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Hi, I'm planning to DIY a 3x4 garden pergola, and i just want to know the timber specs that i should use for the post and frame? My initial plan is to install a 4 post using 100x100mm H4 treated timber and having a square frame around using either 200x50mm H3 treated or 140x45mm H4 treated timber.
Hi @jmm10,
A warm welcome to the Bunnings Workshop community, it is fantastic to have you with us.
As @AlanM52 has mentioned, because it is an unroofed structure, it is unlikely you would need to apply for building approval with your local council, but it is always worth checking to be sure.
As the roof structure is not supporting the roofing material, I can't find definitive information on spans, but using these Woodhouse Span Tables as reference, when supporting a sheet roof with rafters spaced 600mm apart, an F7 rated 185x42mm timber can span 3920mm.
Considering there is no roofing material to support, a timber of a similar size, such as this 190 x 45mm Outdoor Framing H3 Treated Pine would be what I would look at.
If you wanted a more refined finish, then I would also consider the 185 x 42mm F7 H3 Treated Pine Primed LOSP, but it will be at an increased cost compared to rougher H3 framing.
The 100x100 H4 posts will be perfect for this job. Just make sure 1/3 of the overall height of the post is in the ground and a suitable amount of concrete is used.
Let me know what you think and if you require any clarification, please don't hesitate to ask.
Jacob
Hello @jmm10
Both 100x100mm H4 treated pine and F7 cypress sawn posts are good options, but each has different strengths. H4 treated pine is budget-friendly, easy to find, and well-suited for in-ground use because it’s treated to resist moisture, rot, and termites. It’s a strong, practical choice for fences or outdoor structures, though it does contain chemicals, which may not be ideal near edible gardens or water.
F7 cypress, on the other hand, is naturally resistant to termites and decay without needing chemical treatment. It also looks more attractive with its grain and rustic appeal, making it a good option if the posts will be visible. It’s often more stable and eco-friendly, but it usually costs more and might be harder to source in consistent quality. If your priority is affordability and function, go with treated pine. If you prefer natural timber, better looks, and no chemicals, cypress is the better fit.
In regards to your second question, Using a stirrup or concreting the post into the ground both work, but each suits different needs. Stirrups are great if you want to keep the post off the soil to avoid moisture damage and rot. They’re especially useful for untreated or lightly treated timber and make it easier to replace posts later. They also give a clean, raised look and are ideal for decks, pergolas, or other above-ground structures.
Concreting the post directly into the ground usually gives stronger support, especially for fences or retaining walls that face pressure or wind. If you’re using H4 treated pine or similar timber rated for in-ground use, rotting isn’t a big issue if the concrete drains well. This method can feel more solid and is often quicker for large fencing jobs.
If long life and easy replacement are important to you, stirrups are a smart choice. If strength and simplicity matter more, and you're using properly treated posts, concreting into the ground is a good option too. Either method works, you just need to match it to your project and the materials you’re using.
If you need further assistance, please let us know.
Eric
Thanks for the info. My plan is to use a cypress over h4 treated pine because it feels more compact and hard when i check it to your warehouse. Because as far as i know good wood should be heavy.
Just want to ask fallow question in regards to stirrup option, does braces from beam to post is required considering project is a freestanding pergola and i want it to be more secure and stable structure or should i go to concrete into ground?
And does cypress is also good for into ground posts as well or not?
My project will be in the middle of the garden and my plan is to color it with white paint.
