Code Listing for Modified Wood Deck and Rail Products

All load-bearing products used in structural applications must have a code listing to ensure their adequacy for use.

The International Code Council-Evaluation Service (ICC-ES) develops Acceptance Criteria (AC) that provide explicit instructions for obtaining a code listing. This paper focuses on Modified Wood products used in deck and guardrail applications.

Modified Wood Deck and Guardrail Products

The decking market has several Modified Wood products from which to choose.

ICC-ES Acceptance Criteria AC47 (1) defines three types of Modified Wood in Section 3.7.2.

3.7.2 Modified Wood Used for Deck Boards and Guardrail Systems:

For the purposes of this criteria, thermally modified wood is defined as solid sawn hardwood or softwood that has been subjected to an advanced dry kiln process that substantially reduces hemicellulose content, acetylated wood is defined as solid sawn hardwood or softwood that has been subjected to a chemical modification process in which acetyl groups are covalently bonded to accessible hydroxyl sites within the cell walls of the wood, and furfurylated wood is defined as solid sawn hardwood or softwood that has been subjected to a chemical modification process in which furfuryl alcohol is polymerized in the wood micro-structure resulting in a permanent bulking of the cell walls.

All three processes to manufacture Modified Wood described in Section 3.7.2 of AC47 permanently change the mechanical and physical properties of the resultant product.

To obtain and maintain a code listing, Modified Wood decking products must be evaluated (by an accredited testing facility) according to AC47.  The data collected from the testing is used to determine an allowable span for the product. In addition, AC47 includes requirements for ongoing daily quality control and appropriate labeling of the outgoing product.

Currently, it appears that only furfurylated wood has a code listing through AC47.

For thermally modified wood, the process parameters of thermal modification are particularly critical. Depending upon the time and temperature of the modification process, flexural stiffness typically sees minor changes, however; the flexural strength can see reductions in excess of 50 percent (2). In addition, the thermally modified wood can become very brittle (i.e., significant ductility reduction) (2,3). Impact bending strength has been found to decrease by 80 percent (2).

Conclusions

Building Inspectors are obligated to reject the use of any structural-use product that does not have a building code listing. Without an ESR, there is no assurance of code compliance. Should a product, without a code listing, become part of a legal proceeding, those who manufacture, specify, or sell the product have little or no defense.

The bottom line is simple. For those specifying, selling, or inspecting Modified Wood products for decking and guardrail applications – demand to see a code listing document.

References

1. International Code Council-Evaluation Service. Acceptance Criteria for Wood-Based Products. AC47. May 2023.
2. Militz, M. Altgen. Chapter 16, Wood Biology and Wood Products, George-Aufust-University Goettingen, Buesgenweg 4, 37077 Goettingen, Germany. American Chemical Society: Washington DC 2014.
3. Zelinka, M. Altgen, L. Emmerich, N. Guigo, T. Keplinger, M. Kymäläinen, E. Thybring, L. Thygesen. Review of Wood Modification and Wood Functionalization Technologies. Forests 2022, 13, 2022,13,1004.