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How to build a concrete outdoor island bench?

Hi.

We are looking at building an outdoor island bench with a concrete top, poured in place. The island is not directly exposed to weather but could cop a bit, depending on the severity/angle of the weather.

The bench will be roughly 2m L by 1m deep. Concrete top may overhang on the rear side.

I have 2 questions:

-1) If built sturdy, will a timber frame support sufficiently a concrete top, about 3/4cm thick? As i'd prefer to build the frame from timber as opposed to brick.

-2) We are wanting to clad in something like James Hardi stria panelling. What is the best method to prevent moisture getting inside to the frame? And what to build the frame from, to assist with this. Treated pine?

Thanks in advance.

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Comments

EricL

Hello @damianpc

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your concrete benchtop.

In regards to using timber as the bottom support for the legs, I suggest engaging the services of a registered builder. They will be able to design and build you a proper support structure for the concrete benchtop. In regards to cladding the timber support in James Hardie 3000 x 325 x 14mm Stria Standard Profile Cladding, I'm not aware of any reason why it won't work. Simply follow the James Hardie Stria installation guide to make sure that it fits the timber frame properly.

It would be better to leave the bottom unsealed allowing air to pass through the structure of the table. It would give the framework a chance to breath and dry off. As to the type of timber to use, I suggest using 90x45 or bigger and H3 treated for outdoor use.

Let me call on our experienced members @Dave-1, @Nailbag, @Jewelleryrescue and @JoeAzza for their recommendations.

If you need further assistance, please let us know.

Eric

Nailbag

Hi @damianpc

I like the plan and the suggestions by @EricL . My only recommendation to consider would be to build the frame from metal not timber. Don't under estimate the weight of the concrete and long term steel lasts longer without decay. So there will never be an issue with cracking and durablity in the future.

Nailbag

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/48081#Comment_48081

Good Morning @damianpc

That will be a nice looking outdoor island bench 🙂 I like both @EricL's and @Nailbag's suggestions but like Nailbags steel support even better. Mainly for long term weight stability. Even 3/4cm depth of a concrete top is a fair amount of weight to be a base load. Think of how many concrete bags you will need to cover the bench at 3-4cm and you may be surpprised at the weight (even without the water added)

What are you considering having underneath the concrete and left in place long term? Concrete formwork?

One last thought, is the bench going to be sitting on a concrete area or a deck?

Dave

damianpc

https://www.workshop.bunnings.com.au/discussion/comment/86093#Comment_86093

Thanks so much everyone.

Steel sounds great, except that it's not something I've worked with before and wouldn't really know how to go about it or where to start.

Would it be better to go bricks rather?

Underneath the concrete top, I was thinking concrete sheeting or something along those lines.

I was going to poor a small concrete pad in place underneath where it will be.

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/108357#Comment_108357

Morning @damianpc

I havnt worked with steel before as in welding either 🙂 However I have worked with steel as in bolting ect You could treat angle iron the same way as welding but via bolting. It may not have the ridgidity tho, that would depend on how you would construct it.

Sketching up the design of your bench might be an idea, depending on the materials you use it could really help decide the path you want.

With the "small concrete pad" you are installing for the bench. I would also suggest to sketch out the whole area as the concrete footing wont be going places easily 🙂 you could put footings in to support the bench instead of a pad if you want a deck down the track. If you level off the footings that way you could easily reuse them if you decide to shift the bench.

Dave

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/108357#Comment_108357

Hi @damianpc,

A concrete bench with a 2m length is a large span, and the weight of the concrete top needs to be properly supported to avoid cracking or failure over time. Bricks or concrete blocks could certainly be used for the supports, but the key factor is how the weight is distributed.

If your supports are positioned at the ends only, then a 2m span would require significant reinforcement, such as metal lintels running through the concrete. Standard reinforcement rod (reo rod) alone is unlikely to provide enough structural support to prevent cracking or eventual collapse.

