How to support decking on soil?

Hey Folks. I’m building a small 2.4M x 2.4M deck in the garden for our kids cubby house, however I’m struggling to determine what foundation to use. Conventional wisdom seems to be to secure wooden posts into the ground with concrete, which I could still do, however it feels a bit overkill for what was supposed to be a simple project. I also tried to dig one of the holes recently and it was very slow moving, so much so that I’d have to hire a petrol post digger if I was to carry on.

Has anyone had any luck using one of the following as a foundation for a deck on a soil covered area?

- Adjustable Pedestal feet (e.g. Builders Edge Pedestal Feet)

- Concrete Block (e.g. DEKO concrete foundation block)

- Plastic foundation (e.g.BuildTuff Tuffblock) foundation blocks?

The above sound much easier to use, and the adjustable ones in particular would help with the unevenness of the ground, however I’m a bit concerned about the deck not being anchored, and that it could move from side-to-side (or even upwards).. I’m also not sure how I would prepare the ground for using one of the above - do I need the compress it in some way and/or perhaps place the feet on a block of some sort? (Which makes it sound a bit precarious too).

Welcome any thoughts you can share. Concreting feels like the most solid option, but it would also be a bit of a learning curve for a novice like me.

Thanks!

Robin

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Comments

MitchellMc

Welcome to the Bunnings Workshop community @robinlewis. It's a pleasure to have you join us, and many thanks for your question about constructing a deck over soil.

Check out the guide I put together on How to build a deck. In it, I placed pavers on the soil and then pedestal feet on top of them. I used the pavers to disperse the weight over a larger area. It worked a treat, and I've seen zero issues with the deck. The adjustable pedestal feet are fantastic and make it so easy to level your deck. The other options you've suggested are also viable solutions. Any of those will work and certainly beat having to dig holes is solid ground.

The ground gets compacted with a tamper before the pavers go down. In regards to the deck moving, once you get the decking boards on it, it's not going anywhere. There's a significant amount of weight involved, so I don't think you'll have any issues.

Check out the how-to guide. I'm sure you'll find it gives you a great idea of how to construct the deck.

Please let me know if you have further questions.

Mitchell

robinlewis

https://www.workshop.bunnings.com.au/discussion/comment/26281#Comment_26281

Hey Mitchell. Thanks so much for the comprehensive summary! It’s super helpful. A few quick follow-up questions:

1. How much drainage gravel would I need for a 2.4M x 2.4M deck? The guide suggested 50MM depth, however when I punched that into an online calculator it came back as 480KG (or about $250 at current prices at Bunnings). Is there a cheaper way to achieve this?

2. Would landscape paver sand be an okay sand type? If so, can you clarify how much of it I’d need for our 2.4M x 2.4M frame? I’m guessing around 8 bag given your 3M deck had 10?

3. Is there an easy way to calculate how many pedestal feet I’d need? From what I could see in your video you had 3 per joist and 3 each for the left and right side of the frame, totalling 21? Guessing I might need the same?

Thanks again! Hopefully off to Bunnings tomorrow to get the last few bits I need!

Akanksha

https://www.workshop.bunnings.com.au/discussion/comment/75828#Comment_75828

Hello @robinlewis,

Lovely to hear that you found @MitchellMc's advice helpful. He will be back on the site later in the week but in the meantime, let me tag our other resident D.I.Y. expert @EricL who should be able to assist you with your questions.

Also mentioning our experienced Workshop members @Adam_W and @ProjectPete to see if they have any thoughts.

Keep us updated as your project progresses. We'd love to see the final result.

Akanksha

Adam_W

Hey @robinlewis good work! Sounds like a fun project.
I've used TuffBlocks before & found them very useful & easy to use. Certainly save a lot of digging! One caveat however...
Foundations or footings are not just for levelling & support. They are also there to keep a structure firmly anchored to the ground in extreme weather. Uplift from strong wind can easily lift & move a structure if there's no solid anchoring.

