How to build a low-level deck over existing concrete?

[benny4]

Hi I’m looking at starting a low level timber deck in a recently built house but just need some advice if I’m on the right track or someone to steer me in the right direction.

I’m thinking single height subfloor and will be built on existing slab and on ground. I have 160mm height to work with on the alfresco slab but can dig out some additional ground to get more clearance. Currently this is what I’m proposing.

- 120x45 wailer bolted into brick work

- 2/120x45 bearers (pink lines) supported by adjustable feet (on concrete) and posts in ground. I was going to span approximately 1600mm.

- 120x45 joists @ 450 centres in between bearers fixed with joist hangers. Max span 1500mm

- 140x19 spotted gum decking with 5mm spacing. Deck direction North/South looking at my sketch

- I will set deck out for a picture frame boarder

- On the natural ground laying approximately 50mm of stone to help with drainage.

Am I on the right track? Im not 100% sure on the spans and also the section of the deck on my sketch top left there will be a 600-700 cantilever I’m not sure will work.

I have also seen frames set up with just joists at 450 centres supported by feet and posts with noggins nailed in between.

I hope to get a start soon so thanks to anyone who replies!

   

 

 

 

[JacobZ]

Hi @benny4,

That's brilliant to hear, I'm excited to see your project get underway.

The main reasons for leaving a gap are so that there is room for the timber to expand and contract and so water can drain from the top of the deck. A 5mm gap should be adequate for drainage, but monitoring to ensure this gap is kept open from leaves, dirt and debris is going to be necessary.

I'd suggest simply coating the tips of your decking boards with whatever decking oil you are planning to use. Something like this Intergrain Spotted Gum UltraDeck Timber Decking Oil might be a good option. Spotted Gum, due to its tight grain structure and natural oils is already fairly water resistant, but any additional protection you can give is going to be worthwhile.

Keep us updated as the project progresses, I will be eagerly awaiting an update this coming weekend.

Jacob

[benny4]

Thanks @JacobZ 

My next question is I’ve started the framing over the alfresco slab. I’m using 90x45 and have spaced the L brackets approximately 1100-1200mm apart and alternating sides of the timber. Is that enough to hold the timber or should I be adding noggins as well? The overall length of the 90x45 is approximately 3900mm.

I also installed a L bracket close to the end of the timber near the joist hanger not sure if that’s overkill or not but thought it wouldn’t hurt I guess

[JacobZ]

Hi @benny4,

Adding noggins is definitely worthwhile. They help to add strength by transferring the load between your joists as well as helping to prevent the joists from twisting over time. If you have the additional material they are definitely worth adding.

There's no harm in adding additional brackets and I'd encourage you to use as many as you'd like as long as you are meeting the minimum requirements set out in the span tables.

It looks like your deck is coming along nicely, I can't wait to see further progress in the near future.

Jacob