Low level decking

Hi all

I am currently in the process of planning a deck 8050mm x 5400mm but I’m restricted with height (70mm in the concrete slab) and around 100mm at the other end (I have already dug out some soil in this area, so I can have a decent height and I can dig more if needed)

Hoping you can help with a deck design (It's my first ever decking project) as I’m struggling with the specifications regarding the below:

Would 100mm clearance to the ground be enough to use H3 treated pine or do I need to use H4?.
The span between joist and bearers. Would you recommend using a 90 x 45mm or 120 x 45mm or is it better to use a 90 x 35mm or 120 x 35mm considering I only have 70mm height to play with? what span do I need to have?
I’m also thinking about using the tuff block as a foundation system. Thoughts?

Any assistance from anyone would be greatly appreciated!!!

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Comments

MitchellMc

Hi @fcardenas,

I'd encourage you to read through some of the wealth of information and previous discussions on this topic within the community. Low level decking Help by @Sam_Newbie has several responses from knowledgeable members about decks being built close to the soil. I trust you'll also find this article helpful: How to build a low-level deck by @Adam_W.

Most would not consider 100mm enough space to build a deck as it is close to the soil. If you remove the decking and joist thickness, that will leave 36mm to the soil. Creating a moist environment for H3 timber leads to premature rot. You might like to check with your local council on their height requirements as mine would not allow such a low deck where inspection isn't possible. You could build a deck within this height, but whether it is a good idea and how long it lasts is another question.

Since you are already turning the joist on the side as there is not enough room to stand on edge, I would suggest you use 45mm thick timber.

I do not believe Tuff blocks can hold a joist width ways. The gap provided accepts up to 45mm thick timber so a 70 or 90mm wide timber will not fit.

Unfortunately, I think this is a situation of just keep digging. You'd really want at least 150mm of clearance under the timber and even then you'd need to think about laying gravel and adequate drainage.

Mitchell

fcardenas

https://www.workshop.bunnings.com.au/discussion/comment/11081#Comment_11081

Thank You Mitchell!!

I have dug more soil and I have now a clearance of ~220mm.

@MitchellMc can you help with some basic drawings based on the above deck dimensions(8050mm x5400mm)? I understand good practice is to keep about 450mm gap between joists but I'm not sure the bearers and the number of posts I will need.

your assistance would be greatly appreciated!!!

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/68496#Comment_68496

Hi @fcardenas,

To build a traditional joist over bearer deck, you'd need more than 220mm. A minimum bearer width would be 140 x 45mm treated pine and your joists 90 x 45mm treated pine. You'd need at least 300mm, this would include the 140mm bearer, 90mm joist, 19mm decking and a minimum 50mm gap underneath. Please check with your local council regarding how much clearance they require under decks in your area.

With only 220mm clearance, you'd still be looking at just working with joists supported every meter directly into the ground. As you have mentioned, a spacing of around 450mm is normally sufficient. If you run the joists perpendicular to the house, you would require 19 of them to span the 8050mm distance.

Please let me know if you have further questions.

Mitchell

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