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A treated 2/4 weighs much more than a standard untreated 2/4. Does treating do anything for strength or is it purely weather protection?

I always though of treated wood was more heavy duty/strong but maybe it a false sense.
 

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They are the same in strength. You are probably getting Spruce, Pine or Fir depending on where you live. All of them are close the same in strength unless you ordered a higher grade of lumber.
 

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The strength is the same for a particular species whether it is treated or untreated, depending on the incisions treated wood may not be as strong as untreated. Surface incisions will have little effect, deeper incisions may weaken the material.
What incisions?

George
 

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A treated 2/4 weighs much more than a standard untreated 2/4. Does treating do anything for strength or is it purely weather protection?

I always though of treated wood was more heavy duty/strong but maybe it a false sense.
For the same exact species, there is no difference in strength. "Treating", as in pressure treating is primarily intended to protect lumber from insect and fungus infestation. That does very little to cope with weather conditions. Since it's done to softwoods (dimensional lumber) the actual weight may depend on how wet, or the type of chemical used.

Treating wood has been done for years, with the intent to prolong its life. Oils and petroleum products have been used and in many cases successfully.

As for strength in the profile of a 2x4, the strongest are likely from the Hemlock/Fir species. it's difficult to tell when looking at a pile of 2x4's at a box store, what their species is. If that size lumber is needed for strength, it could be special ordered from a hardwood species. There are species that fare better than softwoods in weather exposure, such as White Oak or Ipe.

Pressure treating doesn't stop rotting, as any wood that doesn't have the ability to dry out will rot.






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I beg to differ with Cabinetman.
The goal of pressure treating is to resist rotting due to the action of bacteria and fungi in wet soils. Guaranteed 40 years in the ground here, both plywood and posts. Post penetration is about 1/2", more near the ends. In other places, perhaps insect infestation is a bigger threat.

Shear, brashness and other values can be measured easily for any species, they do differ a little. Moisture content control is another headache for testing. Plus, it's done with weird cuts of lumber like radial, tangential and transverse directions. Quarter-sawn sort of evens out the radial/tangential values. Plus, there's reduced movement on drying.

Every year without fail, my dendrology students looked forward to the labs for measuring wood strength. The numbers are in several books, all we did was to try to approach/confirm the established range of values ("range of values" = it ain't one fixed number.) from the Mean +/- SD.
Three hours of serious busting boards in a 20-ton press. Almost as good as beer & pizza.
 

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in my experience.....

...we could get Southern Yellow Pine or Long Leaf Pine back when, and it was substantially stronger than Spruce. Doug Fir was also better as a construction member, but I don't think DF was pressure Treated...I donno?
This chart shows it to be stronger than the other Pines:

http://www.woodworkweb.com/woodwork-topics/wood/146-wood-strengths.html
 

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rotting potential

that's my read on it. :yes:
 

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Treated only weighs more (to amount to anything) until it drys back out. The lumber is kiln dried and either banded to go, or put in the pressure treatment plant. The treated lumber bundle is banded while it's still soaking wet. At least, that's the way they do for treated Yellow Pine around here. No incisions in it around here. What is available locally will vary in different parts of the country.

I keep some drying here all the time. I don't use it for finish parts, like handrails or steps, until it's dry. I don't like to make something out of it, that I want to keep movement to a minimum, until it has dried for at least a year. I check it with a moisture meter, of course, but if a board is going to move too much, it will do the most of that moving when it is getting close to dry again. So sometimes you don't know until it's close to a year old.

The steps in the link below were made from treated pine that we had drying for well over a year. We pick clear boards to dry. One side is jointed flat, and then planed to desired thickness. In this case the other original finish boards were 1-1/16" thick, so that's what we ran these down to. I don't know if you can tell in the photo, but all the surfaces are hand planed to match others in the old house. Paint doesn't stick well to treated wood, unless it's completely dry. They're dirty because they are used a lot, but haven't been painted since we built them in 2009.

http://www.historic-house-restoration.com/images/windowsx_006.JPG
 

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The box stores only have one kind of 2X4 - "white wood". Just ask them! :laughing:
 
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