How to build a low-level deck?

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.

PXL_20230917_054034077~3.jpg

Find more posts tagged with

Tools and Materials

Decking and Paving

Building and Construction

Comments

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

https://www.workshop.bunnings.com.au/discussion/comment/30276#Comment_30276

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.

https://www.workshop.bunnings.com.au/discussion/comment/77607#Comment_77607

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

https://www.workshop.bunnings.com.au/discussion/comment/102835#Comment_102835

Ok, so assume we could go either way, 140mm joists, or double 90x45mm.

140mm joists would have the deck at the same height as the inside floor. It'd be a step across from inside, onto the deck.

90mm joists would see maybe a 20 to 30mm step down.

Would using 90mm joists, thus, resulting in a step down be a tripping hazard?

https://www.workshop.bunnings.com.au/discussion/comment/118430#Comment_118430

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

https://www.workshop.bunnings.com.au/discussion/comment/127644#Comment_127644

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.

https://www.workshop.bunnings.com.au/discussion/comment/133680#Comment_133680

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

https://www.workshop.bunnings.com.au/discussion/comment/137422#Comment_137422

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.

https://www.workshop.bunnings.com.au/discussion/comment/139984#Comment_139984

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

https://www.workshop.bunnings.com.au/discussion/comment/141785#Comment_141785

OK, so I'll do 90x45. Let's assume I raise the joists by a minimum 25mm (the lowest of a minifoot pedestal). I'll have 25mm off the slab, plus 90mm for the joist and leaving me 25mm to play with for the boards, which will either be 19mm timber, or 23mm composite.

...and the actual thing:

Depending on where I take an actual measurement from, it can vary a few mm here or there which is no big deal.

I'd use a bunch of pedestals to lift and level the joists (depending on the board I use) to the height needed so the boards finish flush with the brick. Angle brackets will be used to fix the joists to the concrete.

For some strange reason, my wife is fixated on a step-down onto the deck. But by step-down, it would be only about 20 mm. Assuming I raise the joists off the slab by a minimum of 5mm using https://www.bunnings.com.au/macsim-fasteners-5-0-x-75mm-window-packer-200-pack_p1040470 and screw the joists directly into the slab, a timber board (19mm) would result in a 26mm lip and a composite board (23mm) would be a 22mm lip, which I don't think is suitable. That little 22/26mm lip might be a tripping hazard.

https://www.workshop.bunnings.com.au/discussion/comment/143137#Comment_143137

Hi @IronStan001

You're absolutely correct, that little lip will become a tripping hazard. You and your family in time could get use to the little step outside, but if you ever have guests over, they will not notice that step and get caught. I suggest thinking about lifting the frame up all the way to the top of the brick and have the decking panel as close as you can to the sliding rail height. In this manner there will be no mini steps to trip over.

I've placed a sample image below to give you an idea of what I'm suggesting, but please let me know what you think.

Please note that the depth measurement for your brick lip is just a guess and you'll need to adjust it to your needs.

If you need further assistance, please let me know.

Eric

https://www.workshop.bunnings.com.au/discussion/comment/144088#Comment_144088

I also suggested the same to my wife that the first board would butt up against the rail (or close enough to it) so the transition from the inside floor to the outside deck is seamless, without any lip or step down. Besides, one of the bricks has a nice chunk taken out of it. Running the first board along the front of the brick will make that chip stand out and very noticeable. Putting the joists at 140mm to allow the boards to cover the brick is ideal.

The length of the concrete slab is 3240mm, then of course, I have a further 1760mm over the soil. Sticking with the concrete area first, along the 3240mm run, how many angle brackets and pedestals would I need per-joist? Keep in mind, I anticipate on having the bearer 1400mm from the concrete, with a 360mm overhang and will probably use MGP10 just for some extra strength and remove the possibility of bounce over that 1400mm gap.

https://www.workshop.bunnings.com.au/discussion/comment/144805#Comment_144805

Hi @IronStan001

At this point of your journey, I suggest drawing up a complete plan with timber framing put into place. I propose thinking about spacing the brackets at 800mm apart and positioning them alternately from one side to the other. If for example using the 3240mm length and placing a bracket at the very start, you will be putting in at least 5 brackets.

