How to build a low-level deck?

[Taco]

G'Day All ,

I am looking to build a low level deck to join a shed , patio and bedroom

Structures are 100mm,150mm, 130mm above ground, so all relativity close in height.

The plan is for excavation and concrete poured then deck added on top - but I wasn't sure what I needed to ask for excavation depth wise to the concreters? ie finished slab height

Just looking for some info so I can have an informed discussion with the concreters

[JacobZ]

Hi @Taco,

Thank you for your question. 

Do you have any photos of the space? Some measurements of the area would also be handy. This will help our members to understand what you're trying to achieve and offer advice accordingly. 

Is there a reason you're planning to pour a concrete slab and then cover it with a deck? I'm happy to offer advice if there is, but it's likely not the best solution.

If the plan is to build a deck over the area, there's really no need to have a concrete slab. Realistically it will be much cheaper, better for drainage and better for the deck's longevity to build the deck on concrete piers.

When there is the ability to excavate the area to create the height required for your deck and then build the deck on concrete piers, I'd always advise using concrete piers instead of a slab.

Allow me to tag @Nailbag, @Dave-1 and @Jewelleryrescue for their thoughts.

Jacob

[Dave-1]

Good Evening @Taco 

I was wondering if you can supply a few pictures of the area you are describing. I am like @JacobZ in my preference to not build a deck on a slab. For me its Mainly drainage reasons but also you are digging a hole and putting concrete money into it

You might still need to escavate the area a little but nowhere near what you would if you went down the path of a concrete slab and deck height.

If you really want the concrete I would suggest to consider drainage installed before the slab goes in, the fall of the concrete should be away from the house (tho that may be an issue with you having three structures next to it) and also some conduits installed for electrical work that you may do in the future (small cost so you have peace of mind down the track)

Dave

[Taco]

Thanks for the reply @JacobZ and @Dave-1  , sorry I should have mentioned my block is full sand so I was concerned that piers wouldn't work in the sand as a builder had mentioned it to me previously,  but def agree your points RE slab and drainage.

[JacobZ]

Hi @Taco,

Thank you for the additional information, 

If the soil is sandy on top, you may have issues with a concrete slab anyway as they need to sit on top of a solidly compacted base to prevent movement and subsidence. 

The NCC Part 3.2.2 Preparation states that -

On loose sand sites or sites subject to wind or water erosion, the depth below finished ground level for footings must be not less than 300 mm.

Using this as an example, if your site is loose sand, you would need a concrete slab that is at least 300mm thick, which realistically is a monumental amount of concrete for a decking project.

Concrete piers may actually handle this issue better as pier holes can be extended down to more solidly compacted soil.

It sounds like the best place to start might be by having a soil survey done to determine the composition of the soil and taking their recommendations on what style of foundation is best for your deck. 

Let me know what you think.

Jacob

[Nailbag]

Hi @Taco 

With the cost of concrete these days, to cover it over with a deck is essentially throwing money away. I would follow @JacobZ advice and have the ground surveyed. Stumps will be a far cheaper alternative and achievable once the specs have been established with a small investment in the research. 

Nailbag. 

[Taco]

Thanks for the replies all, bit to think about - seems like its probably beyond me and will need to go with a builder

[JacobZ]

Hi @Taco,

I understand it can seem a bit daunting, but once the foundation is in place, the deck building itself is fairly simple, just following one step after the other.

There are plenty of people who can help out with guidance on building the deck, but unfortunately, we can't survey the ground conditions for you.

Maybe you could break it down into parts and contract out the foundation design and pour then build the deck yourself. 

Let me know what you think.

Jacob