Will this deck sub floor work?

[Squirt]

Hi guys!  What an awesome forum.  I'd like some advice please.  I'm planning on building a small deck directly over a concrete patio with only 162mm clearance.  I was thinking of using 140 x 19mm Merbau on 90 x 45mm joists and 45 x 90mm bearers dynabolted to the concrete with plastic window spacers underneath to level and clear the concrete sub floor by 8mm, similiar to what can be seen in this terrible YouTube video: https://www.youtube.com/watch?v=QC5d3jDzySQ

My question is, what are the advantages/disadvantages of doing it this way compared to using angle brackets as done here: https://www.workshop.com.au/t5/Outdoor/Merbau-decking-over-concrete-slab/td-p/31230

I've planned the sub floor for my deck, it is completely under cover and the top rectangle is flush into the building with brickwork on all 3 sides, there are also 2 brick piers in the bottom corners.  I will be running the decking vertically as the diagram shows and using a picture frame border.  I was also thinking of using the Camo Fasteners Hidden Screw system but I've heard that 140mm boards are susceptible to cupping unless the joists are closer to 400mm.  I've placed the bearers at 1088mm apart and the joists at 380mm, is this overkill?  Is there a better/easy way to do things?  Thanks for your time.

[MitchellMc]

Welcome to the Bunnings Workshop community @Squirt.

If you are laying the 90 x 45 mm on its side I would be Dynabolting through it like in the video. You couldn't really use angled brackets as you would need long bolts (over 90mm) to fix the timber to the bracket. In the discussion you are linking to they are using 45mm thick timber and in that case it is perfectly fine. There is just no advantage to using an angled bracket that I can see and Dynabolting through the timber is a significantly more positive fixing method.

Let me mention @Adam_W who recently put together the article How to build a low-level deck. He might be able to provide some information as to whether the joists at 380mm is overkill or not.

Mitchell

[Squirt]

Thanks @MitchellMc and @Adam_W.  After going over the plans a few times, reducing the amount of joists leaves me with a span larger than 450mm, so I'll stick to the 380mm spans.  Will I need to place noggins between the joists?  Also, should I use triple grips to attach the joists to bearers, or will framing nails be sufficient to hold it all together?

Thanks again!

[MitchellMc]

Hi @Squirt,

I don't believe noggins would be needed between joists as you've already got them spaced very close together. You could certainly use the triple grips, though I would be inclined to drive a couple of 100mm nails through the ends also.

Mitchell

[Jason]

How is your decking project going @Squirt? We are looking forward to seeing what you can create.

Feel free to also post again if you need a hand with anything. 

Jason

[pete_brig]

Hi @Squirt 

Just saw this thread. " bearers at 1088mm apart and the joists at 380mm, is this overkill?" would depend on looking at span tables (off the top of my head, I think I used 2-3m spacing for bearers but they were 190x??). The size of your bearers will depend on their span i.e. distance between the fixing points to the concrete. You could opt for fewer, larger bearers if you fixed the joists inline using joist hangers--possibly needing larger joists. You could probably omit one or the other & just use larger timber e.g. 140x?? fixed to the concrete but would then need to include noggins. My jerry-builder understanding of using bearers/joists is to reduce the number of stumps typically needed to create an above ground floor i.e. with smaller bearers, more frequent stumps are needed with less spacing between the "lines". So it can come down to cost--is it cheaper to use smaller timber but more of it or larger timber but less of it & then add in the larger/smaller number of fixing points/stumps.

As far as the span for decking goes, I recall the standard is 450mm between joists to avoid "bounce" but probably depends on the timber type (e.g. pine vs hardwood). Cupping results from underfloor moisture so the span & number of fixings (e.g. 3 nails/screws across 140mm rather than 2) plays a part in reducing it but with little airflow it'll probably be a battle. I used 90x19 merbau on a deck with 400mm clearance over an old patio & got some cupping; slightly worse for merbau on a boardwalk with 100-150mm clearance & no airflow. If your budget can accomodate, maybe consider using artifical decking (Ekowood?). Cheers