Pressure-Treated Wood and Corrosion. Is your wood eating your screws?
Summary: Pressure Treated Wood and Corrosion of fasteners has received more attention in recent years. Enclosurists should be aware of the risk of galvanic corrosion of fasteners due to the copper used to preserve the wood. Keep this in mind during design, peer reviews, and repairs. Coated, and Stainless steel fasteners are recommended instead.
Table of Contents
Background
Many of us has done it before at some point in our lives.
Doesn’t matter if it is a shed or a bench or a planter. You wanted to build something that takes 3 days to make, but since you are more capable than most people (and didn’t really think it through), you believe you can do it in one.
Halfway through, you realize that you don’t have the correct wood for the base, but since nothing can stop you now, you decide to leave untreated wood in contact with the soil.
You probably didn’t use those words, but when you decided to shove that 2-by between the dirt and the other 2-by, that is precisely what you did.
Since you are technically minded, you might have even put a piece of plastic under it to serve as “damp-proofing” because you are, after all, an ‘Enclosurist’…
A few months later, the universe rewards you with an education, and you learn a practical lesson on why it is essential to do things the right way.
Untreated wood in contact with wet soil will very often deteriorate beyond redemption.
The solution: Pressure treated lumber
The use of Pressure-treated lumber (PT) is a well-known strategy to reduce the potential for deterioration and wood-destroying insect activity in lumber.
During the 1930’s Chromated Copper Arsenate (CCA) was invented as lumber treatment.
While CCA is excellent at protecting wood (i.e., killing things), arsenic is also excellent at killing people and therefore considered hazardous to human health. (The Europeans were concerned about the Chromium.)
The lumber industry agreed to halt the use of CCA at the end of 2003 and had to find alternative wood treatment. This alternative involved copper. (Think about the greenish color of pressure-treated lumber.)
Copper naphthenate has broad efficacy against wood-destroying insects as well as the fungi that cause wood to decay. It is also less toxic to humans. Copper azole (CA) and Alkaline Copper Quat (ACQ) were used to treating wood instead of the toxic CCA.
Copper and the galvanic corrosion of fasteners.
As we all know, fasteners are used to hold things together.
Framing without fasteners would just be stacking, and stacking in the construction industry is generally not good.
It is particularly bad when you build an elevated stack.
It is really bad when you invite friends over to grill meat and drink beer on top of your elevated stack of wood.
That is the reason we use metal fasteners and connection hardware to transform stacks into structures, e.g. decks for example.
Many fasteners are made of steel and work really well in turning stacks into structures.
What happens when we bring copper into the mix?
“Its like a bad game of rock-paper-scissors where wood beats metal.”
Copper and steel are on different sides of the galvanic series. Copper is more inert, which means steel will become corroded over time in the presence of water.
This means that over time, susceptible fasteners (screws, bolts, and nails) and connection hardware (e.g., joist hangers) used with pressure-treated wood is at risk of deterioration in wet and moist conditions. Even zinc-coated fasteners (G90) can lose the coating and deteriorate.
This leads to connections losing their strength and increasing the risk of failure. You stack turned structure is now turning into a stack again.
The solution?
*Affiliate links below
The construction industry responded to this problem in a number of different ways:
- Hot-dip galvanized fasteners
- Polymer coated fasteners
- Stainless Steel fasteners
Hot-dipped galvanized fasteners
The zinc on hot-dipped galvanized fasteners* serve as a sacrificial layer, so before the copper can eat its way through your fastener, it first needs to get through the zinc.
The amount of zinc is determined by the weight of zinc per square foot of the surface area of a sheet product.
So one way to address corrosion is by raising the standard for galvanized fasteners and hardware used with pressure-treated wood to G185 instead of the original G90.
G185 refers to 1.85oz of zinc per square foot of the surface area of a sheet product (i.e. both sides.)
As a side note, electroplated hardware has a thin coating of zinc and is therefore not nearly as durable as a hot-dipped fastener.
Both fasteners* and connection hardware* are available as galvanized.
Organic Polymer Coated Fasteners
A number of manufacturers developed organic polymer coatings for fasteners.
The coatings are proprietary in nature but generally work in the same way. The protective coating protects the metal underneath from getting in contact with water and the copper ions from the pressure-treated wood.
Certain polymer-coated fasteners are also galvanized prior to the coating process.
Stainless Steel Fasteners
The third solution is stainless steel fasteners*.
Stainless steel fasteners are resistant to corrosion throughout so the fastener’s durability does not depend on a coating.
Stainless steel fasteners are considered the gold standard for fasteners* and connectors but it definitely comes with a price.
Stainless steel fasteners can be five to ten times the cost of other fasteners.
Ask yourself, considering the cost of the entire project, is that really too much to pay for a durable fastener?
Why do you need to know about Pressure Treated Wood and Corrosion?
Whether you are designing an outdoor structure or reviewing a roof edge detail, always be sure to confirm the fastener type every time you encounter the use of pressure-treated wood.
This is a standard on our peer review checklists and it can now be one on yours as well.
References for this post and resources for further reading:
ASTM A123 and G90 Specifications
Modern Pressure-Treated Lumber is Corrosive
*Affiliate links help us pay our hosting fees and enable us to fund our tool habit. If you make a purchase through one of our affiliate links, this blog earns a commission at no cost to you. We appreciate your support, thank you.
