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How to build a 3x4 pergola?

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.

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AlanM52

Hi jmm10,

That timber should be fine and the pergola is defined as an unroofed structure (check with local council) so DIY should be OK without a permit.

Cheers

JacobZ

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

jmm10

https://www.workshop.bunnings.com.au/discussion/comment/53718#Comment_53718

Thanks for the helpful information.

jmm10

https://www.workshop.bunnings.com.au/discussion/comment/53732#Comment_53732

Okay i will check that reference. Thank you

jmm10

https://www.workshop.bunnings.com.au/discussion/comment/53732#Comment_53732

Which is better to use as post, 100x100mm post h4 treated or the 200x75 sleeper h4 treated?

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/89113#Comment_89113

Hi @jmm10,

The 100x100 post is the standard timber used for this type of application.

Being square, they have equal strength in both directions, and it is much easier to dig footings for them as they will just need to be circular.

Let me know if you have any further questions.

Jacob

jmm10

Hi, just want to ask another few questions.

1. Which one is better to use for posts, 100x100mm h4 treated pine or the 100x100mm f7 cypress sawn?

2. For setting the posts, is it better to use the Stirrup rather to concrete the post into the ground?

EricL

https://www.workshop.bunnings.com.au/discussion/comment/54445#Comment_54445

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

jmm10

https://www.workshop.bunnings.com.au/discussion/comment/89455#Comment_89455

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.

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/110530#Comment_110530

Hi @jmm10,

Cypress is a good option as long as it is structural grade.

Keep in mind that the weight can be a bit deceptive as posts are often full of water. If the weight is heavier, it does not necessarily mean it is denser, it could just be full of water. If density and strength is what you are after, then you would be better off using hardwood posts.

Without the input of a structural engineer, I would not advise using stirrups. Concreting in your posts so that 1/3 the overall height of the posts is embedded, will ensure they are adequately anchored and up to the task. For example if the posts sit 2.4m out of the ground, then you will need to use 3.6m posts so that 1.2m can be embedded in concrete.

Cypress can be used in ground if you choose to do so, however you should coat the in-ground portion of the post with bitumen membrane to help protect it from water in the concrete.

If you are painting the structure, make sure you use a quality exterior paint such as Dulux Weathershield or Taubmans Sunproof.

Let me know what you think and if you have any further questions, please don't hesitate to ask.

Jacob

jmm10

https://www.workshop.bunnings.com.au/discussion/comment/110530#Comment_110530

Update. After a long planning i already finished my DYI project by my self, from posts digging up to painting. All materials and tools are all from bunnings. Thank you everyone for sharing ideas. Thank you and more power...

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/123046#Comment_123046

That looks like an extremely solid pergola with some beautiful accents. It looks like a great place to sit and enjoy time with your friends and family.

You have done a brilliant job turning your vision into a reality and I am so glad to know I could help, even in a small way.

Thank you for sharing @jmm10.

Jacob

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