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Pergola Structural Strength

Postby craigfarrall » Mon Apr 18, 2022 11:51 am

I have recently built out a decking and pergola area in my garden. I must caveat this by saying I am a complete novice DIY'er, but like to get involved.

I have attached a few photos of how this looks currently - but I have concerns about the top bit with the rafters from a structural perspective. As you can see, I have used the timber on the outside of the posts (that were concreted in) using carriage bolts. In retrospect, I would have done this differently and maybe cut a piece out at the top of the post, so that those sit on top, instead of to the side to make it even more secure (hope that makes sense) - however I am reluctant to pull all of this down, especially having to take out all of the nails from the joist hangers.

So even though I could / should have done this differently if I was doing this again, my question is what I can potentially do based on what I have currently. A few ideas that I have thought of are:

    Adding further posts that are concreted into the ground at the back only. Obviously I wouldn't want to do this at the front, as I naturally want to keep the big opening as currently.
    Adding triangular corner pieces. Not sure how I would this exactly, with the timber beams being outside of the posts, but I see quite a lot of pergolas doing this. I am personally not a huge fan of how this looks, but if it helps structurally then I am willing to do it.
    Not sure if this will work, but adding another set of timber beams below the ones currently set up around the whole perimeter, which are attached to the posts - not sure if this will help at all though

Any recommendations on what I can do? I don't necessarily feel like this is going anywhere as everything is bolted in, but for example if I were to hang from one of the rafters, there is movement that is concerning me, hence this post.

This does tie into the same question, but I have attached a picture of the carriage bolts. My question on this is I have had to use one bolt only towards the bottom of the beam, as I have the cross beam also needing to be attached by the bolt too, so I added those more towards the top on those. Ideally I would bolt this up twice at the top and bottom - but obviously couldn't do this as the bolts would hit into each other. So if there is a way I can possible sure this up too, let me know.

Please go easy on any novice mistakes made.
Attachments
Pergola3_2.jpg
Pergola2_2.jpg
craigfarrall
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Postby stoneyboy » Tue Apr 19, 2022 9:59 pm

Hi craigfarrall,
Your structure looks fine as it is but I suspect the flexibility is in the joint hangers. Try driving 120mm part thread screws through the outside framing timber and into the end grain of the rafters. This will pull the structure square and stop flexing. You will need an impact driver!
Regards S
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Postby craigfarrall » Wed Apr 20, 2022 7:11 am

stoneyboy wrote:Hi craigfarrall,
Your structure looks fine as it is but I suspect the flexibility is in the joint hangers. Try driving 120mm part thread screws through the outside framing timber and into the end grain of the rafters. This will pull the structure square and stop flexing. You will need an impact driver!
Regards S


Thanks for the reply. I never considered it was anything to do with the joist hangers as that feels pretty stable as they have nails drove in at both sides and underneath - but will definitely give this a try - do you have any specific recommendations on screw to use?
craigfarrall
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Postby stoneyboy » Wed Apr 20, 2022 9:45 pm

Hi craigfarrall,
Use 120x5mm screws with a part thread so you can fit them without the need for pilot holes and they should not split the end grain timbers. I find Reisser are as good as any but if you can find part thread screws that are NOT self countersinking they should be ok.
Regards S
stoneyboy
Rank: Project Manager
Posts: 5330
Joined: Wed Dec 10, 2008 6:44 pm



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