At the end of the concrete slab, it is possible to reduce the distance between the bracket and first pedestal so that you can have a 360mm overhang. I'm a bit conservative when it comes to overhangs and recommend reducing it down to 200mm. This means that the first pedestal will be placed 800mm after the last angled bracket. The last pedestal can be pulled back in so that you'll have the 200mm overhang. But this all depends on how you'll be building your frame and if you're putting in a picture frame and breakers.

I strongly suggest having a look at Low-level composite deck by @oninpena. It's an excellent example of a decking frame going over a sloped concrete base and over soil. Plus, it is built for picture framing and breakers included.

Please make sure to share your plans with the community and any questions you might have.

Eric

https://www.workshop.bunnings.com.au/discussion/comment/145357#Comment_145357

So this is where I'm at, at the moment:

5 angled brackets fixing each joist to the concrete is fine. Then I have the 1400mm jump over to the bearer. What should the joist be fixed to, or sitting on before it leaves the concrete? A bracket? Or pedestal?

https://www.workshop.bunnings.com.au/discussion/comment/145786#Comment_145786

Hi @IronStan001

I suggest a combination of Brighton Masonry 400 x 400 x 40mm Charcoal Mypave Paver and Builders Edge Pedestal Foot. This will save you from having to put any timber pieces making contact with the ground. the full length of the timber piece can be supported by pavers and pedestal. I've placed an image below to give you an idea of how it can be done.

If you need further assistance, please let me know.

Eric

https://www.workshop.bunnings.com.au/discussion/comment/146113#Comment_146113

I can't do that.

The white line is the storm water. At this point, its about 300mm below the concrete. The green line is roughly about where the bearer would be. This is maybe about 200mm below the concrete. The ground is both too low and too loose to use pavers and pedestals, hence why I need something more stable like a bearer.

https://www.workshop.bunnings.com.au/discussion/comment/146389#Comment_146389

Hi @IronStan001

My apologies, I forgot to mention that the area must be first levelled and compacted so that you have a nice flat area to place the paver and the pedestal. Please keep in mind that the area has to have a slope so that any rainwater flowing in that area goes towards the drain. By levelling the area first, it will solve the issue of whether you can use pavers and pedestals together.

In regards to the storm water, you can bypass that area by placing the paver right after it. You also have the option of having one right after the concrete base if you want extra support for that area.

If the area is not levelled and compacted, any footing you place there will eventually settle into the soft ground and your deking frame will sink and bend along that spot.

The other option is to to dig into the ground at 500mm deep and put in H4 100x100 treated timber posts and concrete them into place. This will provide stable footing for your low-level deck.

If you need further assistance, please let me know.

Eric

https://www.workshop.bunnings.com.au/discussion/comment/146615#Comment_146615

"The other option is to to dig into the ground at 500mm deep and put in H4 100x100 treated timber posts and concrete them into place. This will provide stable footing for your low-level deck."

This is my intention. There is too much loose fill / building rubble right next to the concrete. It would require a reasonable amount of scraping away before I get to something that I'd consider firm ground. And if I do get to that point, I've now probably created a drainage problem. Hence, the preference to go with a double 120x45 bearer at 1400mm, supported by 4 posts. The end posts would be 200mm in from the end, the 2 middle posts would be 1450 to 1500mm apart. If the 120mm height of the bearer is a problem, I could still go with a twin 90x45 bearer and 4 posts since that still fits within the parameters of spanning guidelines (according to https://www.demak.com.au/wp-content/uploads/2013/08/tabma_span_tables_v1_0_june_2007.pdf).

So based on that, what option do I have for supporting the joist before it leaves the concrete?

https://www.workshop.bunnings.com.au/discussion/comment/146800#Comment_146800

Hi @IronStan001

If those are the conditions, I suggest using pedestals at the very end instead of an angle bracket. You'll be able to adjust the height of the pedestal should you need it to be higher. My only other suggestion is to add more footings in the middle of the 1760 length to prevent a bouncy frame.

