Q: I have two deteriorating (but still standing) retaining walls made of timber and two even worse, leaning cinder-block retaining walls. I was planning to replace them all eventually, but to spread out the cost, I figured I’d do the cinder-block walls first, making them timber to match the others. I do like the natural look, but one of the landscapers I had come out said timber just doesn’t last like it used to — maybe three years instead of 30! He said it was because they no longer use arsenic as a preservative. He suggested we use stone instead. The other landscapers didn’t mention this and seemed happy to do the timber work. Whom do I trust here?

A: It’s true that arsenic is no longer used as a preservative for wood sold for projects around the house, although it still can be used for some commercial projects. However, the replacement chemicals should ensure that wood lasts for decades, even when in contact with damp soil, which is where the fungi live that cause wood to rot, along with insects such as termites and carpenter ants that do considerable damage.

Grant Kirker, a research forest products technologist at the Agriculture Department’s Forest Products Laboratory in Madison, Wisconsin, who has studied wood preservatives for years, disputed what the contractor told you. That’s “at odds with decades of testing data that would indicate otherwise,” he said in an email.

When building codes require the use of preservative-treated wood, the wood must meet standards set by the American Wood Protection Association, a trade group. These standards expanded in 2015 to include new categories for interior wood in close contact with the ground, such as for sill plates, but the changes didn’t affect the standards for outdoor wood in ground contact, he said.

Over the years, the wood-treatment industry has used different chemicals to make wood more durable. But as the environmental risk when the chemicals leach out of the wood has become more well-known, some types of treated wood have disappeared from lumber yards. Creosote and pentachlorophenol were both banned for residential uses in the late 1980s, although they can still be used for railroad ties, utility poles and some farm uses. For projects around homes, the main replacement was lumber treated with chromated copper arsenate, or CCA, which was introduced in the 1940s. Your lumber retaining walls are undoubtedly made with this type of wood. For years, CCA was touted as a far safer alternative, but as it showed up more and more in decks, fences, picnic tables and even some playground equipment, tests began to show that the arsenic leached into soil. In 2003, it was also banned for consumer use.

Copper-based formulas then took over, with alkaline copper quaternary, or ACQ, and copper azole the most popular. But their downside turned out to be that copper was very corrosive to other metals, including the zinc coating on galvanized screws and nails. Having fasteners rust away is clearly a safety hazard on a structure such as a deck. Although it was possible to build safely by using ceramic-coated, stainless steel or other approved fasteners, chemists at treatment companies kept working.


Today, the type of treated wood you are most likely to find at home centers and lumberyards is micronized copper quaternary (MCQ) or micronized copper azole (MCA). The copper is ground so finely that it gets into the wood cells, which researchers say makes it far less likely to leach out or set up a galvanic reaction that corrodes fasteners. As a side benefit, it leaves wood looking more natural, without the greenish tint that was common with the earlier formulas.

Regardless of the formula, preservatives for wood labeled to AWPA standards must pass tests showing that they withstand termites and rot. After laboratory tests, treated wooden stakes are set out in a field, often in Mississippi, where the soil is usually moist and where termites are abundant. The stakes still need to be strong and intact after three years, at a minimum. That might not sound like much, but it’s enough to rule out formulas that clearly don’t work. (And perhaps it’s what your contractor extrapolated from to say some treated wood lasts for only three years.)

However, researchers also take a longer look. Kirker and three other wood scientists teamed up on a study published in 2013 that looked at what happened to stakes left in the ground beyond three years. Creosote-treated wood, which might seem to have the longest life given its nasty smell and oily look and feel, lasted at least 50 years. But pentachlorophenol and the copper formulas were lasting 60 years or longer. The micronized copper formulas hadn’t been invented yet, but the study did confirm that testing stakes for three years gives a good indication of longevity for a longer period.

If you decide to go ahead with a timber retaining wall, a couple of issues are really important to understand. One is to check the label on the end of the wood to see its use rating, which correlates with how much preservative has to be in the wood. Lumber designed for decking will be labeled UC3, with an A added if it’s coated with a water repellent and a B if it is not coated. But for ground contact, as in a retaining wall, you need wood labeled UC4. After that, you should see an A, B or C, signifying general use, heavy duty or extreme duty.

At home centers, the only ground-contact wood will probably be general use. Lumberyards catering to contractors carry the other kinds, which have significantly more preservative infused into the wood. For example, if micronized copper azole is the preservative, southern yellow pine boards designed for decking or fence posts must have 0.06 pounds active ingredient per cubic foot of wood, while timbers for heavy-duty use must have more than twice as much (0.15) and those for extreme-duty use must have double that (0.31).

If you want a timber retaining wall to last as long as possible, buy wood rated for heavy or extreme duty. It may require a special order, and you will probably need to wait several weeks for it to arrive due to delays caused by the coronavirus pandemic and pent-up consumer demand. It will also cost about 20 percent more than standard ground-contact lumber.

The other critical detail is that regardless of what kind of treated wood you buy, the chemicals only go so deep. If you or your landscaper cuts pieces to length or drills through the wood for rebar or spikes that hold layers together, a paint-on wood preservative such, as Copper-Green Wood Preservative, needs to be added to protect the freshly exposed, untreated wood.