Decking over existing slab

[Johno35]

Hello all,

I've noticed a few threads regarding decking over existing slabs and I was hoping to get some feedback on how well the decking boards have held up against cupping, particularly boards over 136mm wide.

I've got 140mm between the slab and the underside of my deck but worried about potential cupping. I'm wanting to lay 136mm spotted gum using 12g stainless steel screws.

Any advice, tips from experience would be appreciated.

Cheers, John

[EricL]

Hello @Johno35

Welcome to the Bunnings Workshop community. It's fabulous to have you join us, and thank you for sharing your question about decking over existing slabs.

That is an excellent query, it would be interesting to find out how some of our member's low-level decks have fared in the past few months. You'll notice that some of the low-level decks built by our members are extremely low. Having a 140mm gap is great and I feel is sufficient enough to allow air to pass through. 

Please keep us updated with your progress, we look forward to seeing your deck assembled and ready for use.

Let me tag our experienced members @Neo19,  @Backyardtradie, @MGusto, @tombuilds, and @pepsp for an update on their timber decking panels.

If you need further assistance, please let us know.

Eric

[pepsp]

Hi @Johno35 welcome to the workshop community. 

My low level deck is around 190-250mm off the ground over concrete and open grass which I removed and laid down weed mat and gravel. I used spotted gum boards 90 x 22mm, joists at 450mm centres. The undercover section looks brilliant, hardly any wear. The section in the open has had a lot of rain in the last 15 months and there is evidence of cupping on 2 or 3 boards near where a join in the gutter came loose. 

Hope this helps.

Cheers 

Peps

[Johno35]

Thanks for the responses.

I'm fortunate that the section over the slab is undercover, the remaining decking is above ground that slopes away to a depth of approx 450mm. I'm going to allow a 5-6mm gap just to be sure I've got enough airflow, would hate to see any cupping on such a beautiful looking deck. I will post progress photos.

Cheers, 

John

[MitchellMc]

Hi @Johno35,

Good call on the board spacing. I'm currently building a deck over soil and have gone for a 5mm spacing for that reason. If any sides of the deck are not exposed to view, leaving a fascia board off can also assist with airflow.

Consider your screw placement in the decking boards. If cupping is a concern I'd suggest between 15mm and 25mm from the outside edge and no further than 25mm. The boards will cup on their outside edges, like a U shape. The closer you place the screws to the centre of the board the more it can cup, but I wouldn't advise going any closer than 15mm to the edge due to splitting issues. For my deck, I'm using 25mm. I've created some renderings for you below which should illustrate.

Keep us updated and I can't wait to see your finished deck.

Mitchell

[Neo19]

@Johno35 

Unsure if my experience is of much use to you as the merbau boards that I have used for my previous decking are also 90 mm wide like pepsp and are completely undercover on a slab. 

The gap I had to work with was between 170 - 180 mm. Decking still looks as solid as it did when I laid them two and half years ago. 

 I'm currently in the process of making a deck over slab that has 100 mm clearing at it's lowest point then slopes to 200 mm at it's highest point with mostly 90 mm boards and very few 140 mm boards as well. This deck wouldn't be completely undercover, however due to the large degree of slope and very good drainage, I am feeling quite optimistic!!!

Good luck with your decking project!

[Johno35]

Subfloor completed and have begun to lay the decktec joist protection/board laying system. It's a huge deck, 106m2 to be exact, might take a while to lay. Using 135mm spotted gum with 12g square drive screws.

 

 

 

[MitchellMc]

Wow! That's one impressive deck @Johno35. It's going to look amazing once you're done.

It would be well worth considering a self-drilling screw like the Zenith 12G x 65mm Stainless Steel Self Drilling Decking Screw for a deck that size. I've just used them on a much smaller deck, and they saved me a ton of time not having to pre-drill.

Many thanks for the update. We'll be waiting with anticipation for the finished project. 

Mitchell