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Greeting guru’s. last time I asked about a deck I got my head in a spin and ended up doing a knock down and rebuild because “it was easier”.
I now have a new home and a very low area in the front of my house and am wanting to do a low level deck so we can better utilise this space (which is very shaded by a balcony, gets quite a bit of water run off from the neighbours, and contains a storm water pit. ).
the space below is about 5400 long x 2600. The pit is only 60mm lower than the SLAB. I am wanting to have the deck finish about 260mm up from the pit (200mm up from the slab) - which gives me about a 130mm step from the patio onto the deck.
I have so much clay, bricks, debris, and a gas line under the soil that I am trying to avoid doing piers (I am concerned about the amount of work to dig the footing, and am concerned that a mistake will be less forgiving). I have done a lot of clean up to level the ground with a slight slope, and have put a French drain to try and catch the runoff and direct it to the pit. I was thinking of putting builders plastic over the whole area, covering it with coarse sand (for drainage). and then putting supports on the sand (maybe with a paver under each) and a single layer frame so I can meet my target height with the limited clearance.
i am planning on doing composite decking to reduce maintenance in the future.
i am attempting this myself so try and minimise the cost and for the self satisfaction - but I am an over thinker and will be unlikely to start until I know what I’m committed to. Does anyone have advice? (I looked at the span tables - but they always confuse me). Thanks
Hello @qroozn
Thanks for sharing your question about building a low-level deck. I suggest having a quick look at these guides - How to build a low-level deck by @Adam_W and How to build a deck by @MitchellMc. These guides will give you a very good idea of what you need to take into consideration when planning your decking frame.
I also recommend having a look at the TuffBlock 300 x 300 x 90mm Instant Foundation System Deck Support along with Builders Edge 70 - 170mm Bigfoot Xl Pedestal Feet.
These pedestals allow you to build a deck without having to dig into the earth. Here is an excellent example of a deck that is single level, goes over concrete and soil - Low-level composite deck over concrete and soil by @oninpena. The timber used for that decking frame is the 90 x 45mm Outdoor Framing H3 Treated Pine - 2.4m. It's rated for outdoor use and is structurally sound.
I understand that the items I've suggested are not tall enough for your needs, but I suggest building brick footings made from your excess stock to increase the height of the pedestals.
Let me call on our experienced members @Dave-1 and @Nailbag for their recommendations.
I also recommend having a look at the Bunnings deck building guide:
- How to plan out your deck
- How to set out a deck
- How to install stumps
- How to install bearers
- How to install joists
- How to lay decking
- How to add a step to your deck.
If you need further assistance, please let us know.
Eric
Good Evening @qroozn
Sounds like a pretty interesting project, lots of bits to work out for sure.
Are you intending to have the deck over the drainage pit? I suggest against it if you are. Access for that type of pit would always be needed and if unsure I would contact your water authority.
Im not keen on the sand on the ground under your supports for the deck. I figure sand will shift with time and you will end up with an issue.
The air vents for your house are about the height you mention you want the deck at. Are you making sure that they will remain able to breath?
I do like @EricL's suggestion about a brick pier. It may be easier building them then digging holes. I have just dug a 120mm by 650mm by 2000mm ditch for drainage and pavers in clay and it was hard going. Doable but hard 
Black plastic - Another one I am not keen on as it stops the ground drying after its wet. The ground will be under the deck so unlikely to have weeds growing.
With your drainage for the yard, I like the idea of the french drain and would really only cover the drain itself with gravel, mainly to provide an area for the water to drain into and then into the ag pipe. The end of it looks like its level with your pit? The rest of the soil/clay I would rake/slope to create a pathto the pit and pretty much let nature do its job and drain towards the drain itself.
Dave
Hi @qroozn,
Is it possible to get a quick sketch of what you have in your head so far?
I am happy to check everything against the relevant span tables to ensure it is in line with the recommendations.
You're currently looking at a two-tiered frame, with joists sitting on top of bearers. This is usually not necessary with a low-level deck such as yours.
My suggestion is to change your structure to a single tier, with 140x45 used as both bearers (supported vertically) and joists (connected to bearers). Following the Hyne Timber Timber Span Tables, 140x45 pine can span 2600mm when acting as a joist and 1400mm when acting as a bearer. This means you can use two long bearers on either side of the gap and then your joists do not need footings beneath.
See my basic design below. This design will work in an ideal world, but some slight adjustments may need to be made to accommodate your drain.
If this is wildly different to what you are thinking, please let me know. I'm more than happy to work with you to come up with a design that will work for you.
Jacob
i have been struggling to find a tool to help me draw up what I am trying to do. So here is a rough sketch. The width of the area is 2550 wide, but I was thinking of making it 2400 to optimise timber (and to keep it off the house a bit for termite barrier etc. I am only planning on doing the 2550x5250 bit down for this exercise.
Thanks @JacobZ .
I have a clearance of about 250-300mm height to work with (so 140 bearers +90 joists + 20 composite decking = 250 ) seemed to be a good height to work with.
also given its composite, I understand i need more joists... so 90mm seemed more economical for joists.
attached is roughly what i think i need. (based on 140x45 bearers and 90x45 mm joists).. I might just need to move a few thing around obstacles.(and depending on if i am going to do picture framing).
Hi @qroozn,
Your plan looks solid to me, and it will certainly ensure everything is well-supported.
Having a two-tiered deck frame means you will have to use a Pryda Joist Strap 162mm everywhere that the joists run over a bearer.
140x45 bearers can span 1400mm between stumps, so you have some room for movement to accommodate the drainage grate.
The column will be a bit trickier to accommodate. At the current spacing, I am fairly confident based on your measurements that the central bearer will run just outside the column. This means that at least 1 joist will have to butt into the column.
See the below images of a potential method of dealing with this that will be nice and solid. Note that I have hidden the bearers and stumps for ease of viewing.
You can use a Pryda Joist Hanger - To Suit 45 x 90mm where the joist butts into the side of the blocking against the column.
You'll also note that I have added rim joists to the ends of your joists. They are a must and should be taken into account when purchasing materials.
Let me know what you think and if you have further questions please feel free to ask.
Jacob
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