ASSY carbon steel fasteners are manufactured with zinc passivated or yellow chromate surface coating to resist corrosion, in spite of this the carbon steel fasteners are only intended for use in dry service conditions. Dry service conditions are met when the average equilibrium moisture content of wood remains below 15% over a year without exceeding 19% at any given time.
If these conditions are not met for the full life span of the structure, the structure needs to be designed assuming wet service conditions. In the case of wet service conditions, it is necessary to use the corrosion-free ASSY A2 Stainless Steel fasteners: A2 Ecofast, A2 SK, and the A2 Kombi self-tapping screws. While ASSY A2 Stainless Steel screws are suitable for wet service conditions, the use of CLT in wet service conditions is generally restricted. In North America, the use of CLT in wet service conditions is only possible when specifically permitted by the CLT manufacturer, as per NDS 10.1.5 and CSA 086 8.3.2. The use of glulam and timber is permissible in wet service conditions but capacities of wood members and connections are reduced by a service condition factor (CM or KS). In wet service conditions, it is also important to account for contact corrosion if the connection includes two metals with different electro-chemicals.
Corrosion issues may still arise even in sheltered outdoor applications due to wind-driven rain, splashing, or poor air circulation that leads to there being high moisture content in the air. The wood must be protected from direct wetting and be allowed to dry effectively in case of accidental wetting. When possible it is a good idea to factor accidental wetting into the design, for example in a location such as the underside of a bridge. The bridge may initially be protected from direct wetting, but cracks formed in the deck over the service life of the structure may allow rainwater to penetrate through later on. Engineering judgment and careful consideration of both the surrounding conditions and structural detailing is required when specifying screw types.