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How to build low profile deck over soil?

Growing in Experience

How to build low profile deck over soil?

Hi everyone. I’m hoping someone could help me on this.


I’ve been thinking to deck the whole area of my backyard (see photo). The area is around 25 m2. I have a few questions:

1. Should I still dig the grass or just put a weed mat on top?


2. The clearance from the finish level (tiled area) is around 130-150mm. I am planning to use 90x45 H3 for my joist and bearers and Merbau for my decking. I will also use Builder’s Edge Pedestal and put some pavers and blue metal underneath to play with my level as some areas are not even.

Will this be okay?


3. In terms of connecting the timber to the existing concrete, what is the best way to do it?


4. Would you recommend to connect the deck to the blockwork retaining wall as well? 

5. I will space the joist at 450mm and planning to put the Builders’s Edge Pedestal at 1000mm spacing. Is this enough?


6. What size of screw should I use for my timber and when I bolt the timber to the concrete?



Bunnings Team Member
Bunnings Team Member

Re: Low profile deck over soil

Hi @Mdsyd,


It looks like you ended up going with the 90x45 for your framing. Is this correct?


There's no issue with running a joist over the top of the grate, but you shouldn't place a paver and toughblock on it. This would be transferring the weight of the deck onto something not designed for this purpose.


If you could advise the dimensions of this space, I may be able to advise a slight redesign that would allow the deck to be fully supported and fall in line with advisable bearer spans and advisable joist spans.


I've done a very basic mock-up of a potential solution using the 1200mm level as a scale for size. The red line is an additional bearer, and the yellow dots are decking supports. 



You would then run the joists perpendicular to the bearers as you would have originally, they would just be split into two separate sections.

This should work based on the size of the level, but the overall dimension would need to be confirmed. 


Allow me to tag some of our helpful members to see if they have any other ideas, @Dave-1, @DIYGnome , @Nailbag.


Let me know if you have any further questions.




Growing in Experience

Re: Low profile deck over soil

Yes 90x45. Size of the corner is 1100 x 1250mm.


Yes, I’m aware not to put something on top of it. I only place the paver and builders edge to help me lift it while I screw the other side. 

Home Improvement Guru

Re: Low profile deck over soil

Evening @Mdsyd 

Going through your project I have some concerns over covering that drainage grate. From memory councils are like the waterboard and dont like having their drains covered. I wonder if you could factor in either the deck being able to be opened like a trap door or another grill that can sit in/on your wooden frame to allow inspection and that can be lifted so they can access the drain if needed. And yeah as soon as you cover it is when they will need access lol It happened to me at a sewage inspection point that I was trying to grow grass over as I saw it as an eyesore... They came out in the middle of the night and stripped the grass and cleared a blockage without a concern. Thats why I am suggesting that access can be made to that drain cover.



Becoming a Leader

Re: How to build low profile deck over soil?

Hi @Mdsyd is your intention to deck over the grate as this wouldn't be permitted by any council. At the very least I would advise making that area a non-fixed removable hatch slightly larger than the grate dimensions but still inline with your decking boards.


Regards, Nailbag

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