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How’s it going,
We’re building a timber fly over / fixed to wall pergola 5m x 3m. We’ve used H4 100 x 100 treated pine posts for the upright posts (x 3) and support beam (x 1). And we’re going apply 2-3 x diagonal supports to every corner to brace the structure.
Someone suggested to me that we should have used F7 90 x 90 structural treated pine instead.
Whilst I’m aware that H4 posts wouldn’t necessarily have the longevity of the F7 grade, and that it may tend to warp over time as opposed to the F7 grade not doing so; my main concern is if it’s structural integrity is just as good?
Welcome to the Bunnings Workshop community @Bookluck. It's wonderful to have you join us, and many thanks for your question about treated posts.
It sounds like you might have used non-structural landscaping posts, which aren’t graded for structural applications like this. The main concern here isn’t just longevity but that H4-treated posts aren’t verified for load-bearing capacity like MGP10 or F7 structural timbers. Structural timber is specifically graded to ensure it meets strength and stability requirements, whereas landscaping timber can have natural defects that may affect its integrity over time.
If you’re unsure about the suitability of your posts, it might be worth consulting a structural engineer or builder to assess whether they will be sufficient for your pergola.
Please let me know if you have any questions.
Mitchell