It would be beneficial to post a sketch of how you plan to support the bench, including details on how the concrete top will be reinforced. Given the weight and potential safety risks if the bench is incorrectly built, I would strongly advise enlisting the help of a professional—even if just for guidance on structural integrity. A structural engineer or builder could provide valuable advice to ensure your design is safe and durable.

Mitchell

Nailbag

https://www.workshop.bunnings.com.au/discussion/comment/108357#Comment_108357

Hi @damianpc

As this is going to be a first time project and one that will be technically challenging, I would go with the great advice from both @MitchellMc and @Dave-1 and either change tact on the build materials and or size, or at the very least get some professional advice.

Regards Nailbag

damianpc

https://www.workshop.bunnings.com.au/discussion/comment/48081#Comment_48081

Thanks for that. I've tried to sketch out what I was thinking. It's basically a 3 dimensional mini stud wall. Using 75 x 200 treated pine. Cross bracing both front to back and horizontally left to right. It's a little hard to sketch it the way it is in my head.

EricL

Hello @damianpc

That is a good sketch of the timber support. I suggest bringing your sketch to an engineer or builder and explaining to them what you plan to build and put on top of the timber frame. Since you have the length, width and height of the benchtop you're planning to build. The engineer and builder should be able to give you advice in regards to the timber base and the proper construction method for the concrete bench top.

If you need further assistance. please let us know.

Eric

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/86106#Comment_86106

Morning @damianpc

Nice sketch 🙂 I dont see a problem with making that up yourself. Tho one suggestion I will make is that once made you put the concrete bags (un-used) on top of the frame work and then see fow solid it feels. Just a precaution and I would do the same with a frame I was building. I did it with my Bench seat 🙂 Yeah threw myself at it and flopped at the points that wernt supported by wheels just to test it 🙂 I figure if you can stress test it and it passes then its go ahead time. Just rem someone will also be sitting on the table several times over the lifespan of it 🙂 BBQ's are BBQ's 🙂

Dave

damianpc

https://www.workshop.bunnings.com.au/discussion/comment/108375#Comment_108375

Thanks everyone for your very helpful advice.

Another option I had considered was pouring the slab, maybe 200/300mm, and setting in the concrete post stirrup anchors, and using 90 x 90mm treated pine as the uprights.

Then using cross bracing between all uprights as well (without cross bracing)

See rough pic below, dotted lines of stirrups is what would be set within the concrete.

EricL

https://www.workshop.bunnings.com.au/discussion/comment/121491#Comment_121491

Hello @damianpc

That's definitely an option you can explore, but please make sure to have it certified by a draftsperson or engineer to make sure that it can carry the weight of the concrete benchtop.

If you need further assistance, please let us know.

Eric

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/121491#Comment_121491

Evening @damianpc

Thats a solid support and the cross bracing will definently strenthen it. It may end up looking chunky. You could draw up a welded table and top and then go down to your light industrial area and see a welder/metal manufacturer would be able to weld it for you? I have those type of stirrups for my pool deck and they are solid as a rock with a lot of weight above. They are cross braced as well. The difference between the deck and the table is you dont see the feet of the deck 🙂

Dave

damianpc

https://www.workshop.bunnings.com.au/discussion/comment/129711#Comment_129711

Thanks for that. Good thing is I'm going to clad the whole face with hardi groove or similar. So you shouldn't see the inside.

With your stirrups were they set in the concrete or bolted in afterwards. I'm potentially looking at bolting in once the concrete is set, maybe using chemical anchors.

Cheers

Dave-1

https://www.workshop.bunnings.com.au/discussion/comment/134898#Comment_134898

I set the stirrups in the concrete and then adjusted the heights of the posts acordingly. The ground slopes so there were a few things to factor in for me.

Pool deck stage 1 step 2 will show you how I concreted the posts in. The holes were 500odd mm deep from memory.

Bolting to a slab would work as well and then you could remove if wanted to a lot easier.

Dave

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