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How to build a composite deck on sloping ground?

Rusany

Dear community,

I’m starting to plan a DIY composite deck for my backyard. The area is approximately 3.7m × 6.8m. I’ve been researching the tools, materials, and steps involved, but as this will be my first time building a deck, I’m feeling a bit overwhelmed and would really appreciate some advice before moving forward.

The existing ground is sloped and mostly concreted, with the outer/lower side paved. The right side is about 45cm higher than the left side, while the inner side near the house is around 20cm higher than the outer side.

My main questions are: Would adjustable pedestal feet be suitable for this type of sloping surface and able to achieve a level deck? Or would there be a better structural solution for this layout?

I also have some recycled timber salvaged from my demolished garage. The timber is classified as H4 or H5 treated pine. I’m considering reusing it for the joists/frame — would this be appropriate? If not, would you recommend a timber frame or a metal frame instead? The deck will be under a patio and attached to two walls of the house, but it will still be exposed to rain and lawn irrigation.

Any advice or practical tips regarding framing, levelling, drainage, or general considerations for a deck of this size and site condition would be greatly appreciated.

Thank you in advance!

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MitchellMc

Hi @Rusany,

It sounds like you’re doing the right research before jumping in, especially for a first deck build. A composite deck on a sloping site is definitely achievable as a DIY project, but the amount of fall across your area does mean the subframe design will be one of the most important parts to get right.

Based on the height differences you mentioned, I’m not aware of any pedestal feet that would accommodate the full 450mm fall on their own. Even the larger adjustable pedestal systems are generally only suitable up to around 150–170mm adjustment. On the higher side of the deck, you would likely need to construct a secondary subframe or raised support structure first to reduce the gap before adjustable feet could be used effectively.

Because the area is already mostly concreted and paved, you may find a raised frame system on fixed post supports to be a more practical solution than relying entirely on pedestals. The good thing is that your existing hard surface may reduce the amount of excavation required, but you’ll still need to ensure there’s adequate drainage beneath the deck and that water can’t become trapped against the house or framing.

The recycled H4/H5 treated pine could potentially be reused for parts of the subframe if it’s still straight and in good condition. H4 and H5 treatments are typically only used for in-ground applications, so it would be unusual to see them used in a garage.

For the frame itself, both timber and steel can work well with composite decking. Timber framing is usually the more approachable option for DIY builds and easier to modify on-site, while steel framing can offer better long-term stability and reduced movement. Since the deck will still be exposed to rain and irrigation, whichever frame type you use, good airflow and drainage underneath will be very important.

Before getting too far into planning, I’d strongly recommend checking with your local council regarding permit requirements for a deck of that size and potential finished height. In many cases, once a deck reaches certain heights or spans, permits and engineering details may be required. If approvals are needed, that can heavily influence the design and build method from the beginning.

You might also find this guide really helpful as a starting point, as it walks through the general process and considerations for low-level deck construction: How to build a low-level deck.

If possible, it would also really help to see a few photos of the area, particularly showing the slope and surrounding surfaces. That will give members a much clearer idea of the site conditions and allow for more specific advice around support options, framing layout, and levelling methods. Let me know if you need a hand uploading some images.

Please let me know if you have any questions.

Mitchell

Rusany

Hi @MitchellMc,

Thanks for your response and the helpful advice. Please find attached a photo of the current site where I am planning to build the deck. I am expecting the finished deck to be aligned with the existing step and edge.

Kind regards,

Rusany

MitchellMc

Hi @Rusany,

You can certainly use 90 x 45mm timber and pedestal feet closer to the door, but as you move further away, you'll need to use larger 140 x 45mm and posts to achieve the needed height. I've created the below rendering to illustrate.

Mitchell

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Rusany

Hi @MitchellMc,

Thanks for the timber details and the render. They provide excellent clarity and have given me a lot more confidence moving forward.

I just have a question regarding the framing layout: as shown in the render, the joists are interrupted at each bearer. Is there a specific structural advantage to this layout, versus using continuous, single-span joists across all three bearers? Also, what joist-to-bearer connectors or fasteners do you recommend to properly secure them? And, which timber size would you recommend for the posts?

Thank you,

Rusany

MitchellMc

Hi @Rusany,

You could certainly do a joist over bearer system, which would be easier. The only benefit to having the joists set within the bearers is that when you get to the shorter end, you can continue with the same layout futher. If you go joist (90mm) over bearer (140mm), you'll need to switch layout when you hit 230mm in height.

For joist-to-bearer connections, you'll be using https://www.bunnings.com.au/pryda-joist-hanger-to-suit-45-x-90mm_p0294207 . For the posts, https://www.bunnings.com.au/88-x-88mm-1-8m-f7-h4-kd-treated-pine-premium-post_p8423171 or https://www.bunnings.com.au/90-x-90mm-2-4m-post-f7-h4-treated-pine-premium-kd-2-4m_p8422817 .

Mitchell

Rusany

Hi Mitchell,

Thanks a lot for the explanation and the links. They are very helpful. I am just wondering whether there is a significant quality concern or huge compromise if I am using all 90x45 H3 for the subframe? Or larger sized timber for bearers/posts is a must and should not be compromised? And 26 is a sufficient quantity of timber (3.6m length) needed for the site (26m2 on a slight slope as mentioned above)?

Kind regards,

Rusany

JacobZ

Hi @Rusany,

You can certainly use 90x45 H3 for all of the joists and bearers, as long as they are used within the limits of the relevant joist and bearer span tables. When used as a bearer, a 90x45 can span 900mm between posts. When used as a joist, they can span 1200mm between support from either a bearer, post or pedestal. They would however not be suitable for the posts. You need to use 88x88 or 90x90 for your posts, like those Mitchell mentioned above.

Just as a rough guide, without getting too precise, you'll need four bearers runs that are 6.8m in length. This would mean 8 x 3.6m lengths. You then need a joist every 450mm, so roughly 16 to cover the full 6.8m length. This gives me 24 lengths of 3.6m, but it is always a good idea to have a spare or two just in case. I think 26 is a fair estimate for how many lengths of timber you'll need for the basic frame. Remember you can always return the timber as along as it has not been cut or grab an extra length or two if you need them.

Hopefully this helps to clarify. Let me know if you have any further questions.

Jacob