How to build deck over gently sloping ground (about 17% slope)?

Sebastian90

Hi all,

I’m about to build a deck in my mum’s backyard in Adelaide, and I'd appreciate some tips on managing the slope beneath it, especially since the house foundations aren't very deep.

About the project:

• Deck size: Roughly 5 metres by 2.5 metres.
• Underneath height: Previously, there was a deck here, and underneath was dirt with just enough height to duck under—it wasn't used as a functional space.
• Ground slope: The ground slopes down away from the house, approximately 37 cm drop over 220 cm distance (about a 17% slope).

I’ve calculated the slope roughly as:

• Percentage: ~17%
• Angle: ~9.5°
• Ratio: around 1:6 (every 6 cm horizontal distance drops by about 1 cm).

What I'd like help with:

• Safely levelling or working around this slope without disturbing the shallow house foundations.
• Managing drainage properly to keep water from pooling near the foundations.
• Creating a tidy, low-maintenance space under the deck suitable for occasional use.

Extra context:

• Plenty of paving bricks available onsite.
• Do I need some kind of retaining system?
• Looking for not the perfect solution but rather effective and affordable.

I've attached some pictures to help show what I mean. I'd really appreciate any suggestions or experiences you can share.

Thanks in advance!

MitchellMc

Hi @Sebastian90,

It sounds like you have a good setup, as water will naturally run down the slope rather than pooling near the house. To further manage drainage, you could install sub-surface agline drainage to collect and direct water further down the slope, helping to prevent any potential issues.

Since you're building an elevated deck and planning to connect it to a bricked wall, you'll need to consult an engineer. Keep in mind that you can't use a wailing plate to connect to a brick facade wall, so you'll need expert advice on how to properly attach the structure to the house and determine the required footing size and depth for the sloped site. It would also be worth checking with your local council to see if approval is needed and whether plans need to be submitted.

For the space under the deck, crushed rock is a great option for keeping the area neat and usable while also preventing soil erosion from rain falling through the decking. Paving is also another option since you have bricks available.

Please let me know if you have any questions.

Mitchell