How to construct low-level deck over concrete?

[muzza1]

Hi, It's been really useful reading posts about low level decks here, but every one is a bit different so I'd appreciate some guidance with my project.

Intending to make a wooden deck for the front porch area of our older style cottage in Christchurch NZ. Thinking the deck can come up to the bottom of the front door sill, which is 120mm from the concrete/ground at the smallest clearance point. The concrete falls away from the house at around 2°, so with a level deck it would be 230mm from the ground at the entrance, and so needs a step.

I've read about using 90x45 framing timber as joists, haven't selected boards yet but maybe they will be 21mm thick, leaving only a small air gap maybe around 10mm. Then with a step up to 230mm at say 115mm, there's not much height to work with. So my first question is can I use 75x45 joists? What would the spacing be?

Suggestions on boards to use are welcome, often Kwila is the go-to around here...

Fixing the joists to the concrete ( which is a bit uneven), could be L brackets or maybe pedestal feet, with dynabolts.

I've drawn up a model of the deck to help convey what I'm thinking, and some photos as well. Would be great to get your ideas!

Many thanks

Muzza

[MitchellMc]

Welcome to the Bunnings Workshop community @muzza1. It's terrific to have you join us, and many thanks for your question about building a low-level deck.

Structurally-wise, it would be preferable to use 90 x 45mm joists, but you'll get better airflow with the 70 x 45mm timber. You can use 70 x 45mm, provided you place the supports at the required intervals. For 90mm, I typically recommend not spacing your supports more than 1000mm apart, so, for 70mm, I'd shoot for every 750mm. Joists are spaced at 450mm centres.

By taking your overall height of 120mm minus 70mm (joist) minus 19mm (decking), you're left with a 31mm air gap closest to the door. Unfortunately, there doesn't appear to be an adjustable pedestal foot to suit 31mm. I suggest using angle brackets for this portion. There is the Nurajack SE2 50-75mm For Timber Joist, which would be perfect for the step-up side, where the gap under the joist will be 71mm. 

Kwila, or Merbau, as we Aussies would call it, is also the go-to over the pond. It's a great timber to use on a deck, as it is robust and resistant to the elements. You could also consider a composite decking timber from Ekologix. Composite decking has a few significant benefits, such as minimal yearly maintenance.

Please let me know if you have any questions.

Mitchell

[muzza1]

Thanks @MitchellMc for your great advice! I was hoping you would answer! Ekologix looks to be a perfect option for us. Low maintenance is ideal and concealed fastening was something I was interested in too. 

I'm not sure about your comment "... perfect for the step-up side, where the gap under the joist will be 71mm." The step up side is 230mm high, minus 19mm (decking) minus 70mm (joist) would be 141mm gap, can you clarify?

With the high side at 230mm, I'm thinking a half height step at 115mm. The air gap will be reduced but hopefully still enough, even with Ekologix thickness of 23mm and joist 70mm, would be gap of 22mm. The 115mm step would be fastened to an area of asphalt, maybe only 20mm thick or so, will dynabolts still be a reasonable approach for anchoring to asphalt?

I've noticed at the sides of deck construction there can be 2 joists together, like this project: https://www.workshop.bunnings.com.au/t5/Featured-Projects/Low-level-hardwood-deck-over-concrete/ta-p..., would I need to do this?

When looking at construction stages, is it a case of assembling the joist framing sitting on the ground, then lifting up to the desired height, maybe to a string level, and then fastening to angle brackets and Nurajacks?

Thanks again, this is gold:)

[MitchellMc]

"I'm not sure about your comment". Sorry, and I'm unsure how I ended up with a 71mm gap @muzza1. You're correct; it would be a 141mm gap. There's a whole range of Nurajacks, and the Nurajack SE4 120-170mm For Timber Joist looks to suit that height. I've also just realised the Nurajack SE0 28-38mm For Timber Joist would suit the small gap. I'm just going to check that the price on those smaller ones is correct, as it seems a bit unusual that they would be so much more expensive than the largest version.

Bit concerned about that step height of 115mm. 230mm would be in the realm of a standard step height. Maximum step height should be not more than 225mm, but the minimum should also not be less than 130mm. Creating a step that is too small in rise can be as much of a trip hazard as one that is too tall.

As long as the step is connected to the deck structure, fixing the asphalt with a Ramset 10 x 100mm Galvanised AnkaScrew Masonry Anchor should work. A Dynabolt might crack the asphalt.

A double joist is needed on two sides of the deck to run a picture frame. The frame decking board is run over and parallel to the double joist, and then the decking boards run perpendicular to the joists and frame pieces. The double joist is just long enough to nail off the ends of the decking boards. Check out this guide on How to build a deck. In Step four, I take you through this process, and then in Step seven, you'll see the decking boards being fixed off.

Typically it's easiest to construct the frame on the ground and then lift it to the desired height.

Mitchell

[MitchellMc]

Hi @muzza1,

Those Nurajack SE0 28-38mm For Timber Joist - 5 Pack are actually $81, just pop in your local store, and the price should come up for you.

Mitchell