Personally, at the least, I would be looking at putting concrete footings in the corners, and perhaps one in the centre too, and then use easier options, like TuffBlocks.

EricL

https://www.workshop.bunnings.com.au/discussion/comment/75828#Comment_75828

Hello @robinlewis

Thanks for sharing your question about your low-level deck. The gravel under your deck does not have to be very thick. Its main purpose is to hold the weedmat down and facilitate drainage. An easier way to calculate your needs is to keep in mind that every 20kg bag of drainage gravel can cover an area of 500mmx500mm at a 50mm depth.

If you were to reduce the depth to 25mm you would be able to cover an area of 1000mmx500mm per bag. This means that you would only need two bags per square meter. Since you are covering an area of 5.76 square meters, you would approximately need 12 bags. If for example you were to use the Ki-Carma 20kg 10-20mm Drainage Gravel covering the area would cost $131.88.

The calculation for sand is very similar, for example, if you were to use the Ki-Carma 20kg Coarse Landscape Sand. You would need two bags per square meter with a depth coverage of 30mm. Please keep in mind that these figures are estimates and you might not need as much to level your area.

Generally, you would need to consult a span table in order to figure out where to put the footings. Looking at the table you'll see that the maximum span for a 90x45 MGP10 is 1.6m. At this footing distance the deck frame would be very bouncy, in order to reduce that bounce and make the deck frame well supported, I suggest reducing the distance between posts and adding more footing.

Let me tag @MitchellMc in case he has any extra information to add regarding his pedestal foot placement.

I've placed a span table below from one of our suppliers as a reference.

If you need further assistance, please let us know.

Eric

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/75828#Comment_75828

Hi @robinlewis,

@Adam_W raises a great point about using Tuffblocks in high-wind areas. It's always best to check with your local building authority, but concreting in posts in the corners and centre to tie your project down is a great choice.

Here's the installation guide, which will show you how many foundations you need for your deck. When using 90 x 45mm joists, it's advised not to exceed a foundation spacing of 1500mm. Typically, I stick around 1200mm max spacing to ensure a solid deck.

Please let us know if you have further questions.

Mitchell

robinlewis

https://www.workshop.bunnings.com.au/discussion/comment/101741#Comment_101741

Thanks everyone for the super useful responses. I’m pleased to say that we got the posts in last night alongside the pedestals. All seems to be ready for starting to lay the decking, except the weed matting which I’ll have to add tomorrow.

My plan, like the video, is to do a picture frame finish on the top, and then build some stairs up to it. Do you guys have any videos, guides or advice for building stairs?

I’m hoping for the final product to look something like the attached picture (in the same colours). If you’ve any advice on how best to achieve that type of professional finish it would be really appreciated. I’m currently planning to use treated pine and simply paint the white and greys. I did look into using prefab boards like Ekodeck, but they seemed to be comparatively much more expensive.

If you’ve any thoughts about how to install spotlights into the stairs that would be amazing too. Thanks!

EricL

https://www.workshop.bunnings.com.au/discussion/comment/117849#Comment_117849

Hello @robinlewis

You most certainly can paint the pine decking panel; however, I recommend letting the pine panels season for at least six weeks to let all the tannins out of the pine timbre. You can shorten the waiting time by using Intergrain 1L UltraPrep Tannin And Oil Remover.

In regards to your decking steps, I suggest having a look at these discussions - Merbau patio step by @edwardjones. It's basically a miniature frame with decking panels on it. You'll notice that they anchored it to the ground with the same 90x90 timber post to make sure it does not shift or move when stepped on. How to build a back door step? by @Jolly181 and How to build front yard tiered decking steps? by @frediynewbie.

Here are some handy step-by-step guides regarding lights:

The key to a fantastic finish is to make sure that you've prepared the area and materials as best you can. Make sure to use good brushes and rollers when painting so that you will get a nice uniform finish.

Please make sure to wear personal protection such as gloves, goggles and a paint mask when working on your low-level deck.

If you need further assistance, please let us know.

Eric

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