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How to install a semi-permanent gazebo on sloped concrete?

I would like to install a Mimosa Mullaloo Aluminium Gazebo in my backyard. The gazebo is 3.6m x 3m.

The concrete in my backyard has some slope to it to let water drain out. What do I need to do to ensure the gazebo legs are levelled across the concrete?

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EricL

Hello @husainZahid

A slight fall in the concrete for drainage is completely normal, so the goal is simply to pack the gazebo legs up to a consistent height rather than trying to “fix” the slab itself.

The usual approach is to position the gazebo exactly where you want it, then use a long spirit level (or even better, a laser level if you have access to one) across the base rails to see how much height difference you’re dealing with between the high and low sides. Once you know that, you can bring the lower legs up to level using non-compressible packers or shims. For something permanent like a gazebo, you want to avoid timber offcuts because they can rot or compress over time. Instead, use large plastic packers or even thin galvanised steel plates, anything that won’t degrade outdoors.

If the slope is minor (say 5–15 mm), stacking packers under the base plates works well. If it’s a bit more significant, I propose engaging the services of professional.

Once everything is level, you should anchor each leg into the slab using the suggested anchors from the installation manual. Drill through the base plates (with your packers in place), fix them down tight, and make sure nothing shifts as you tighten. If you’ve used packers, it’s a good idea to keep them fully supported under the plate so there are no gaps that could cause movement. In most backyard situations, though, a combination of careful levelling, durable packers, and solid anchoring will give you a stable, level gazebo that sits properly despite the fall in the concrete.

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

If you need further assistance, please let us know.

Eric

Nailbag

Hi @husainZahid

First you need to establish the fall height from the house to the side fence which, looks to be considerable. As the posts need to be fixed to the ground then based on any warranty concerns, then I would fix stirrup brackets with suitable height sides to the concrete with the sides facing nth/sth. This will allow you to plumb the posts to the gazebo so it's level. Then secure the posts using galvanised bolts, washers and nuts.

@EricL or one of the other Bunnings team members maybe able to confirm if drilling holes through the posts will void any warranty? And of course, contact your local council re: any permit required to install one due to structure type as per the supplied manual. If permitted, they may have specific requirements for fixing to the concrete

Nailbag

husainZahid

Thank you @EricL amd @Nailbag for writing back. To my very limited knowledge, I think Stirrup brackets are more suitable for timber posts but the gazebo has aluminium legs with base plates. Would it better to use stirrup brackets or plastic shims?

MitchellMc

Hi @husainZahid,

Would you be able to confirm the height difference at all? That would be very helpful in determining appropriate fixing methods.

Regarding @Nailbag's question, about drilling into the posts, the requirements of the gazebo are that its mounting brackets are directly fixed to an immovable concrete base. I believe the only real method to comply with the engineering would be to constuct a raised concrete pad for the higher legs. You would need to use construction grout and form up the pad around four threaded studs that have been secured into the underlying slab with Chemset.

Please let me know if you have any questions.

Mitchell

husainZahid

Thanks @MitchellMc. I will confirm the height difference either later today or tomorrow and report back.

husainZahid

@MitchellMc I measured the slope and found a difference of 221mm from left to right.

Dave-1

Good Evening @husainZahid

Love the idea of a Gazebo :) Your slope adds a bit of complexity to the installation is all. Solveable for sure, just have to add an extra step in or two.

I was thinking of high wind post supports but they ned to be embedded in concrete. I found this when I was looking at how to install them

Pryda Post Anchor Guide.pdf

The guide shows a range of post supports, Yours being that 221mm difference in height is a fair distance to jack up one side. This is what I wa sthinking of but it being one of 4 support points makes me a little cautious in using it

https://www.bunnings.com.au/pryda-300x100mm-m12-full-stirrup-post-anchor_p0503000

If you do use a support like this you will have to use packers on one side of it to keep it vertical. The suggestion that @MitchellMc has made is one I do feel comfortable with.

Dave

husainZahid

@MitchellMc @Dave-1 would it be a good idea to use some height adjustable deck pedestals (see attached photo for reference)? If not, I think the safest option would be build 2 concrete plinths to raise the right side legs of the gazebo. Would you be able to share any guidelines or videos for building the concrete plinths?

EricL

Hello @husainZahid

Thank you for sharing that information about the height difference, I believe Mitchell's suggestion of building a concrete pad would be ideal for your build. Please always check your local council's rules and regulations in regard to building footings for gazebos before starting. In regard to the pedestal footing, these are geared more towards low-level decking applications. It would not be ideal to use this on your gazebo post.

Generally speaking, you would need to build formwork to create the plinth. The plinth would need to have an anchoring method that would secure it to the concrete base. This could be generally accomplished by inserting reo bars into the concrete base and steel mesh placed into the formwork. I don't have a video guide for this type of build. To make sure that the footing complies with regulations, I propose engaging the services of a professional to properly build your plinth.

Let me tag my teammate @MitchellMc for his thoughts.

Eric

Dave-1

Good Evening @husainZahid

I am afraid that they would be unsuitable for this type of use. That 220mm is too high. The concrete plinth that @MitchellMc has suggested would definently be the way to head towards to secure the structure. Remember you are installing a gazebo that is designed to be installed on a flat concrete slab. Modifying it has caution attached to it and you dont want a weak point on one leg.

Dave

husainZahid

Thanks @Dave-1. Would you be able to point me to a video tutorial on building concrete plinths?

MitchellMc

Hi @husainZahid,

In essence, the process involves drilling appropriately spaced holes into the existing concrete to a depth of at least about 100 mm, then inserting and securing threaded studs into those holes using the chemical adhesive. It’s important to plan the finished height carefully so the studs protrude above the final surface of the plinth. This allows you to attach your mounting bracket later with nuts.

After the studs are set, you’d form a simple timber box around them to create the shape of the plinth; since the ground is sloped, the bottom edge of the form can be cut to match. Then you mix your construction grout and fill the form to just below the top of the studs. Once it has properly cured, you can remove the form, leaving a solid plinth with the studs ready for fixing.

I've created the rendering below to illustrate.

Mitchell

Dave-1

Good Morning @husainZahid

I just had a look but nothing jumped out at me that suitability shows a process of how to attach a concrete plinth to an existing slab. However the steps that @MitchellMc has shown just above are pretty much the steps that you need to do to ensure you have a suitable connection for the plinth to the slab so it holds the leg of your gazebo. If I come across a suitable link I will post it up. :) Any video that shows adding rebar/threaded rod into a slab will help you see the steps that you need, that plus @MitchellMc's guide will fill in the blanks.

Dave

husainZahid

Good morning @MitchellMc and @Dave-1 thank you very much for your guidance and detailed steps for building the plinth. I really appreciate it.

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