Alternative to low profile decking pedestals for low-level deck build?

[jeffl_au]

Hi there,

Planning to build a low profile deck (110mm) height at its lowest point, roughly 35sqm.

So I will be using 90x45 H3 MGP10 as the joists.

Part of the deck is over concrete patio, and part will be over dirt.

I am planning to have a max span of about 1m, 450mm apart.  The part that is over concrete, I am planning to dynabolt galvanised angles to the patio.   For the part that is over the dirt, I am seeing products like the tuffbuild blocks,  and deka blocks and a few others that can support the frame in the middle these are about $17 each.  Just wondering is there any problem with just stacking a few pavers to support the frame as oppose to buying these blocks?  I plan the have concreted posts around the edges so the deck won't move laterally.

thanks!

[Jewelleryrescue]

Hi @jeffl_au 

I was planning a timber deck half off and on the ground level and staring to gather the hardware to make it all then it hit me why dont I just add some form work and steel and pour in a slab that will never rot never have termites or spider haven under it.  You said you can concrete posts well this is the same just mix more cement. or have it delivered.  

just angle concrete down 1:20 so water runs off you still have timber on top,

This  may  not suit your plans and I wish you great project  works

[MitchellMc]

Welcome to the Bunnings Workshop community @jeffl_au. It's amazing to have you join us, and many thanks for your question about supporting a deck.

You could certainly use pavers under the joists, and they would substitute for those blocks. If you have a look at this article on How to build a deck, you'll see that I've used a paver and then a Builders Edge 35 - 60mm Smallfoot Pedestal Foot. That's going to cost less than blocks and more than pavers. However, they allow you to adjust the joists to find level. Unless you level the ground perfectly, then you'll need some type of adjustment. I found the additional cost of the pedestal feet well worth it due to the ease of adjustment. If you do go with the paver option, I suggest picking up some plastic shims to pack out under the joists if needed.

Please let me know if you have any questions.

Mitchell

[jeffl_au]

Hi guys,

Building a super low profile deck (using 90x45), planning to use a combination of things to support the deck frame.  The part that is over concrete, I am planning to use galvanised angle brackets and dynabolt to the ground.

Questions:

1. What length and size dynabolt do I need? 12mm and 40mm? or something longer?

2. The concrete area is 2900 x 4400, my joist runs along the 2900 and I m planing 450mm spacing all along the 4400.  For each joist, can I get away with just angle brackets on either end (i.e. 2900 apart), and use some plastic packers for vertical support in between?  Ground clearance is only going to be 2-3cm

3. Do I really need to bolt every joist to concrete?  Or can I just bolt say every 4 joists to prevent lateral movement, and then just have something else (plastic packers or adjustable pedestals) to bear load?  - Wanting to avoid the number of holes I put into the concrete.

[MitchellMc]

Hi @jeffl_au,

Ramset 10 x 50mm DynaBolt Plus Hex Nut Bolts would be sufficient. I don't think you'll readily find 12 x 40mm. 

Your 90 x 45mm joists must be supported at least every metre. How you do that is up to you, but the packers underneath the joist don't offer any rigidity to the structure or stop twisting or bowing of the joists. For your 2900mm width, I'd recommend an angle bracket at either end and two in the middle. Between those, you can pack underneath for additional support. 

Creating a deck from 90 x 45mm joists is already pushing the limits for traditional deck building. In my opinion, every joist should be adequately fixed to the concrete. Say, for instance, a joist decides to bow upwards away from the concrete due to it not being fixed across its length or at all, as you're suggesting. You'll then have a floating deck in every sense of the word, and when stepped on, it will flex considerably and lead to a bouncy deck. Having a well-fixed structure to lay your decking onto is essential.

Please let me know if you have further questions.

Mitchell

[jeffl_au]

Hi guys,

I see a most of the ground level decks are built with no bearers and all joists supported by shims or blocks (e.g. tuffblocks or adjustable height pedestals).

My question is, since the joist is just literally sitting on these support, wouldn't over time when the timber shrinks a little or perhaps the dirt its sitting sinks or shifts little, the joist will no longer be sitting on the support - leading to flexing on the deck?

thanks,

Jeff 

[MitchellMc]

Hi @jeffl_au,

You'll typically find in floating deck support systems like the Tuffblocks that you are required to compact the ground before using the supports. I like to recommend compacting the ground and then placing a paver larger than the support on the compacted ground. This spreads the load over a larger area and helps prevent compression of the soil. 

I believe we have likely not seen this type of issue arise, as any timber shrinkage (which should hopefully be minimal) is likely offset by the weight of the deck continuing to hold it on the support. When a specific support subsides, and the others remain solid, you might see a gap between the joist and support. However, the next time it rains and the deck timber is saturated, it's going the sag to that new lower position and then set in place when it dries.

I have a floating deck next to a pool where the soil has a fairly bad subsidence issue. This is due to an aged leaking pool that was built on a slope which fill was used to build up. I manually tamped the ground and laid pavers for my supports. I haven't seen any issues using the adjustable pedestal feet in a year or so since it's been built.

If you expect to have issues, then a deck bolted to posts in concrete footers is probably going to be the more solid option over a floating arrangement.

Please let me know if you have further questions.

Mitchell

[jeffl_au]

Hi guys,

Looking at a ground level deck project, where I don't have enough clearance (110mm) for typical horizonal bearers.

So its basically going to be all joists, running from concrete to dirt.

I know one of the solution is using deckblock/pedestal for every single joist.

But that would be a lot of work,  I m thinking for every pedestal/block .. I will follow this process:

1.  Compact the ground. 

2.  Make a little concrete pad, 250x250x100 (no mesh... just concrete, the idea is to avoid the sinking of the ground?) 

3.  Set a paver on top, get it level

4.  Unstall the block/adjustable pedestal. 

5.  Do this about 30 times.

Are there other clever ways to support these joists?

Is there some sort of composite bearers that can work in ground? 

thanks!

Jeff

[MitchellMc]

Hi @jeffl_au,

That sounds like it would work @jeffl_au. I'm unsure whether creating a concrete pad of 250 x 250 x 100mm would be better than just laying a 300 x 300 x 40mm paver on the compacted ground. Creating the pads would be an additional step, and the effort might not be worth it. 

Unfortunately, in a single-level deck system, the joists must be supported at certain increments. There's no real way to get around this.

Mitchell

[jeffl_au]

Just trying to clarify the ground prep process for using products like Tuffbuild blocks for deck footing.

I have already marked the space dug out the top soil. I m thinking of the following process.

1. On the rows where I will be using the blocks (block rows) compact the ground.

2. Lay weed mat.

3. On the block rows, lay paver sand.

4. Compact the pavers sand on top of the weed map.

5. Place and level the block with more paver sand.

6. Cover rest of the area with crushed rock.

Thoughts on this process? Anything I can make easier? Thanks!