Why can’t I install B-deck directly onto wood trusses?

Good news! You can install type b metal roof deck on wood trusses, but if the wood has been chemically treated, you will need to install a physical, water-resistant barrier between the wood and the metal roof deck to prevent accelerated corrosion.

Metal decking attached to wood trusses Why do I need a physical barrier between the metal deck and pressure treated lumber?

Current formulations used for pressure treating lumber that are intended to be more environmentally friendly have proven to be highly corrosive when in direct contact with steel due to the dissimilar metal that is present. Some of the more corrosive treatments are ACQ (Alkaline Copper Quaternary) and CA-A or CA-B (Copper-azole).

What physical barrier should I use?

While there may be other suitable options, the Steel Deck Institute performed tests with the Grace Ice and Water Shield and found that it served as an effective barrier between the metal deck and pressure-treated lumber.  For more information, please see the SDI position statement entitled Steel Deck Products in Contact with Pressure Treated Lumber that can be downloaded for free here.

What fasteners should I use?

Stainless steel fasteners provide the best corrosion resistance, but they are also the most expensive option.  Hot-dipped galvanized fasteners perform better than zinc-plated fasteners (which should not be used), but you should consult your fastener supplier or manufacturer for specific recommendations.

As always, please ASK D-MAC when you encounter an uncommon application for metal deck.  We’ll try to keep you out of trouble!