Monday, 5 May 2014

Sheet Steel Products and Pressure Treated Wood

Many buildings will include wood members in applications such as sill plates, splash boards, strapping, purlins, door or window bucks, and posts. In some of these end-uses it is a requirement that the wood be chemically treated (pressure treated) to extend the service life.

Designers and builders need to be aware that changes in the available wood preservatives may impact the durability of any connected steel components or fasteners.

Effective January 1, 2004 the Environmental Protection Agency (EPA) banned the use of Chromated Copper Arsenate (CCA) as a preservative in treated lumber for residential construction. This was done in an effort to reduce the use of chromate and arsenic thereby mitigating the potential health and environmental problems. The wood preservative industry has been switching to alternative waterborne compounds including Sodium Borate (SBX), Alkaline Copper Quat (ACQ), Copper Azole (CBA-A and CA-B), and Ammoniacal Copper Zinc Arsenate (ACZA).

Unfortunately, research has indicated that ACQ, CBA-A, CA-B and ACZA, the new generation copper-based products, are more corrosive to galvanized steel than the former CCA. Since ACQ is becoming the predominant preservative in use, the discussions in this paper will refer to it exclusively.

Here are the recommendations for using sheet steel building products and pressure treated wood.

Sheet Steel Roong and Siding

In the construction of a wood frame building (i.e. pole barn), pressure treated wood is commonly used for the framing poles, roof purlins and skirt boards. The balance of the building wood components (i.e. trusses, strapping and girts) are from non-treated lumber.

It is common practice to attach the roong or siding to strapping or sheathing (i.e. plywood or OSB) that are in turn attached to the pressure treated wood structural members. Strapping and sheathing are generally not made from pressure treated wood and standard installation practices can be followed. The connection of the strapping or sheathing to the pressure treated framing requires added consideration.

It is recommended that all galvanized or painted sheet steel roofing or siding products be separated from ACQ pressure treated wood with Ice & Water Shield membrane or similar product. The use of roofing felts is not an adequate separator.

Lightweight Steel Framing

The following are options for cold-formed steel framing that should be considered:
  • Isolate the steel and wood components
  • Avoid use of pressure treated wood

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