When using pressure treated lumber there are some things you need to know about how it effects metal fasteners -- in order to ensure your safety and the longevity of your home. 

First, you need to know that today's pressure treated lumber is far different than the treated lumber used and sold just a few years ago.  In 2002 the Environmental Protection Agency coerced the companies that produce pressure treated lumber to 'voluntarily' change their treatment methods.

The old treated lumber used an Arsenic-based preservative solution, and was known as CCA treated lumber  (chromated copper arsenate).  The new chemical formula is known as ACQ treated lumber (Alkaline Copper Quaternary).  Unlike the old formula, ACQ contains no arsenic but has high copper content. 

Switching to a high copper content created a major problem.  If you remember your high school chemistry, copper is extremely conductive.  That means ACQ lumber has a very corrosive affect on most metals -- like the nails and screws that are used to hold boards together.  The government and lumber industry initially ignored this potential safety hazard.  They started paying attention to it after people fell to their deaths when second and third story decks across the country began to collapse due to fastener failures caused by ACQ eroding the nails that held the decks together. 

ACQ is still in common usage today, in fact the majority of treated lumber on the market is ACQ.  Because of its corrosive nature, you must take special steps to ensure any metal that touches the ACQ treated wood will not develop corrosion problems. 

There are three main categories of metal material used when building something with ACQ.   

1)  Fasteners

“Fasteners” is a category that includes nails, screws, bolts, nuts, washers, anchor-bolts, etc.  It’s especially important that any fastener touching ACQ wood will be able to resist the corrosive ACQ preservative, so that it continues to “hold things together” for the life of your house. 

Galvanized fasteners are often used.  To be compatible with ACQ lumber they must be “Hot Dipped Galvanized” and display the code “G-185” (which refers to the thickness of the galvanization).  Different brands have different designations for this—such as “ZMAX” or “Triple Zinc”—but the brand names will vary by region.  It’s safest to confirm that the “G-185” code is also present.

Some companies produce metal fasteners that are coated with a special material that will also hold up fine.  The coating almost looks like a paint, or a thing plastic coating, and come in several different colors.  Deck screws often have a resistant coating, rather than being galvanized.

Stainless Steel fasteners can also be used, because good grades of stainless steel are considered virtually corrosion-proof, and in some locations (around salt water, for instance) this may be the preferred fastener material. One downside to stainless steel is the cost, because it is substantially more expensive than other options. 

Copper fasteners are also immune to corrosion by the ACQ formula, but are not a very practical alternative to use in building a home.

Avoid the fasteners that are labeled “electro-galvanized,” or are marked “G-90” or “G-60.”  They were designed to be compatible with the old CCA preservative, and won’t hold up to the new ACQ wood.

Not too paranoid -- Hot dipped galvanized G-185 nails and screws.

Somewhat paranoid -- Coated screws.

Very paranoid -- Stainless steel fasteners.

2)  Connectors

Metal connectors, such as joist hangers or post brackets, must also be able to withstand the corrosiveness of the ACQ chemicals. 

Some people feel that simply using the connectors with the thicker galvanization is sufficient—other people go to greater lengths.  The steps you take to ensure the longevity of your connectors will be largely determined by your level of paranoia. 

Not Too Paranoid --  Hot dipped galvanized G-185 connectors.

Somewhat Paranoid -- Use hot dipped galvanized connectors AND physically prevent the ACQ lumber from touching the connectors by using some sort of barrier.  Some people line joist hangers and beam cradles with 6-Mil plastic sheeting to prevent the wood from touching metal.  For instance, placing a layer of plastic in between the back of the joist-hanger & the beam it hangs from will prevent the hanger from touching the ACQ wood.

For the joist that goes into the joist hanger, just wrap the plastic on the bottom and sides of the joist (trimming off excess plastic after the joist is hung).  If you go that route you should not limit air circulation any more than you have to, and try to leave the end grain of the wood exposed to air. 

Quite Paranoid -- Use “Vycor Deck Flashing” as a barrier between connectors and ACQ lumber.  Vycor is a thick adhesive sheet of tar-like material.  It is substantially thicker than 6-Mil plastic.  Follow the procedures outlined on the product websites.  “Grace” is the manufacturer of the flashing, and “Simpson Strong-tie” is a manufacturer of metal connectors.

Fully Paranoid, and Willing To Forgo That Second Bathroom  (or—fully paranoid and recently won the lottery)  Use stainless steel connectors and fasteners for all ACQ lumber applications.

3)  Flashing

Any metal flashing used with ACQ lumber will also need to be carefully chosen.  The most common flashing material, aluminum, is not compatible with ACQ.  Either stainless steel or copper will be much more corrosion-resistant.  A plastic product should also work fine.

There's another corrosion issue that you should be aware of.  If two pieces of dissimilar metal touch, that can cause corrosion and eventual failure.  So, if you choose to use G-185 joist hangers, you’ll also need to use G-185 nails to fasten them with—not stainless steel.  If you opt for Stainless Steel post-brackets, remember to buy Stainless Steel bolts—not G-185 bolts.   

On any given construction site you're bound to have different versions of the same fasteners.  Make sure everyone working at your site knows not to simply grab the nearest nails to just “get the job done,” but instead that they look for the right nails for the specific wood they are working with (treated or non-treated).
 
Even if you follow all the proper rules regarding metal and ACQ you might still see some corrosion in the future.  If that happens don’t panic.  The thicker galvanization on the G-185 products is there for a reason.  The common adage is, a certain amount of white corrosion is to be expected -- it’s the rusty red discoloration that causes concern because that indicates corrosion of the steel UNDERNEATH the galvanization. 

One last piece of advice, "Trust But Verify."  It is important to confirm for yourself that the fastener or connector you’re about to buy is the correct one.  The kids down at the “Big Box” building supply stores may not know how to steer you in the right direction, and they may never have heard of the corrosive issues caused by ACQ wood. 

Normally the packaging will say "for use with ACQ treated wood," or "ACQ compliant" (or something to that effect).  If you have doubts then you can always call a manufacture to double check if your selecting the right product.  Their number is usually right on the box, and it only takes a few minutes to confirm you're buying the right item.

For more information about treated lumber and fasteners see the following resources:

Fastener Corrosion: Testing. Research, and Design Consideration

Corrosion of Fasteners in ACQ Treated Wood

 A Still Grainy View of ACQ-Treated Lumber