Author Topic: Sub Structure for Deck  (Read 1515 times)

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Offline DannyG

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Sub Structure for Deck
« on: August 14, 2018, 01:34:39 PM »
Just to continue on my theme of posting stuff about anything other than camping I was hoping to get some advice about a deck, or more precisely the bearers and joists.

Firstly the deck is on the ground.

I have attached a render of the area but its basically a 6m x 6m main deck with a little 5.4m x 2m extension.

My main question is that according to Aust Standards I need to use double bearers. Is this right? Its not over 1m off the gorund, it is on the ground or around 300mm above the ground.

I was going to use 140mm x 45mm bearers on 90mm x 90mm posts that will be concreted into the ground via post stirrups. Then use 90mm x 45mm joists, 450mm apart as per my flooring requirement. I have used the AS to work out the amount the spacing for the bearers and posts.

I have no problems using double bearers but it seems like an extra cost for a lot of overkill??

Any builders or others that have built decks care to enlighten me? Perhaps show some pics of your decks sub structure?? :)

Thanks for any help.
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Offline WilSurf

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Re: Sub Structure for Deck
« Reply #1 on: August 14, 2018, 02:15:42 PM »
Interesting to know as I want to install decking over a paved area.
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Offline GBC

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Re: Sub Structure for Deck
« Reply #2 on: August 15, 2018, 04:16:55 AM »
Have you got a link to the doc?

Offline Rumpig

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Re: Sub Structure for Deck
« Reply #3 on: August 15, 2018, 08:07:19 AM »
Only time I would double something up, is if it's size for a single one wasn't up to the task of spanning the distance required. If it's just a bearer, then wack an extra post in to reduce the spans for something like a deck sitting low to the ground. I'd be more concerned about ground clearances and timber types used if practically sitting on the ground, aswell as keeping the structure with a clear gap of things like your house in case termites get into it......but sure you've likely thought about that already if it's next to a house.
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Offline #jonesy

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Re: Sub Structure for Deck
« Reply #4 on: August 15, 2018, 09:11:33 AM »
On the decks I built close to the ground I used 90x45 bearers (double) and 90x45 joists. That way it gave me reasonable ground clearance. Other areas were 140x45 so I didn't have to dig too may holes as the ground was rock.
I bolted up the bearers to the posts and propped them over the holes and poured the concrete. I found it easier to get the levels that way. If it is low enough you could even put your double bearers direct into the stirrup.
« Last Edit: August 15, 2018, 11:29:31 AM by #jonesy »
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Offline DannyG

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Re: Sub Structure for Deck
« Reply #5 on: August 15, 2018, 09:55:43 AM »
Thanks for the replies.

Here is a link to the URL for the Australian Standards.
https://www.ozisuzu.com.au/files/85884.pdf

And here is a link to a good resource for working out bearer spaces, post spaces and bearer/joist sizing. It looks complicated initially but when you read it, its actually quite simple to follow.
https://www.softwoods.com.au/blog/blog-diy/build-deck-step/
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Offline tryagain

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Re: Sub Structure for Deck
« Reply #6 on: August 15, 2018, 11:33:31 AM »
Generally, if it's in the AS it's there for a reason. My initial thought that unless you know why it's in there and why this doesn't apply to you then I would probably just do it. As Rumpig mentioned, it might be easier to reduce the spans with an extra support though as that should then reduce the strength requirement.

Offline #jonesy

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Re: Sub Structure for Deck
« Reply #7 on: August 15, 2018, 01:28:48 PM »
I took a pic of one of my bearers are actually 190x45 (x2)
In the spot they are way overkill which was a add-on,  but further in and around the spans are bigger.  It was easier to use them than sit down no go through all the calculations.

Try a few different scenarios to work out your cost. Bigger timbers mean bigger spans. That means less bearers,  posts,  holes etc.
 I used to have a computer program that did all the calculations and work out the prices
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Offline DannyG

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Re: Sub Structure for Deck
« Reply #8 on: August 15, 2018, 01:31:01 PM »
I took a pic of one of my bearers are actually 190x45 (x2)
In the spot they are way overkill which was a add-on,  but further in and around the spans are bigger.  It was easier to use them than sit down no go through all the calculations.

Try a few different scenarios to work out your cost. Bigger timbers mean bigger spans. That means less bearers,  posts,  holes etc.
 I used to have a computer program that did all the calculations and work out the prices


Thanks, what do you have between the bearers and joists? Want me to send you my plans so you can figure out the most cost effective way to build my deck? :D
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Offline Marcus73

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Re: Sub Structure for Deck
« Reply #9 on: August 15, 2018, 03:46:14 PM »
Thanks, what do you have between the bearers and joists? Want me to send you my plans so you can figure out the most cost effective way to build my deck? :D

Geez not bad are ya?
Do you have a go fund me page too?


