Materials for Wood Pergola Kits

Materials for Wood Pergola Kits

We had some big decisions to make when starting Maine Porch & Pergola Company. The selection of materials for our wood pergola kits was one of the most important. We wanted to research this thoroughly. Our biggest concerns were rot (fungal decay) resistance and insect resistance. Woodworking stability, appearance and costs were also concerns, as was structural strength is also a concern.

We hunted for information and recommendations from forestry professors, wood scientists, lumber companies, trade groups and professional builders. We did this by talking with them, studying research papers, reading web articles, and scouring blogs and forums.

Rot and Insect Resistance

Here is what we found on materials for wood pergola kits:

  • Hardwoods: Teak, old growth Cypress White Oak, Black Walnut and American Mahogany are the most rot and insect resistant hardwoods. These are followed by imported Mahogany, Cherry, young growth Teak and young growth Cypress in the moderate-resistance group. Other than White Oak, pretty much all of the commonly available lumber-yard hardwoods are in the low-resistance group (e.g., Maple, Ash, Beech, Birch, Poplar).
  • Softwoods: Old growth Redwood and old growth Cedars (white, yellow and red) are the most rot and insect resistant softwoods. These are followed by young growth Redwood and young growth Cedars, Tamarack, Douglas Fir, and old growth Pines, which provide moderate-resistance to both rot and insects. All other untreated softwoods that are commonly available at the lumber-yard are in the low-resistance group (e.g., Pine, Fir, Spruce).

These findings are based on research cited in the USDA Forest Service 2010 Engineering Handbook, as well as anecdotal experience that dates back decades. But according to Professor Gene Wengert of University of Wisconsin, the problem is that as old growth forests are logged out, the quality of young growth lumber for even the highest rated species can be all over the place. This is because the proportion of sapwood (new outer rings of a tree) vs. heartwood (older inner rings of a tree) has been increasing in commercially available lumber. And it’s the heartwood that provides the best rot and insect resistance.

Conventional Wisdom on Cedar and Redwood is NOT Reliable

These concerns about cedar and redwood became most evident when reading blogs and forums where professional builders were comparing notes on fence posts. These are perhaps the most rigorous testing-ground for rot and insect resistance. According to these hands-on experts, years ago you could trust Cedar and Redwood to last several decades or more.  Today this is no longer the case. Longevity may be as low as 10 years or even less.

This calls into question whether today’s Cedar and Redwood really are worth their high cost (see below). Many retail lumber providers will not be able to answer questions about old vs. young growth or sapwood vs. heartwood. This makes it very hard to know exactly what you are buying for a wood pergola kits.

It’s not clear to what extent these quality concerns apply to hardwoods, but hardwood forests in developing countries are very susceptible to over-harvesting. This bring up environmental and sustainability concerns.

Cost is Important

Cost in a business like ours is very important. We are always seeking the best trade-offs between rot resistance, insect resistance, strength, appearance, wood working stability, sustainability and cost when selecting materials for wood pergola kits. Here are some cost comparison ratios to consider.

Suppose you were going to buy $100 worth of commonly available Spruce-Pine-Fir 2×4 dimensional lumber. The same amount of similar grade lumber in other selected species would cost approximately:

  • Regular Pine-Spruce-Fir – $100
  • Atlantic White Cedar – $210
  • Alaskan Yellow Cedar – $332
  • Redwood – $468
  • Western Red Cedar – $595
  • Imported Mahogany – $880

Pressure Treated Lumber

Then there is pressure treated (PT) lumber. Treating lumber dates back to ancient Greece, where bridge wood was soaked in olive oil.

PT lumber is specially treated to resist rot and insects. It is generally trusted to be rot resistant for 20-30 years or more.  This is well beyond even the best hardwoods, Cedar or Redwood. This “20-30 years or more” is anecdotal, however. Wood treaters resist providing specific duration warranties due to site variability, installation and maintenance factors.

PT lumber is also relatively cheap. Extending the cost comparison above:

  • PT Pine-Spruce-Fir – $113

One important attribute of treated lumber is that the treatment process is done under uniform quality control. This ensures that every piece is treated to the manufacturers specifications. So there is no variation and uncertainty in rot and insect resistance which you find in today’s Cedar and Redwood.

