pressure treated lumber Archives - Charles Buell Consulting LLC

With all the recent deck collapses around the country it may be time once again to re-visit deck construction methods and materials.

Actually we have pretty much figured out the “methods.” It is getting builders to follow the proper method that is the problem with newer decks now. Many of the failures of older decks were the result of poor practices at the time of construction but may not have “technically” been against the codes of the time. Sorting all that out could be a nightmare in itself.

We clearly know HOW to build decks safe now–there is simply no excuse for catastrophic deck failures like we have been seeing in the news.

So if we have figured out the “methods,” what about the “materials” the decks are made of?

Modern decks are generally required to be constructed of rot resistant materials. Around here, that generally means “ground contact” pressure treated lumber.

I started thinking about the adequacy of this type of lumber when inspecting a deck recently. As near as I could tell the deck appeared to be about 20 years old. One of the support beams had a crack in the side of it. This is not unusual for wood beams–they often check without being a structural issue. Since many of these cracks will only show up after installation, we cannot simply say no checked wood can be used in deck construction.

What was frightening about this beam was that the crack had fungus growing all along it.

Those of you that have experience with ground contact pressure treated lumber and for those that do not, it is typically only the outer ½” or so of the beam that will be saturated with the preservative, leaving the center vulnerable to wood decay/rot and wood destroying insects. A check that goes deeper than the preservative will lead to rot of the interior core of the beam if a supply of water is provided to it. This is not hard to do in the Northwest.

treatedbeam2 — This is how much of the center of a ground contact pressure treated 6 x 10 can “disappear”

That is what was happening to this beam. It was hard to tell how much of the strength of the beam was still present but it could be anywhere from a lot to very little. For example the untreated part of a 5-1/2” x 11-1/4” beam is approximately a 4×10. Clearly if the entire center portion of the beam turns to mush, the beam has lost virtually all of its strength.

So what is the solution?

The solution would be to require that all structural members of decks exposed to weather be constructed of “foundation grade” pressure treated lumber.

I can argue that all flat surface deck and roof structures should be constructed of pressure treated lumber. It is not a matter of if they will leak but when they will leak, especially if they are walking surfaces.

This type of pressure treatment is certified to be treated all the way through the lumber.

This may be an idea that’s time has come.

Here is some good additional information about Pressure Treated Wood.

Charles Buell, Real Estate Inspections in Seattle

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Not all pressure treated lumber is created equal

This post is not meant to be a complete tutorial on Pressure Treated Lumber.

A complete presentation of all of the information is left better to manufacturer’s websites or like this one from the Southern Pine Council. I just want to introduce you to the fact that not all pressure treated lumber is the same. It seems most people think that “pressure-treated” is “pressure-treated.” The reality is that not only are there several different “grades,” of pressure treated materials, there are different types of chemicals that can be used to treat the wood as well.

One thing that will be true of them all is that each piece of lumber will have a tag stapled to it that explains what it is.

The tag will tell you how much preservative is in the material, what type of preservative was used—-along with a bunch of other information—-including the year it was treated and where.

It is important for the installer/builder to be familiar with what all this labeling means so that the correct materials get used for the right job.

The Grade of pressure treatment refers to the “amount” of preservative that is required to be forced (under pressure) into the piece of wood—-per board foot. These percentages run from 2.5 ounces per board foot for pilings used in salt-water (marine) applications, to .25 ounces per board foot for some deck railing and surface installations. When you get up into the higher number of ounces per board foot there are only certain species of wood that will actually accept that much preservative—-Southern Yellow Pine for example. (Don’t tell that to the NW timber industry however.) For all its other great characteristics, Douglas Fir is lousy at being pressure treated and Hemlock is likely not much better. For most purposes, wood that is going to be in contact with the ground will have a saturation level above .40 ounces. This is very typical of most deck structural posts, beams and joists. For wood meant to be buried—-as in Permanent Wood Foundation Systems (PWF’s)—-we want to see .60 ounces per board ft.

The problem is that because most people—especially “Weekend Warriors”—-don’t know the differences between all these grades of treated wood, we end up with the wrong grades being used in the wrong places. Another factor is that materials designed to be buried are only available at real lumber yards. You are probably not going to find them at Weekend Warrior hunting grounds like Home Depot, Lowes, or OSH.

Here is a picture of your garden variety .40 Hemlock pressure treated 6×6. Because the treatment of this wood “species” (at this grade) can’t penetrate all the way to the center of the wood—-the center just rots away.

So when you are using pressure treated lumber—make sure you have the correct grade and that the weekend warrior has good information.

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Charles Buell, Seattle Home Inspector

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