How to build a low-level deck over concrete?

[Halima]

Hi team,

Looking to tackle a DIY deck at my parents backyard.

Some details of the space:

- roughly 24m2 (7mx4m)

- Working area is concrete slabbed. Level is variable in certain parts

- Clearance from the internal space to the concrete slab is around 100mm only

- The planned deckspace is covered with an awning - still yet to decide if I'll work around the 4 posts or work right up to it

My thoughts:

- From what I can understand the internal to slab clearance available would be a ideal for 'low profile' deck 

- I'm thinking of using adjustable pedestal feet that will accept the joist frame directly (no bearers). I saw that there's also pedestal feet that will accept blockers

Questions:

- Are gal nails or framing screws better?

- is there any significant feature advantage with pedestal feet that will accept blocker pieces? does it even matter?

- H3 treated wood is reccommended for above ground/outdoor use but what size timber pieces should I be considering and how far apart are the spacing between joists at their centre? I'm sure there's a best practice/building code to this question I'm not familiar with where to start looking

- How do I know the spacing requirements for the pedestal feet?

- Can anyone improve on what I'm thinking of doing or provide other ideas?

thanks in advance!

[Halima]

Thanks Mitchell, I like the idea of the 45mm thick for that extra bit of strength. I cant seem to find any joist hangers suited for this dimension wood however- what options can I use to fix these? I noticed in another thread you suggested a pryda 40x50 hanger however they dont seem to exist a product listing anymore

[MitchellMc]

Hi @Halima,

You can screw through the rim joist into the ends of the joists with 125mm bugle screws. This might prove to be a bit of an issue where the rim joist is against walls. Typically, you'd attach the joists to it and then move them all into position. That might require a helper, considering the size of the deck.

Mitchell

[Halima]

Thanks Mitchell

70x45 sounds like a goer. I cant see any joists specfically for this dimension timber however. Could you suggest a joist hanger and L bracket?

for the L bracket I found this from pryda

https://www.bunnings.com.au/pryda-pergola-angle-bracket-88-x-63-x-36mm_p1071032

Would this be appropriate for the job

[MitchellMc]

Hi @Halima,

70mm timber is typically not used as joists as generally 90mm is the smallest considered. You won't often find joist hangers or brackets to suit 70mm being used as joists. Does the above method of screwing through the rim joist into the ends of the 70mm joists to support them not sound like it would work for you?

Take a look at this guide on How to build a deck. In step three, you'll see how I've used screws to connect the rim frame in the corners and attach the joists to it. No brackets or hangers are used, just the bugle screws.

Mitchell

[Halima]

apologies just noticed my previous reply came through so doubled up there!

the west section of the rim joist will be hard against the house where the back door is located so perhaps pre screwing with bugle heads as suggested might be the only way, unfortunately at the lowest section I only have 90mm clearance so the larger joists are out of the question. 

Plan is then to fix the west rim joist to the house via masonry screws (10mmx100mm) and after level have everything else referenced from that.

Does the pryda L bracket I suggest work as a supporting piece? unsure of what gauge I am expected to use as part of the support structure

[MitchellMc]

I am unsure how you would use the L bracket to attach your joists @Halima. I would suggest creating your rim frame, installing the joists within it with bugle screws, then shifting the deck frame into position with the help of a few friends, lifting it to the correct height on the adjustable pedestal feet, levelling and then attaching it to the wall where needed with Dynabolts.

Mitchell

[Halima]

Thanks Mitchell. Sounds like thats the way I will go.

L/angled steel brackets are for support along the length of the joints - fixed to the concrete because clearance is quite low. ie. No room for pedestals

[MitchellMc]

Thanks for clarifying @Halima. For brackets that support the joists and connect them to the concrete, our members typically go for a more heavy-duty bracket. The size of the bracket depends on how high the joist is above the concrete; you want the mounting hole to be about midway up the joist.

Mitchell

[Halima]

Thanks @MitchellMc 

I was on the lookout for cheaper alternatives thanks for that bracket suggestion! and yes will keep positioning in mind. for fixing the bracket to concrete will m10 dynabolts around 50mm suffice or should I go for 100mm? and what about bracket to joist, will a red tip screw like I am going to use for the joist hanger suffice or should I opt for something else?

https://www.bunnings.com.au/pryda-timber-connector-screw-12g-x-35mm-hex-head-pack-50_p0084754?region...

[MitchellMc]

Since these brackets will essentially be bearing the entire weight of the deck, I'd suggest going with the thicker versions @Halima. The Pryda brackets are not designed for such use. M10 Dynabolts would be fine. I'd go with the 75mm as the 100mm might be a bit excessive. 

Our members either bolt the bracket to the joist or use coach screws.

Mitchell