A.Chris Paterson, an engineer in Simpson Strong-Tie’s research and development department, responds: Because new wood preservatives such as ACQ-D and CA-B are roughly twice as corrosive as CCA-C, galvanized metal connectors and fasteners used with PT wood require thicker protective zinc coatings. So in addition to offering “continuous” G90 hot-dip galvanized coatings (where a layer of zinc is applied to both sides of sheet steel at a coating weight of 0.90 ounce of zinc per square foot) for its standard products and G185 (1.85 oz/ft² of zinc) coatings for its ZMAX connectors and fasteners, Simpson sells post hot-dip galvanized HDG connectors with approximately 2.0 oz/ft² zinc coatings.
In the continuous hot-dip galvanizing process, coil and sheet steel is coated with molten zinc prior to fabrication. Post hot-dip galvanized products are dipped in a molten zinc bath after they have been fabricated.
How much corrosion resistance a connector needs depends on where it will be used and the type of preservative used in the lumber (see chart). For example, in a dry interior environment — the kind of location where STHD straps would be found if they were installed correctly, with the strap above the concrete and behind the housewrap and siding — our testing has shown that a G90 coating offers plenty of protection as long as you’re using dry (less than 19 percent moisture content), ammonia-free ACQ-D lumber, or wood treated with sodium or zinc borate. But if the treated ACQ lumber has a moisture content higher than 19 percent, or if the ACQ also contains ammonia (more common on the West Coast, because the ammonia solution allows ACQ-B to penetrate difficult-to-treat species like Douglas fir) and has a retention level lower than 0.40 pcf (pounds of preservative per cubic foot of wood), then you should use HDG STHDs and fasteners, which are available by special order. Alternatively, you could use the G90 STHDs along with an approved peel-and-stick barrier membrane, such as Grace Vycor Deck Protector — but be sure to use HDG fasteners. For ACQ with retention levels of 0.40 pcf or greater, stainless steel connectors and fasteners should be used (though Simpson STHDs aren’t available in stainless steel).
For more information about Simpson’s connector coating recommendations and barrier membranes, go to strongtie.com and look for technical bulletins T-PTWOOD08-R and T-PTBARRIER08-R.