The Right Fastener for the Job: Navigating Service Environments in Mass Timber

When selecting fasteners for mass timber projects, understanding service conditions conducive to corrosion—such as wood with high moisture content—is crucial. These factors directly impact fastener durability, making them a top priority for designers to consider.

In this article, we’ll explore dry and wet service environments and recommend suitable fasteners for each to ensure your connections stay strong and durable.

Approximately 6-minute read.

The first step in designing mass timber connections is understanding the environment your structure will face. Will the connection remain indoors in a dry, conditioned space, or will it be directly exposed to water or high levels of humidity for prolonged periods?

In short, assessing the anticipated moisture content (MC) of the wood in service is essential. It determines whether carbon steel or stainless steel is the right material for your fasteners, as each offers varying levels of corrosion resistance.
 

Dry Service Conditions

In both the U.S. and Canada, a dry service environment for wood connections means the timber’s MC is not expected to exceed 19% at any time throughout its service life. Canada goes a step further by requiring climatic conditions (i.e., temperature and relative humidity (RH)) that ensure the average equilibrium MC will remain below 15% on average over a year.

Typical dry service environments include:

• Most indoor or enclosed structures (Figure 1A)
• Most exterior structures where the wood is sheltered from precipitation (Figure 1B)

Figure 1. Common dry service environments

For dry service conditions, MTC Solutions provides carbon steel fasteners with regular corrosion resistance:

• Case-hardened partially and fully threaded self-tapping screws (STSs)
• Case-hardened self-drilling dowels

💡Pro tip: Ensure timber’s MC doesn’t exceed 19% during fabrication (unless stated otherwise in the construction document) to prevent connection strength loss due to wood shrinking or swelling.
 

Wet Service Conditions

If a wood connection is expected to experience an MC greater than 19% during its service life, it falls into the “wet” service category. This significantly increases the risk of corrosion, impacting fastener durability and longevity.

Common wet service conditions include:

• Exterior connections exposed to precipitation (Figure 2A)
• Sheltered connections permanently exposed to outdoor air in regions with prolonged high RH (e.g., Olympia, WA) (Figure 2B)
• Indoor spaces with high RH levels (e.g., indoor pools and greenhouses) (Figure 2C)

Figure 2. Common wet service environments

For wet service conditions, we recommend fasteners with enhanced corrosion resistance, such as:

• A2 Ecofast and A2 SK stainless steel fasteners—ideal for typical wet environments (contact our Technical Support Team for large orders)
• A4 stainless steel fasteners—best for extreme corrosive conditions like those involving saltwater, aggressive chemicals, pressure-treated lumber, or submerged wood members (custom orders only; contact our Technical Support Team for assistance)

⚠️Why electroplated carbon steel screws shouldn’t be used in wet service conditions:

• The electroplated zinc coating on carbon steel screws isn’t thick enough to protect against galvanic corrosion over the design life of the structure.
• Carbon steel screws, with a core hardness greater than 360 HV, are susceptible to hydrogen embrittlement (HE) in wet conditions. Learn more from our articles on internal and environmental HE.

⚠️Why hot-dip galvanizing (HDG) isn’t used for corrosion protection in STSs:

• HE risks: STSs are made with high-strength steel, making them unable to safely complete the acid pickling process needed for HDG without the risk of internal HE.
• Exposure to high temperatures: As case-hardened steel fasteners, STSs cannot survive the high temperatures experienced during HDG without adverse effects on their mechanical properties.
• Thread precision issues: Thick HDG coatings (typically 43–86 microns, per the American Galvanizers Association) dull the cutting surfaces of STSs, affecting their self-tapping ability.
• Seating challenges: Thick HDG coatings can interfere with proper driver bit engagement in the screw head.
• Higher torque: The rough surface texture of HDG coatings increases drive-in torque, which can increase user fatigue and prevent STSs from being fully installed.

Figure 3. Service conditions and fastener options at a glance

For tips on protecting your connections from corrosion, check out our other article, Tips for Preventing Fastener Corrosion in Mass Timber Connections.

💡Note: Manufacturing complexities can affect the cost and availability of carbon and stainless steel fasteners in different diameters and lengths. For more details, contact our Technical Support Team and read our article on preventing fastener corrosion.
 
 

Choosing the right fasteners means accounting for service conditions and corrosion risks. By following best practices, you can ensure durable, high-performance mass timber connections. Contact our Technical Support Team for fastener recommendations tailored to your project’s environment—or for any other questions you might have! 😉