Don’t Get Caught with a Floating Coping
Donald E. Smith, CCS
June 2009During the meeting of AWCI’s Interior & Exterior Steel Framing Committee at the AWCI Convention in March, the question was raised about the use of pressure treated lumber as a nailer on the top of a parapet because the chemicals used to treat the lumber corroded the steel runner at the top of the parapet. There is nothing like having a floating coping at the top of a wall. Don Allen of the Steel Stud Manufacturers Association was at the committee and offered some good advice about how to handle the problem. I thought it would be helpful to fill in a little more background on the subject.
As you may be aware for many years the primary treatment used to treat dimension lumber for both commercial and residential project was Chormated Copper Arsenate. If I correctly recall, there were two types of treatment: one for above ground exposure and one for below grade exposure. While the treatment was used to prevent premature deterioration from termites, other insects and fungal decay, it also was used to prevent decay from exposure to water and weather. We all know what happens to untreated wood exposed to the elements. CCA treated lumber was used extensively for deck construction as well as fencing with great success. All was well until someone, a homeowner, decided to sand his deck to improve its appearance. As the story goes, the homeowner’s toddler was playing on the deck while the sanding was in progress, and in the course of events ingested some of the dust generated by the sanding. Damages were claimed and the production of the CCA treatment was suspended.
In place of the CCA treatment the industry introduced new treatments. The waterborne treatments currently on the market include Sodium Borate (SBX/DOT), Chormated Copper Arsenate Type C (CCA-C), Alkaline Copper Quat Type D Carbonate (ACQ-D Carbonate), copper Azole Type B (CA-B), copper Boron Azole Type A (CBA-A) and Ammoniacal Copper Zinc Arsenate (ACZA). There are also many variations of these treatments available. The most common treatments are ACQ and CA. Testing has indicated that ACQ, CA and ACZA are more corrosive to galvanized steel than the old CCA treatment. This has caused significant concern to the manufacturers of wood fasteners, connectors and metal plate connected trusses. There is also an impact on steel framed buildings.
So what is being done about the situation, and what can you do to ensure you don’t get caught with a floating coping? Based on information contained in Issue Paper No. 4 from the Steel Framing Alliance, increasing the galvanized coating thickness on the steel to a G90 coating, which is not available for the framing members commonly used in this situation, has not been shown to effectively counter the corrosive effects of ACQ, CA or ACZA treatments. SFA Issue Paper No 4 has three recommendations to follow for using pressure treated wood in conjunction with steel framing:
First, use lumber treated with SBX/DOT. The issue paper says to specify this treatment, but since we are not specifiers of construction materials you need to be aware of the material specified for blocking by the architect/engineer. If in fact the treatment is something other than SBX/DOT, that means it’s time for an RFI. You might also want to discuss this with the general contractor since you might not have the blocking in your contract. In fact, I’m willing to bet that it’s not in your contract. You can identify pressure treated wood by its green color; however, the treatment type might not be included in the grade stamp. The last PT wood I purchased, the treatment process was on the SKU tag, which you will probably never see.
The second recommendation is to isolate the wood from the steel framing member. This can be accomplished with a polyethylene barrier or other non-absorbent barrier material. You will have to ensure that the integrity of the barrier is maintained during the construction process. The other caution is that the fasteners normally used—self-tapping screws that are not hot dipped galvanized—are not recommended for use with pressure treated wood. In a previous column I addressed the requirement by California building code officials that stainless screws would have to be used for wood structures using pressure treated wood due to failures of wood decks.
As usual, I have saved the best for last. The final recommendation from SFA is to avoid the use of pressure treated wood altogether. Since the wood blocking is encapsulated by the metal coping, it does not need to be treated. Building codes do not require a sill plate used in a steel framed structure, and when needed, relatively inexpensive barrier products can be used. Wood top plates are generally not required either. Avoiding the use of pressure treated wood provides the greatest assurance against a floating coping.
Donald E. Smith, CCS, is AWCI’s director of technical services. Send your questions to firstname.lastname@example.org or call him directly at (703) 538.1611.