· Old School
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.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.
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.