Fastener Follies: Avoiding the Complications of Corrosion

A creak here, a groan there—the familiar orchestra of an aging deck. Though in most cases these noises are innocent, some may betray a deeper problem. Beneath your feet could be impending disaster, a backyard platform poised to plummet to the ground below—even if the wood comprising it is completely sound.

Corroded metal fasteners have been responsible for several deck collapses across the country, and tragically, decks seldom fail when they are unoccupied. Researchers at the Forest Products Laboratory (FPL), in cooperation with the United States Department of Transportation and the Federal Highway Administration, have been investigating metal fasteners, and their corrosion problems, for years. They found that although wood is generally not corrosive, copper-based wood preservatives can react with the metal components of the deck, and lead to compromised structural integrity.

Corrosion of a galvanized joist hanger and nails supporting a wood deck treated with a copper-containing wood preservative. This deterioration would be easily spotted during a visual inspection.

In 2004, changes in regulations saw an influx of wood treatments with increased copper content. Although effective at preserving the wooden components of external structures, they increase the incidence of corrosion.

When two dissimilar metals (for example, the nails in a deck and the wood’s copper coating) come into contact with one another, the electron exchange between the two materials begins the corrosion process. In addition to producing unsightly rust, this significantly weakens the metal.

Fortunately, there are several steps you can take to mitigate these hazards. Because dry materials do not react with one another, FPL stresses that, “proper moisture management is the most important thing one can do to reduce corrosion of metals in treated wood.” This includes preventing moisture from seeping in through the ends of wooden components (where it moves into the timber up to 10 times faster than from other directions) and designing roofs and overhangs so that they do not drain onto lower structures. Researchers maintain that, “if the wood is kept dry, both the wood and fasteners can last for centuries.”

Illustration of the importance of roof overhangs for protecting wood from biodeterioration and corrosion. The right side of the beam is protected by the large roof overhang, whereas the left side is exposed to rain.

Illustration of the importance of roof overhangs for protecting wood from biodeterioration and corrosion. The right side of the beam is protected by the large roof overhang, whereas the left side is exposed to rain.

Isolating the metals from one another is another step one can take. The most common way of doing this is through non-metallic coatings, such as those found on some screws or bolts designed for exterior use. Extreme care must be taken however when using coated metals in construction, as the coatings can be easily damaged during the installation process.

Finally, avoiding metal-on-metal contact altogether is a surefire method to prevent corrosion, but the hardest to implement. Although copper preservatives and metal nails are sometimes unavoidable neighbors in deck construction, being aware of metallic washers used on dissimilar metal bolts, or metal signs hung by metallic screws, can help put a damper on the corrosion process. Using non-conductive washers with metal signs or joist hangers, for example, can significantly decrease the speed of the corrosion and extend the life of the metal by decades.

Preventing corrosion is a multi-billion dollar industry in the United States, with over $100 billion spent in protective coatings alone. It is a problem as old as the material itself and wherever moisture and metal are found, corrosion is sure to follow. By utilizing proper construction techniques, moisture management, and, of course, regular inspections of your deck or home, your fasteners can last a lifetime—and your deck and family can be spared the tragic results of corroded metal fasteners.

For more information, please see the FPL’s Guide for Materials Selection and Design for Metals Used in Contact with Copper-Treated Wood.

Solid Research on Shaky Science: Building with Wood in Earthquake-Prone Regions

Nepal 2015—Japan 2011—Chile 2010.

In the past decade, these nations, and many others, have been host to some of the most destructive earthquakes in recent memory. Along with the inestimable human and emotional toll these events take on communities around the globe, the costs associated with reconstruction efforts are equally astronomical, often edging into the billions of dollars. Although we can’t effectively predict or stop earthquakes from occurring, we can be ready, and minimize the impacts of these damaging seismic events.

CLT concept and use in a nine-story mid-rise building in London.

CLT concept and use in a nine-story mid-rise building in London.

From relatively stable ground in Madison, Wisconsin, researchers at the Forest Products Laboratory (FPL) are searching for better ways to build more resilient, taller, safer, and cost effective wooden structures for use in earthquake-prone areas of the nation. For the wood building community, the most viable tall building construction solution incorporates the use of cross-laminated timber (CLT).

A CLT panel consists of multiple layers of kiln-dried lumber boards stacked in alternating directions, and bonded together with structural adhesives. The end result is an inexpensive, strong, solid, rectangular panel that can be used for building walls, floors or roofs.

CLT has already established itself as an important building material in Europe, but is relatively new to North America. Wooden buildings over eight stories tall, which incorporate CLT into their design, have sprung up in areas of low seismic activity to include Sweden, Australia, and the United Kingdom. Experts believe that CLT could also be a cost-effective and environmentally friendly alternative to traditional construction materials for buildings up to 125 feet tall.

FPL, along with the Coalition for Advanced Wood Structures, is developing seismic design perimeters for CLT use that will meet or exceed both design and safety codes. The team hopes that the project will lead to the development of a performance-based seismic design (PBSD) methodology to investigate the feasibility of three prototype systems. This PBSD would allow for the construction of buildings in earthquake-prone areas up to 14 stories tall using CLT components.

Researchers believe that this project will lay the ground work for buildings with elongated natural periods, near elastic behavior, and an increased resiliency to the high forces and accelerations of seismic events.

The growing trend of urbanization has increased the need for taller buildings across the country, including in areas that are crisscrossed by tectonic boundaries like California’s San Andreas Fault or the Cascadia Subduction Zone of the Pacific Northwest. At the same time, more emphasis has been placed on environmentally and fiscally responsible construction. Properly rated CLT construction holds great potential for cities like Los Angeles or Seatte—urban areas that have already witnessed tragedy in the past, but hopefully, when and if the next disaster occurs, can be headline-making cities for their successful implementation of safer building techniques.