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Offline DannyG

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Re: Sub Structure for Deck
« Reply #10 on: August 15, 2018, 04:12:45 PM »
Geez not bad are ya?
Do you have a go fund me page too?


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No but I’ll happily take donations on patreon ;D
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Offline Moggy

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Re: Sub Structure for Deck
« Reply #11 on: August 15, 2018, 06:33:01 PM »
Hyne timbers used to have a program that would give you timber sizes, you simply entered the data. Try Googleing Hyne.com.au

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Offline #jonesy

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Re: Sub Structure for Deck
« Reply #12 on: August 15, 2018, 09:19:37 PM »
Just sit down and work out the joists spans for 90, 140, 190 etc
So 6 metres width
90 - 1000 bearer spacings. (7 bearers)
140 -2400 spacing (4 bearers)
190 - 3500 spacing (3 bearers)

So basically bigger joists will cost more,  but you save on bearers and stirrups.

Then work out the same for bearers. Bigger beaters will mean less holes saving on stirrups and concrete.

It's a bit of fiddling but could save a few dollars.
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Offline DannyG

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Re: Sub Structure for Deck
« Reply #13 on: August 16, 2018, 10:11:40 AM »
Hyne timbers used to have a program that would give you timber sizes, you simply entered the data. Try Googleing Hyne.com.au

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We only use Mac, they only have a PC option so I might pull out an old windows laptop and check it out, it looks interesting.  Thanks.

Just sit down and work out the joists spans for 90, 140, 190 etc
So 6 metres width
90 - 1000 bearer spacings. (7 bearers)
140 -2400 spacing (4 bearers)
190 - 3500 spacing (3 bearers)

So basically bigger joists will cost more,  but you save on bearers and stirrups.

Then work out the same for bearers. Bigger beaters will mean less holes saving on stirrups and concrete.

It's a bit of fiddling but could save a few dollars.
.
 

Thanks Jonesy Ill do some calculations and see what I come up with.

I am a little restricted for height and I dont want to have to excavate too much dirt to gain more space. But until the house is built I wont know for sure what I am left with.
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Offline GBC

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Re: Sub Structure for Deck
« Reply #14 on: August 16, 2018, 10:29:10 AM »
Thanks for the replies.

Here is a link to the URL for the Australian Standards.
https://www.ozisuzu.com.au/files/85884.pdf

And here is a link to a good resource for working out bearer spaces, post spaces and bearer/joist sizing. It looks complicated initially but when you read it, its actually quite simple to follow.
https://www.softwoods.com.au/blog/blog-diy/build-deck-step/

I just searched AS 1684 for any mention of compulsory double bearers and there isn't one. The only mention of double bearers is that they are allowed if you cannot source a single section large enough to do the job. I'd send a query to softwoods direct about that line because it sounds weird to me.
Your softwoods doc mentions that they only sell F7 pine for decking. If you are sourcing better graded timber you might be able to step back in section size and/or increase spacings.
What decking material are you using? What will it span?
What bearer spacing are you proposing to use?
Do you have a physical restraint height between the deck level and the ground?

Offline DannyG

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Re: Sub Structure for Deck
« Reply #15 on: August 16, 2018, 10:49:25 AM »
I just searched AS 1684 for any mention of compulsory double bearers and there isn't one. The only mention of double bearers is that they are allowed if you cannot source a single section large enough to do the job. I'd send a query to softwoods direct about that line because it sounds weird to me.
Your softwoods doc mentions that they only sell F7 pine for decking. If you are sourcing better graded timber you might be able to step back in section size and/or increase spacings.
What decking material are you using? What will it span?
What bearer spacing are you proposing to use?
Do you have a physical restraint height between the deck level and the ground?


You are correct. It does talk about double bearers to gain the required thickness if nescasary but in my case I believe I have the required thickness.

4.2.1.4 Double bearers (spaced bearers)
The required breadth of larger bearers may be obtained by using spaced double bearers.
Spacer blocks shall be placed between the bearers and, where relevant, at supports, at the
intervals specified in Table 4.1 (see Figure 4.3).


I think the AS changed in 2010 so perhaps double bearers was mandatory prior to the change, hence the document from softwoods?

I am undecided on the deck material yet but I think it will come down to either Ekodeck from Bunnings or another composite decking from these guys...http://www.ezywood.com.au/products.html
The bunnings one is solid and allows 450mm joist spans, the other one is hollow and requires 350mm joist spacings.

So far I am proposing to use 1.5m bearer spacing using 145x45 h7 treated pine and to have the posts also spaced 1.5m apart. But this is of course subject to change if I am advised otherwise.

