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Help with designing a pergola

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Help with designing a pergola

Hi all, I am seeking answers to 3 questions i have in regards to building a detached Pergola.


The size of the detached pergola will be 5m x 4m 


Q1. Based off the pergola size (5m x 4m) is it better to attach the beams (240 x 45mm) to a 4 x post structure or with 6 posts? 


Q2. Research has shown two ways to set up the beam structure as being either:

●1 x beam (per single side) cut into the posts as a rebate at either end ; OR ●to have 2 beams, one placed at the front and one at the back of the post.  Which is better method support?  

Both would be use a coach bolt and nut to fix. 


Q3. With regards to knee braces would you fix each direction off the post to better secure (meaning post to beam and post to rafter) or only along the post to the beam? 


These are the 3 questions stumping me before moving forward. Support in answering is greatly appreciated. 


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Retired Team Member

Re: Help with designing a pergola

Welcome to the Bunnings Workshop community @flitegeoff. It's fantastic to have you join us and hear a bit about your project.


@DIYgals, @sandy_d and @tom_builds may be able to offer their thoughts, and our resident D.I.Y. expert @EricL will be happy to help when he's back on the site over the weekend.


Look forward to seeing updates about your pergola.




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Re: Help with designing a pergola

Hi @flitegeoff,

Sounds like you have an exciting project coming up.

In order to be able to provide better advice sepcific to your project I have a couple of questions:

What roof design are you planning on using (gable/skillion etc.) ? This will have an impact on the maximum span between posts.


Regarding the knee braces in each direction are you talking about bracing at an intermediate post where the beam runs either way from the post and a rafter runs off perpindicular to it? Or does your design only have beams on the long sides? If you have beams on all sides then you would brace from post to either beam in each corner and then from post to beam in each direction from intermediate posts (bracing to the rafter from intermediate posts will add extra stiffness but not necessary)


As for attaching the beams, I'm a fan of rebating them into the post, and mitre cutting them to meet at the corner of the post. I think it looks neat and professional, while also making it a little easier to assemble. 



Re: Help with designing a pergola

Hi Tom 

Thanks for reply. 

In regards to you query the Pergola roof will be flat with no roof sheeting to go on top. At most i might put a shade cloth. 

I plan to make rafter spacing at 600mm gaps. 

I hope this further info can allow you to answer Q1 for me stated in original post. (Note- If rafter spacings are to be wider than 600mm to support 4 posts instead of 6 posts please advise). 


Second part to your query is the beams will run along the long side with regards to  knee brace. 


Lastly I agree with rebating the rafters over the beams as they definetely give a better finishing look to the structure. 


I look forward to your reply Tom or someone else in your team. 

Thanks Geoff 


Re: Help with designing a pergola

Thanks for getting back to me with those clarifications @flitegeoff 


With regards to your design, you will be looking at 6 posts, as 240 x 45mm treated pine supports a maximum span of 3600mm when in a continuous span @ a roof load width of 3000mm (your roof load width is 2000mm - which is half the rafter length - but the span table is in multiples of 1500, with rounding up required). 600mm spacing for rafters will be fine with 140x35mm members, and increasing the rafter spacing to 900mm won't allow you to reduce the number of posts to 4 unfortunately.

I would also suggest using knee braces to both the beams and the rafters at the corner posts to provide extra rigidity.


Best of luck as you move forward with your project, and update us with some photos when it's done.

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