For more information on, see this Research In Progress report.

 

Home Wreckers in Search of Moisture: Tips for the Homeowner

The work of FPL’s Durability and Wood Protection Research Unit is broad in scope and includes studies into damage and contamination by decay fungi, mold, and termites. All these household pests are attracted to excess moisture, which can result from inadequate surface drying of condensation, leaks in pipes and foundations, poor ventilation, or flooding.

Homeowners are increasingly concerned about moisture management and indoor air quality. However, chronic moisture problems in a home can lead to more than poor indoor air quality—persistent high moisture can lead to a cascading biological succession from mold to decay to termite damage.

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Blue-black color on walls shows evidence of mold growth. (Photo used with permission from A&J Specialty Services, Inc.)

Mold

Contamination with mold can render a home unlivable, and cleanup may require gutting the entire structure. In some cases, cleanup costs for toxic molds can equal the value of the home!

  • Mold occurs on the surface of wood exposed to excessive humidity or wet/dry cycling.
  • Visible mold growth is a good indicator of damp conditions or excess moisture.
  • Water vapor in humid air will not wet wood sufficiently to support decay fungi, but it will permit mold growth.
  • Mold, though unsightly, causes insignificant strength loss to structural wood components.
  • Common mold fungi can cause allergic symptoms; however, some molds (Stachybotrys sp.) produce mycotoxins, which cause illness and make homes uninhabitable.
  • New York City Department of Health and the U.S. Environmental Protection Agency have established guidelines for the assessment and remediation of mold fungi in indoor environments.

Joint Dimensions in Caulking: Yes, They Are Important!

The Ins and Outs of Caulking by the late Charles Carll reminds the homeowner that joint dimensions matter.

In narrow joints, a given amount of differential movement between substrates translates into relatively large strain rates in the sealant.

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A perimeter sealant joint around a contemporary flanged window. The joint is of appropriate width at 0.25 inch. The joint had been in service for roughly 3 years when the photo was taken. The joint is mostly intact, but some adhesion failure is evident at lower right corner. As is common in residential construction, neither bondbreaker tape nor sealant backer was used. Joint failure is likely the result of three-sided adhesion, an unprimed siding edge, and other-than-ideal sealant width–depth ratio (sealant depth exceeding width).

The photo shows a 0.25-inch-wide perimeter butt sealant joint around a residential window, which was in accord with the window manufacturer’s installation instructions.

Industry standards call for sealant joint depth that vary with joint width and sealant type. A generic rule for joints up to 0.5 inches wide is that joint depth should not exceed joint width. Minimum acceptable joint depth varies with the sealant type, and sealant manufacturers rarely if ever provide minimum depth recommendations to retail customers.

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Cross-sectional sketches of butt sealant joints.

With butt joints, some minimum depth dimension at the substrate surfaces is necessary for adequate adhesion. The hourglass shape of the sealant cross section that can be seen in the above graphic is considered desirable, as it provides the greatest possible adhesive-bond area at substrate surfaces and provides a region of relatively low stiffness at mid-width of the joint. Tooling of sealant (to be discussed in an upcoming post) results in surface concavity that provides in part for the hour-glass shape of the sealant cross section. With sealants that shrink during cure, concavity of the cured sealant joint surface is likely to be accentuated, and as a result, sealant depth at joint mid-width may be less than anticipated.

When using sealants that shrink, making some trial joints to identify cured sealant depth at joint mid-width can be instructive.

Butt and Fillet Joints

The Ins and Outs of Caulking defines butt sealant joints and fillet sealant joints. A butt sealant joint is a joint in which sealant is applied between two approximately parallel substrate surfaces that are either edge-to-edge or face-to-edge.

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Cross-sectional sketches of butt sealant joints.

A fillet sealant joint is a joint in which sealant is applied over (not into) the intersection between surfaces are approximately perpendicular to each other.

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Cross-sectional sketches of fillet sealant joints.

In a well-executed butt joint, the sealant does not adhere to any rigid material at the back of the joint nor does it adhere in the root of the joint. If sealant adhesion occurs at the back of a butt joint or in the root of a fillet joint, stress concentrations will occur in the sealant when there is differential movement between substrates. Joint failure will thus be likely, even when a high-performance sealant is used.

To prevent adhesion behind butt joints or in the roots of fillet joints, use non-rigid sealant backers or bond-breaker tapes. In commercial construction, caulking tradespersons are familiar with non-rigid sealant backers and bond-breaker tapes, and part of a tradesperson’s skill involves his or her ability to fit joints with backer or bond breaker (or both) before application of sealant. Unfortunately, residential construction contractors and home owners rarely pay attention to prevention of three-sided adhesion in butt joints or to sealant adhesion at the roots of fillet joints.

Hardware stores and home centers may sell sealant backer rods, but the variety of shapes and sizes is usually limited and virtually none of these retail businesses sell bond-breaker tape. An internet search will typically locate a handful of online merchants that market bond-breaker tapes to the general public. In retail home centers, backer rods are usually stocked with weatherstripping rather than with caulks and sealants.

No professional consensus exists on how long sealant joints in residential construction can be expected to remain functional. Professionals commonly believe, however, that the service life of residential sealant joints is usually shorter than 20 years. Manufacturers’ warranties of multiple decades of sealant joint performance only provide for replacement of the caulking material. Cost of application labor is not covered by the warranties, nor is the cost of repairing damage sustained as a result of a failed sealant joint.