Home & Garden Columns

About the House: The Merits and Problems of Pressure-Treated Wood

By Matt Cantor
Friday November 03, 2006

The construction world is in love with novelty. Every year, trade shows display the latest inventions and materials with promises of low cost, easy installation and life-long service. Of course, these things never turn out to be as true as presented and the buyer must always beware.  

I’m an old fashioned guy and I tend to like the time-tested and proven-by-abuse. I figure that if something can go wrong it will (Call me Murphy). That might make me a cynical crank (probably true) but it also makes me a great shopper. That’s another thing. I hate to do anything twice or to spend money on something that turns out to be a boondoggle. And so, with all of this in mind, let us turn, dear reader, to the latest in a seemingly endless series of new materials that may be causing unforeseen problems. 

This one is actually quite old and valuable in many ways but not without some serious concerns. It’s what I call Poisonwood and the industry calls Pressure-treated wood. Your house may contain some and if it does there are a few things that are good to know. If you’re building a house now or in the future, it’s a really good issue to explore since it’s being used widely in construction today and presents some special issues. 

Pressure-treated woods are used in places where fungal decay or insect consumption is at its highest. These places include ground contact or contact with damp concrete such as where the bottom of your house meets the foundation. This “mudsill,” which bolts to the foundation is now often made of pressure-treated wood.  

Many decks are built using this material as well, although I generally see it being used for the understructure and not the visible top components. In some parts of the country, foundations are actually made from this material alone (instead of a concrete footing) and other structures that may experience dampness can also be built or repaired using this innovative material. Actually, pressure treatment of woods isn’t all that new. Processes still used today go all the way back to the 1830’s and many other processes were developed around the turn of the last century.  

Materials used include copper (most methods involve copper in some form), ammonia, chromium and arsenic. Arsenic-containing compounds are used less today as a result of voluntary changes in the industry based on EPA studies that found elevated levels of arsenic in soils near construction (ergo my rather nasty term-poisonwood).  

There have also been concerns about workers sawing or handling these woods. CCA (chromated copper arsenate) isn’t used widely any longer as a result of the studies I just mentioned but can still be found in shingles, shakes, wooden foundations and some commercial construction.  

The most common form of PT (pressure-treated) wood seen today in the west is CA or Copper Azole. Although this seems somewhat healthier, there’s a fascinating thing happening with woods treated in this way …. they eat metal for lunch. This seems to have something to do with the copper/steel reaction that we also see with plumbing systems but the specific chemistry is beyond me.  

What I do know is that the lumber industry is aware, the hardware industry is concerned and a lot of contractors are not tuned in. I don’t blame the contractors. They’re busy cleaning up so that you’ll give them that progress payment on the kitchen remodel. It’s hard to stay current on all the issues. Also, the cities don’t seem too focused on the issue but then again, when inspectors have to see 20 houses in a day, how can they pay attention to new errata such as this. 

Here’s a little useful information on the issue and what you might want to do. First, take a look at your house with special attention to mud-sills, decking and other pieces of wood that are exposed to moisture. You’re looking for PT lumber and we’re going to look at the hardware connections. PT lumber tends to be greenish from the coppery treatments and can vary from light to dark. Some types of PT are bluish but you’re not likely to see those ones.  

Most PT has roller marks from a process called incising (like those front teeth of yours) in which the surface of the wood is punctured in longish slits that allow the chemical preservative to penetrate more deeply into the wood and help it to last longer. Most of what we’re protecting against in this process is fungus and there’s nothing dry about the rot that is eventually going to eat your stair stringer.  

When you find some of this wood, take a look at the hardware that holds it in place or keeps other things attached to it. Mudsills may be bolted down to a foundation and it’s worth looking at the bolts, washers (or square “bearing plates”) and nuts to see if they have a lot of visible corrosion. I’ve seen some relatively new construction in which the rust had grown prolifically in a short period and wondered if these connections were going to be providing much strength if an earthquake struck ten years from now. 

A bolt may have a great deal of excess thickness but a non-galvanized bearing plate may have long since become too thin and weak to do its job. My biggest concern with PT wood is for decks or balconies that are high enough to represent a falling hazard. I’d want to be sure that the hardware used for my 25’ high deck was really corrosion resistant and wouldn’t weaken over time. 

As I said, the industry is aware and has specific recommendations for hardware connections that involve use of PT wood. Hardware companies like the, ever astute, Simpson™ have gone further to provide analysis of the various levels of corrosiveness initiated by differing PT woods (there are at least 5 current methods of pressure treatment in use). They’ve also given us some special hardware to be used, such as their Z-max double galvanized hardware and stainless steel for the deeply worried. I’m just concerned enough so that I don’t want to see any conventional hardware or nailing being used connection pressure treated woods. I’ve seen the corrosion and I’m convinced that this is an issue that’s just beginning to present itself. Part of the reason for this is the change in formulations being used and part because PT wood is seeing much wider use than in the past. 

If your carpenter is using this wood, get them to wear a mask when they’re sawing and ask if they’re aware of the need for special hardware. These actions alone will be a good start but I’d like to take one more step and it’s a big step …. backward. 

Before PT woods were commonly being used, most settings that presented the same fungal propensities were addressed through the use of naturally pest resistant woods like Coastal Redwood. Although many of us have concerns about excessive cutting of Redwood, it is a fast growing tree that can be well forested and provide sustainable use (if the birthrate isn’t too out of control). It’s also non-toxic and very effective against both fungal damage and many insects (due to its tart, tannic taste). A good dense piece of Redwood heartwood can be quite effective and not lead to any discernable metal corrosion as the years roll by. 

If you want to spend big bucks on other fungally resistant woods, there is also the Western Red Cedar, Merbau, Huon Pine (used for building ships) and Ironbark, but you’ll only end up draining your bank account. So, If you’re building, talk to your contractor about these issues. I don’t think that pressure-treated woods are a bad choice any more than I think that an angiogram is a bad medical procedure. It’s just a matter of knowing what you’re getting and how best to proceed to get the good stuff and avoid the bad. 



Got a question about home repairs and inspections? Send them to Matt Cantor at mgcantor@pacbell.net.