But PT lumber has its drawbacks. Regular PT lumber can be very wet when you purchase it at the lumber yard. Raised grain, splitting, checking, twisting, warping, and shrinkage can occur as it dries out. This makes using it a dicey proposition when appearance is important as is the case with our wood pergola kits.

Our Solution – KDAT Pine

So, what did we come up with for our wood pergola kits … kiln-dried-after-pressure-treated-Pine (KDAT Pine). The key is that the pressure treated Pine is kiln dried again after treatment. We didn’t know that this option existed until speaking to Maine Wood Treaters which agreed to make it for us on a special-order basis. They use a micronized copper preservative called MircoPro. We think KDAT Pine provides the best compromise in cost, rot resistance, insect resistance, strength, appearance, wood working stability, sustainability, and safety. Extending the earlier cost comparison:

  • Regular Pine-Spruce-Fir – $100
  • PT Pine-Spruce-Fir – $113
  • KDAT Pine – $148
  • Atlantic White Cedar – $210
  • Redwood – $468
  • Western Red Cedar – $595
  • Imported Mahogany – $880

The most important attribute of KDAT lumber when compared to regular PT products is that it is dry. Because it is dry, KDAT lumber can be sealed/primed and painted right away to protect it from natural weathering such as raised grain, splitting, checking, twisting, warping and shrinkage. KDAT lumber can also be stained if a customer is looking for a more natural finish for their wood pergola kit. Just be sure to test a sample with the intended stain product first.

Improved Safety

The wood treatment processes used today are considerably safer that those outlawed decades ago which used arsenic. Most of today’s processes are based on formaldehyde-free copper-based wood preservatives. It is our understanding that the wood preservatives used in treated wood available to consumers have been registered by EPA for general use, which means that EPA has determined it is relatively safe for most, if not all, consumer applications. However, EPA does warn about contact with food, situations where a lot of treated saw dust is being inhaled, burning of treated saw dust, and use of treated saw dust for animal bedding. Otherwise, EPA safety warnings for these materials are the same as for untreated lumber (eye protection, splinters, etc.).

Different people perceive safety in different ways. EPA assurances aside, anyone who has worked with wet treated lumber knows that there is skin contact with wood preservatives. Most manufacturers suggest wearing gloves. This is another advantage of dry KDAT products over normal PT lumber. The kiln drying process that takes place after treatment greatly reduces the potential for subsequent chemical leaching. This is because the water that is used to infuse the preservatives is removed from the wood. And once this water is removed, the preservatives are largely trapped in the wood. Additional sealing with a high-quality primer will further guard against leaching and minimize skin contact with preservatives.

Use Categories

American Wood Protection Association “Use Categories” are a shorthand method of describing the various hazards to which wood products may be exposed. These ratings have to do with the amount of preservative retained per volume of wood. The higher the Use Category number, the higher the rot and insect resistance. The most common Use Categories are:

  • UC1 interior dry
  • UC2 interior damp 
  • UC3 exterior above ground
  • UC4 exterior ground contact

When building a wood pergola kit, we use UC4 “ground contact” grade regardless of whether there is direct contact with the ground. For buried posts we use UC4B “ground contact heavy duty” grade.

What about Hemlock?

Then there is the question of Hemlock. We like to use Hemlock for our rough cut unpainted [wood pergola kits], such as the North Woods. A very economical and strong wood, Hemlock is a long-time favorite of timber frame barn builders, where the foundation posts are buried directly in the ground. Extending the cost comparison from above:

  • Rough Cut Hemlock – $92

While the USDA Forest Service 2010 Engineering Handbook places hemlock in the low resistance group, another notable study by the USDA Forest Service, Forest Products Laboratory reports Hemlock sapwood in its moderate rot resistance group. This may be confusing, but the number of very old hemlock timber framed barns still standing in New England is incontrovertible.

Conclusion About Materials for Wood Pergola Kits

There is no single right answer here for the best selection of materials for wood pergola kits. As explained above, the conventional wisdom on Cedar and Redwood is NOT reliable. We like KDAT Pine because it provides the best compromise in cost, rot resistance, insect resistance, strength, appearance, wood working stability, sustainability, and safety. But if a customer asks for a different wood choice, such as Redwood, Cedar or imported Mahogany, we will gladly switch materials, and advise him or her on the proper maintenance practices for their wood pergola kit.

Enjoy Outdoor Living!


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