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How to replace a large porch deck?

I am in the process of replacing a porch that had seen better days. I will be changing the orientation of the subframe so that decking runs outwards from the house rather than perpendicular.

I note in the past that @BIM_Engineer has been extremely helpful in confirming proposed plans from an engineering perspective.

The total porch dimensions is 15.5m x 2.7m - it will be installed as an independent structure rather than directly tied into the house - it will be tied into the existing roof support posts, but these will likely not be used to support the porch structure directly.

The proposed plan is a single bearer (140x45) notched into 90x90 posts - 3 supports per bearer - accepting that one is required every 1.4m - so really only need 2 but 3 seems more logical.

Then looking to run 90x45 joists - over bearers that will be spaced 1.4m apart - joists spaced with 450mm centres.

So there will be approximately 10-11 bearers over the 15.5m span.

Does this sound appropriate for this project?

I am in two minds about doubling up the bearers - but not sure it's required - presumably it would reduce the number of bearers required by expanding the span?

The general ground clearance / height off the ground is approx. 30cm - 60cm

It would be great to confirm if this is a sound approach.

Thank you

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Comments

MitchellMc

Welcome to the Bunnings Workshop community @Loungeact. It's brilliant to have you join us, and many thanks for your question about deck construction.

Your plan sounds adequate for the project.

"I am in two minds about doubling up the bearers". Doubling up the bearers would increase their span, but one is adequate for your 2700mm width, provided it is supported at the ends and in the middle. It doesn't reduce the number of bearers required, as the 90 x 45mm is good at a 1400mm span. The only way to reduce the number of bearer locations would be to double up the joists, so they can span further.

Please let me know if you have any questions.

Mitchell

Loungeact

https://www.workshop.bunnings.com.au/discussion/comment/51742#Comment_51742

Awesome! I'm glad to hear that I'm the right track, and that my gut feeling about single bearer was okay.

Any suggestions on the best way to tie the new structure into the existing support posts that support the roof? 2700 will take the bearer from flush against exposed bearer of the house to half way in on the existing post. I was thinking of then attaching 140x45mm on the end of the bearers in essence like a facia to finish the off...and running the final joist against the post (inside edge) so that deck boards run just past the inside edge of the support posts of the roof.

Hopefully that makes sense.

JacobZ

https://www.workshop.bunnings.com.au/discussion/comment/88034#Comment_88034

Hi @Loungeact,

I've had a good think about how finishing the bearers halfway in on the existing posts could work, and I can't see an easy way to do it without having a bearer run flush on each side of the post, so the fascia has a structural member to attach to. Do you think you could sketch what you are thinking for the finish so we can have a look?

I've had a play around in SketchUp to see what things will look like, and there are some intricacies due to your use of a two-tiered deck frame.

The solution I came up with, which I think would look nice and give you a few more options for finishing, is to run the bearers flush with the outside edge of the posts, then use a 240x45 attached to the bearers so its top edge sits 90mm above the bearer, level with the joists. You can then notch your decking boards around the posts with a slight overhang.

This would tie the posts in with the deck and as long as nothing is screwed directly to the existing posts and there is expansion gaps around the decking boards, then the structures are still separate.

This is obviously only one of a few different options, but I think it would be a nice finish.

Let me know what you think.

Jacob

Loungeact

https://www.workshop.bunnings.com.au/discussion/comment/109639#Comment_109639

Apologies for the very tardy reply. That is a great looking solution that you've managed to sketch. I'll see if I can sketch what I was contemplating, and compare the two ideas.

On another note, I'm deciding between concreted in supports for the posts vs using the likes of Tuffblocks? Pros vs cons? Evidently to use the Tuffblocks the space need to prepared appropriately with a solid foundation.

Thank you

MitchellMc

Hi @Loungeact,

Tuffblocks would be fine on the lower end, but personally, I'd switch to concrete footers down the far end where the deck is higher above 50cm. Concrete footers provide lateral support and will help keep the structure rigid, especially when you hit 60cm, as it's not really a low-level deck anymore.

Mitchell

Loungeact

https://www.workshop.bunnings.com.au/discussion/comment/52446#Comment_52446

It's been awhile since I sought advice on this project, and it's not got started yet...to be fair it's a project for someone else, so not high on my priority.

I'm confident to commence with the advice and ideas that were previously provided.

I like the design concept by @Jacob and if I choose to do so can secure it into the existing porch posts.

Last few questions...

Is it best practice to notch in the bearers to the posts, is it necessary or can they be affixed on the outer aspect of the posts? I've also seen examples of adhering a bearer to either side of the post, but this doesn't seem to be the suggested method in Australia.

I've noticed some bolt the bearers to the posts, others use screws and others suggest nails are the best option?

Lastly looking for guidance as to how to get started - would it be beneficial Installing the 240 x 45 timber fascia to the existing porch posts and use this as the final level for joists to align with, and therefore can work backwards of that level to set heights for posts / bearers?

Thanks in advance

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/91078#Comment_91078

Hi @Loungeact,

Given this is a low-level deck porch, there’s really no need to notch out the posts. Those posts are already in place to support the roof, and they weren’t designed to have additional timber removed from them for bearers. I’d recommend simply using them as attachment points for your fascia and bearers, fixing them with either bolts or heavy-duty screws. Across the span between the posts and the house you’ll still need additional support foundations to comply with the span limits of the timber you’re working with. While you could notch a post slightly, I wouldn’t remove any significant amount of timber, as their primary role is to carry the roof structure, not act as part of the deck frame. If you were set on notching or altering them, you’d really need an engineer to confirm it’s safe.

As for setting out, fixing your 240 x 45 fascia to the existing porch posts is a smart way to establish the final joist height. Once that fascia is in position, you can work backwards to set the heights of your bearers and any additional posts or footings, ensuring everything lines up neatly.

Please let me know if you have any questions.

Mitchell

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