Screws for Pressure-Treated Wood
Info About Fasteners to Use With New ACQ and CA Treated Lumber
The new pressure-treated lumber is corrosive to some screws and fasteners. Here's what you need to know when building with treated wood
Sitting at the picnic table on your backyard deck, staring across the rows of fences dividing one suburban yard from another, you’d almost think that wood was a greenish colour. After all, in the last few decades countless outdoor projects have been built with green-tinted pressure-treated (PT) lumber.
But the PT lumber from a few years ago isn’t the same as the stuff on the building centre shelves today. The once ubiquitious formulation, chromated copper arsenate, or CCA for short, has relatively recently been replace with alkaline copper quaternary (ACQ) or copper azole (CA) mixtures. (It’s the copper content in all three that give the lumber that decidedly unnatural shade of green.)
Over the years a number of studies and reports suggested that the arsenate (which is in the same family as arsenic) could leach out of CCA treated-lumber, potentially poisoning the soil in everything from playgrounds to vegetable gardens.
While the PT industry continually denied there was any reason for concern – and produced studies of their own to that effect – manufacturers eventually gave in to public pressure and voluntarily phased out CCA wood for consumer lumber sales by the end of 2003.
Good news for concerned parents and gardeners. But there’s an unfortunate side effect to the new formulations: They’re extremely corrosive to certainly types of nails, screws, and other hardware. The culprit is the same copper that gives the wood its green hue. Both ACQ and CA use higher amounts of copper to replace the arsenate in CCA wood. The problem is that water (from rain or your hose) will pick up trace amounts of copper and, if it comes into contact with plain metal screws, it will begin to corrode them. Obviously, corroding screws holding your deck railing in place can be a serious hazard.
So what should you use? Stainless steel is the number one recommendation for fastening PT wood. Next down the list (though likely more popular because it’s significantly cheaper) is hot-dipped galvanized fasteners. (The zinc coating on electric galvanized fasteners is too thin to last the typical lifetime of a deck.)
The easiest way to make sure you’re getting the right stuff is to look for packaging that indicates the product is “ACQ approved” or specifies that it is hot-dip galvanized.
For connectors like joist hangers and post anchors look for brands that say “triple zinc” coated or G185.
What if you’ve already built a deck, for example, using the wrong type of fastners? A contact at Georgia-based ACQ-preservative manufacturer Osmose, recommends that people who have used the wrong type of fasteners either replace them with stainless or ACQ-approved fasteners, or at least beef up the existing ones, particularly with projects where fastener failure could cause injuries. With less critical projects, like garden walls, monitor the hardware for signs of red rust and replace them as needed.
