Found this info......sounds like critical structure has a better grade and more chemical components and "critical structure" is a more robust treatment than "ground contact."
While both terms relate to pressure-treated wood, "critical structure" refers to a higher grade of treatment used for structural components that need extra protection against decay and rot, typically in situations where the wood is deeply buried in the ground or supporting significant weight, whereas "ground contact" indicates a standard level of treatment suitable for wood that will be directly in contact with soil, but not necessarily carrying heavy loads; essentially, "critical structure" is a more robust treatment than "ground contact.".
Key points to remember:
Treatment level:
Critical structure lumber has a higher concentration of preservative chemicals compared to ground contact lumber, making it more resistant to decay and insect damage.
Application:
Critical structure lumber is typically used for structural components like deck posts, pergola supports, or utility poles that are deeply buried in the ground or supporting significant weight, while ground contact lumber can be used for things like fence posts or landscaping timbers that are in contact with the soil.
Building codes:
Many local building codes may require "critical structure" treatment for certain structural applications, especially when the wood is heavily exposed to moisture or ground contact.
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Forna very small price difference, I am going to look at getting critical structure rated 4x4 posts.
I need two of the 4x4x12 and probably four of the 4x4x10