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I have an alfresco area which I'd like to deck. The joists (Green lines) will most probably fix straight into the concrete and ideally, I'd like to go 1500mm beyond the concrete (Red line). I have a storm water pipe (White line) about 800mm out from the concrete. 800mm out from the concrete, the ground is at its lowest point, which is about 200mm lower than the alfresco. With regards to the footings and bearers, what are my options?
Community manager's note: Check out How to build a low-level deck for expert advice.
Hi @IronStan001,
Your options depend on what calibre joist you'll be using. If they are large enough, you'll be able to span the 1500mm distance and just have supports at the end of the joists. If you were looking at something smaller like 90 x 45mm joists, then you'd need a support midway. That will get a bit awkward having the cement footer so close to the pipework.
How high can the deck be above the concrete slab? You need to check timber span tables but 140 x 45mm should be large enough to fully span that gap, but it means there would be a step up onto the deck.
Please let me know if you have any questions.
Mitchell
What sized joists would I need to span the 1500mm gap? Would double 90x45mm joists be sufficient? 140mm joists would be level with the brick.
You'll need to verify that by checking the applicable timber span timber from suppliers @IronStan001. Typically, 90 x 45mm will have a max span of around 1500mm, so 140 x 45mm should easily cover your 1500mm, especially if you have the supports slightly in from the end. Although 90 x 45mm might be able to span the 1500mm, I don't recommend spans over 1000mm as it can lead to a spongey and bouncy deck. However, if you doubled them up, that should be fine.
Mitchell
I'm not sure I'm quite following @IronStan001.
What sized joist are you using for the deck on the slab, presuming that's the "alfresco" area you're referring to? Ideally, you'll consider the level when installing, and the deck will transition out over the soil at the same height.
Mitchell
Sorry, I mean the transition into the deck from inside. In the first pic, the 140mm joist finish level with the brick and as such, the deck boards would be level with the internal floor. In the 2nd pic, a 90mm joist would result in a small step down of about 30mm. I transitioning from inside to outside I dont think would be an issue, but outside to inside with that 30mm lip might be a tripping hazard. The size of the joist would be a TBA at the moment, depending on what will let me span across the 1500mm gap.
Hi @IronStan001,
It's best to get the area level. This is achievable with the 90mm, you'd just elevate it above the slab on pedestal feet.
For the area over the soil, you can then switch to 140mm to get the larger span needed to jump the PVC pipe. Or, you could do the whole deck in 140mm if budget allows.
Mitchell
I don't think 140mm joists are suitable, since I only have 140mm max from the brick to the slab. Since I'd need to raise the joists just above the slab to allow for drainage as needed, I'd then probably need to rip along the length of the joist to shave off what I need.
The entire length of the deck is just shy of 5m, so if a continuous span of 90x45 allows the jump from the concrete to the bearer, I should be OK.
Hello @IronStan001
Thanks for posting that render, the 90x45 should work nicely. As mentioned by @MitchellMc you can have your low-level deck at the same height as your flooring by using pedestal feet. You can also elevate it by using steel angle brackets. I suggest having a quick look at this discussion - Low-level composite deck over concrete and soil by @oninpena. This is an excellent example of a single level frame that is elevated using brackets and pedestals.
You'll notice that the decking frame also goes over soil which is supported by pavers and adjustable pedestals. If for example your interior floor height is 160mm based on that measurement, you'll be able to calculate the gap underneath. I only mention that height as it looks like your interior floor is higher than 140mm. If we take away the thickness of the composite decking 23mm and the 90mm x 45mm framing it will leave you with a gap of 47mm.
Since you've already started with your render, I suggest finishing the decking frame so that you'll an idea of the number of materials you'll be needing to complete your project.
If you have any other questions we can help with, please let us know.
Eric