If you need further assistance, please let me know.

Eric

https://www.workshop.bunnings.com.au/discussion/comment/146946#Comment_146946

Would using double joists rather than extra footings be sufficient to reduce bounce? The more I can avoid digging footings, the better. There is A LOT of rock underneath.

https://www.workshop.bunnings.com.au/discussion/comment/147069#Comment_147069

Hi @IronStan001

If anchored very well together it should theoretically be strong enough. Another possible way that comes to mind is to create two frames, one over the concrete and another over the soil.

Eric

https://www.workshop.bunnings.com.au/discussion/comment/147176#Comment_147176

Actually, since I'm not using pedestals, I could probably do 120x45 joists which I assume would be way more than enough to not have any bounce over the 1400mm gap. I'd just have to find another method to support joist at the end of the concrete. If the gap is 25mm, I could squeeze a pedestal in there.

https://www.workshop.bunnings.com.au/discussion/comment/147269#Comment_147269

Hi @IronStan001

I'm thinking that with a separate frame the heavy weight of the double frame will not affect the frame on the concrete side should there be any movement from the frame on soil side.

Eric

https://www.workshop.bunnings.com.au/discussion/comment/147355#Comment_147355

Would a taller joist like a 120mm eliminate the prospect of bounce? My original idea of using at minimum, 90x45 F7, I'd get a maximum continuous span of 1500mm. MGP10 is up to 1700mm. 120x45 however for my needs, starts at a minimum of F5 with a 2200mm span. Then F7 @ 2400mm and MGP10 at 2700mm. Even using 120mm MGP10 is almost double the span I need. I think to save a bunch of hastle, the 120x45 would be better wouldn't? It would make the joist structure simple and straight forward.

https://www.workshop.bunnings.com.au/discussion/comment/147437#Comment_147437

This is what I have so far. 120x45mm joists at 5000mm long. Fixed at 20mm above the concrete (or whatever it needs to be level with the brick) using https://www.bunnings.com.au/dunnings-40-x-80-x-40-x-5mm-m12-galvanised-angle-bracket_p1070088 (150mm in from the ends, spaced at around 980mm) on alternating sides.

Depending on what the height is at the very edge of the concrete at the jump to the bearer, I might be able to slip a pedestal under there for support, which might also require shifting the angle bracket over by another 50mm. The bearer will be at 1400mm using a double 120x45mm supported by 4 posts.

@EricL and @MitchellMc, thoughts?

https://www.workshop.bunnings.com.au/discussion/comment/147500#Comment_147500

I guess my next design question is, how do I handle the column with downpipe to get a joist there?

https://www.workshop.bunnings.com.au/discussion/comment/147557#Comment_147557

Hi @IronStan001,

You basically just need to create a bit of framing to support any decking boards that can't span across to the last joist due to the drain impeding them. I've added an image below that should help illustrate this.

You'll likely need to place a pedestal foot under that structure to support it.

Please let me know if you have any questions.

Mitchell

https://www.workshop.bunnings.com.au/discussion/comment/147605#Comment_147605

Would I perhaps use a ledger board that's fixed to the slab?

https://www.workshop.bunnings.com.au/discussion/comment/147651#Comment_147651

That's certainly an option, @IronStan001 and it could be an easier option.

Mitchell

https://www.workshop.bunnings.com.au/discussion/comment/147695#Comment_147695

I'd think a ledger board, one either side of the downpipe. A board that's 200mm tall should be ample. At least 180mm of that will be below the brick, so any screws or dynabolts would be going into the slab for a strong fix. The joists could be secured using joist straps:

https://www.bunnings.com.au/pryda-joist-strap-162mm_p1121966?region_id=116395&gclid=EAIaIQobChMIrNLo8oPAgQMVKalmAh2mBwmZEAQYASABEgLEdPD_BwE&gclsrc=aw.ds

Does that sound suitable?

Child Item

Child Item