Abstract Summary: This blog post discusses the concerns and suggested solutions to the problem of galvanic action between wood preservatives (such as copper) and steel in damp locations, such as roofing assemblies.
Years ago I was working on a call center in San Antonio which had a difficult but constructible ‘flat’ roof design.
On the project I had a roofing consultant, who was putting together the specifications, roofing drawings, and providing a general envelope review. In my review of the detailing, I noted that the blocking needed to be treated. The roofing consultant noted that he would never put treated wood into roofing assemblies where metal was being used (such as fasteners, coping, etc.), as he noted it as a risk. Being that he was a certified roofing designer, who was I to argue?
Traditionally in roofing assemblies the standard of care has been to include treated wood nailers and blocking as indicated by manufacturer details, though these are typically shown as ‘by others’. As architects, we tend assume we hold less risk if we follow manufacturer details – but does this also cover items indicated ‘by others,’ such as wood blocking? In the mid-2000s there was a switch from the wood preservative CCA to ACQ as due to the concerns of chromate and arsenic in CCA. ACQ however has elevated levels of copper, which now presents the risk of galvanic action between preservatives in wood (primarily copper) and steel or aluminum in both miscellaneous metals – such as flashings, copings, and fasteners – which could fail due to corrosion.
According to the National Roofing Contractors Association, these corrosion related concerns may outweigh the benefits of preservatives in wood blocking:
NRCA now is of the opinion that the corrosion-related concerns regarding the use of the current generation of treated wood (ACQ) possibly outweigh the benefits treated wood provides as a component in roof assemblies. In many instances, the use of nontreated, construction-grade wood is suitable for use in roof assemblies as blocking or nailers, provided reasonable measures are taken to ensure the nontreated wood remains reasonably dry when in service. Where a specific construction detail provides for a secondary means of waterproofing, NRCA now considers the use of nontreated, construction-grade wood to be an acceptable substitute for treated wood. Construction details depicting such a secondary means of waterproofing are provided in the Construction Details section of The NRCA Roofing and Waterproofing Manual, Fifth Edition. (http://www.nrca.net/roofing/-Special-Report-Use-of-treated-wood-in-roof-assemblies-February-2005-260)
A similar concern is shared by the Steel Framing Alliance, who found through the AWPA Standard E12-94, that ACQ had an accelerated rate of corrosion (by a factor of 2) compared to CCA, with the worst culprit being ACZA, or ammoniacal copper zinc arsenate. (http://www.steelframing.org/PDF/issuepaper/pressure.pdf).
Even the 2012 International Building Code requires specific fasteners where used with preservative-treated woods – see section 2304.9.5.3 “Fasteners for fire-retardant-treated wood used in exterior applications or wet or damp locations,” though per trade literature, copper and bronze should probably be avoided:
Fasteners, including nuts and washers, for fire-retardant-treated wood used in exterior applications or wet or damp locations shall be of hot-dipped zinc-coated galvanized steel, stainless steel, silicon bronze or copper. Fasteners other than nails, timber rivets, wood screws and lag screws shall be permitted to be of mechanically deposited zinc-coated steel with coating weights in accordance with ASTM B 695, Class 55 minimum.
So, being the case, what can be done to resolve the issue? Three specific strategies tend to be supported by multiple sources:
- Avoid the use of treated wood (or any wood) in roofing assemblies. New products, such as the Hickman Edgebox use galvanized metal and lightweight concrete, can be used in lieu of wood nailers, eliminating the problem of corrosion all together. However, note that alternates should be reviewed by third-parties and roofing manufacturers for acceptability and compliance with warranties. There is also the option to use non-treated lumber in roofing assemblies, as was recommended by both the NRCA and my call center roofing consultant.
- Use stainless steel fasteners and metal where metals are exposed to treated wood. In other locations, use roofing material as a break between the treated wood and metals.
- Use a lesser corrosive, but still available, treated wood option, such as SBX. Note that SBX however does not hold up as well in wet conditions and may not be available in all locations. If using SBX, please do your due diligence before specifying.
Written by Ryan Collier, an architect at Integrated Design Group
- North American Deck and Railing Association “ACQ Wood Preservative and Fasteners – Are they Compatible?”http://nadra.org/industry_news/acqarticle.html
- Simpson StrongTie “Preservative Treated Wood FAQs”http://www.strongtie.com/productuse/PTWoodFAQs.html
- International Building Code