural work. While kiln-dried wood is useful for some applications in the landscape, pressure-treated wood is more commonly used where exposure to moisture or soil are likely.

The California Supplemental Exam has several questions on construction detailing and questions about wood may be on the exam. Know the common wood species used for landscape construction and their properties.

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John is a landscape architect who is currently preparing to take the California Supplemental Exam to become licensed in California. He is currently a licensed professional landscape architect in Georgia and Florida. John graduated from California State University, Pomona with a BSLA degree in landscape architecture in 1997 and has extensive practice experience in residential and commercial landscape design.

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