What joist size to use for low-level deck?

[Zog]

Hi everyone. I have assembled the framing of the modular decking for my low level deck (see photos). What is in the photo is essentially a walkway, the main deck will be under the pergola and slightly higher. I've gained some confidence from what I've done so far and am considering using tuffblocks and building a framework rather than using the modular decking for the main deck. My question is just about the joist size. I'm not going to use a bearer and joists system, just a framework of joists, mainly because the Pergola is already there (before I bought the house) so I need to preserve the ceiling height of the pergola as much as possible.

I'm unsure about joist size and span of tuffblocks. The Tuffblock website has an article where they're using 2"x6" (45x150) joists, I've seen joists of 45x90 used in Australian based articles and the joists of the Good Times modular panels from Bunnings are 45x70. I'll probably use 140x19 merbau decking.

The pavers will be staying, they're not perfectly level, a small part of the deck will be on soil (the far sides in the second photo).

The total size of the main deck will be about 6.4m x 7.4m or 47.36 m2. It's a lot bigger than any articles I've seen about building a  low level deck. Any advice will be much appreciated.

[JacobZ]

Hi @Zog,

You can reduce the size of the joists to 90x45, but this would mean you need to increase the number of supports you have. According to the Hyne Joist Span Tables, 90x45 timbers can span 1400mm in a single span and 1800mm in a continuous span.

These Builders Edge 25-40mm Minifoot Pedestal Feet can be used at 25mm high with the 90mmx45mm joists and 19mm decking boards which will make the height of the deck 134mm.

These changes would alter your design, but with the increased number of supports, you wouldn't lose any strength in the deck and you would maintain more height overhead.

Let me know what you think.

Jacob

[Zog]

Hi @JacobZ 

I think it's a good idea, the last thing I want is to put all this work into it only to find it's too high. Also, it occurred to me that 45x140 at over 6 m will be pretty heavy, possibly too heavy for one person. as I'll be doing most of the work alone 45x90 might be more practical.

There's only one more thing I'm not sure about - the concrete footing for one of the pergola posts sticks out of the ground about 10cm (see photo) and is about 30cm in diameter (the photo makes it look smaller than it is). This is because the yard originally sloped and was higher at this point (only at that point). After checking the depth of the footing, I levelled the ground. As the deck will be lower than this I have to find a way to hide it. I'm thinking of putting a step on the edge of the deck or putting a bench there. Another idea is to put a framing box kind of thing at the bottom of every post (see photo). what are your thoughts?

[EricL]

Hi @Zog 

I'm leaning more towards the bench seat as it will not become a potential tripping hazard. Plus, building a bench seat is like automatically having extra seats whenever you have a party.  

If you need further assistance, please let us know.

Eric

[Zog]

Hi @EricL 

That's a good point, I'll rule the step out. Thanks

[Zog]

Hi,

The Builder's Edge Mini foot 25-40mm and small foot 35-60mm pedestals are rated to 220KG while the Builder's Edge Big foot 70-170mm is rated to 800KG. Does the load rating affect how far apart they're placed ? with 45x90s the span is recommended to be 1800mm (continuous). I'm guessing not as 220kg is a lot of weight but thought I better check.

Also, is the Outdoor Framing H3 treated pine good enough for a low level deck? when is the F7 and H4 etc necessary?

[MitchellMc]

Hi @Zog,

The load ratings for the feet would typically exceed the capabilities of the timber and its span. As long as you stick to having a foot as per the required span of your joist, their capacity won't be exceeded. However, personally I recommend 90 x 45mm being supported every 1200-1500mm. 1800mm is not the recommended span it's the max span it's capable of and you generally don't want to build at the max span as you can end up with a springy deck.

F7 is a different structural rating used for hardwoods, and the H3 MGP10 Pine is fine for above-ground joists. H4 is required for timber in the ground, or that has contact with it.

Mitchell

[Zog]

Hi @MitchellMc 

Thanks Mitchell, I'll be getting started within the next week and all the advice has been a big help.