The house is not yet build but it is a waffle slab so I am told Ill have around 300-400mm from the ground to where I want the decking to be with out too much excavation.
« Last Edit: August 16, 2018, 10:53:38 AM by DannyG »
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Offline GBC

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Re: Sub Structure for Deck
« Reply #16 on: August 16, 2018, 10:59:08 AM »
Have you also thought about steel sub floor? There are plenty of wind up adjustable bearer/joist systems and probably reasonably priced. Its been a while since I did domestic stuff. I have put down a few plastic timber decks commercially. They expand and contract on the joints a bit so they don't reward craftsmanship. They look ok though and they are lasting well. Not cheap.
All of the span tables are derived from the old CSIRO handbook which was converted to metric by the Tradac guys - they are getting more complicated and saying less these days.

Offline DannyG

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Re: Sub Structure for Deck
« Reply #17 on: August 16, 2018, 11:10:48 AM »
Have you also thought about steel sub floor? There are plenty of wind up adjustable bearer/joist systems and probably reasonably priced. Its been a while since I did domestic stuff. I have put down a few plastic timber decks commercially. They expand and contract on the joints a bit so they don't reward craftsmanship. They look ok though and they are lasting well. Not cheap.
All of the span tables are derived from the old CSIRO handbook which was converted to metric by the Tradac guys - they are getting more complicated and saying less these days.

I have thought about steel, mainly because the house is being built on a steep hill so there was going to be an opportunity to hang the deck out over a 4m drop. The builder was going to incorporate the steel into the slab.

But as it turned out I was only willing to pay for one row of deep piers under the slab so the house is now set back a couple of metres away from the drop, so the deck is basically on the ground, albeit right at the edge of the drop off so thats why I have now gone back to wood thinking it would be cheaper.....but ill investigate steel.

You can sort of see the drop off here.

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Offline GBC

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Re: Sub Structure for Deck
« Reply #18 on: August 16, 2018, 11:18:01 AM »
https://www.stratco.com.au/products/building-construction/tuffloor-flooring/

Got a local Stratco? Send them your rough plan with some dimensions and they will detail it and price it up (should).

Offline #jonesy

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Re: Sub Structure for Deck
« Reply #19 on: August 16, 2018, 12:48:58 PM »
I just searched AS 1684 for any mention of compulsory double bearers and there isn't one. The only mention of double bearers is that they are allowed if you cannot source a single section large enough to do the job. I'd send a query to softwoods direct about that line because it sounds weird to me.
Your softwoods doc mentions that they only sell F7 pine for decking. If you are sourcing better graded timber you might be able to step back in section size and/or increase spacings.
What decking material are you using? What will it span?
What bearer spacing are you proposing to use?
Do you have a physical restraint height between the deck level and the ground?
Supplement to AS1684 table 49. Specifies double bearers.
https://www.scribd.com/doc/309660524/AS-1684-2-2010-Supplements-Non-Cyclonic#
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Online achjimmy

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Re: Sub Structure for Deck
« Reply #20 on: August 17, 2018, 11:32:55 AM »
Just redoing my deck. Got the span tables from onesteel and all the bearers will be duragal . Proper RHS steel and span distances are good. Won’t rot (previous deck) won’t burn going to use rolled steel joists . Prices compared to timber are reasonable and every piece comes straight  ;D
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Offline WilSurf

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Re: Sub Structure for Deck
« Reply #21 on: August 17, 2018, 01:26:33 PM »
Just redoing my deck. Got the span tables from onesteel and all the bearers will be duragal . Proper RHS steel and span distances are good. Won’t rot (previous deck) won’t burn going to use rolled steel joists . Prices compared to timber are reasonable and every piece comes straight  ;D

That's something for me to think about.
I want to install decking around our spa, on top of the pavers.
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Offline prodigyrf

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Re: Sub Structure for Deck
« Reply #22 on: August 20, 2018, 12:02:58 AM »
You want straight in timber then always use LVLs for flooring- https://www.chhsoftware.com/designit/
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Offline prodigyrf

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Re: Sub Structure for Deck
« Reply #23 on: August 20, 2018, 12:33:01 AM »
That's something for me to think about.
I want to install decking around our spa, on top of the pavers.


Did that for a client once and they wanted to keep it low so we used 50x50x1.6 square galv joists and a heap of window packers with construction adhesive for levelling and then wingteks for the decking with dampcourse between the galv and decking although you can use bituminous paint on the steel if you're not fussed about spillage.
https://www.ebay.com.au/itm/TRADEMARK-PACKERS-Mixed-40x75mm-500PK-WINDOW-PACKERS-SHIMS/223031818945
https://www.buildex.com.au/products/steel-frame-housing/6-on-site/16-wing-teks/
Construction adhesive to the packers and pavers/concrete is just to hold the galv joists in place while you fix the decking as it's not going to move once it's all decked

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Offline Nomad

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Re: Sub Structure for Deck
« Reply #24 on: August 21, 2018, 10:24:27 PM »
I am going through thh process of having a double bearer replaced at the moment due to rot............

I currently have 2 x 245 x 45 (by 5 metre) treated timber bearers failing due to exposure to weather, I think they were holding water between them...

Had the engineer over, who wants some cash so is doing me a fee proposal to look at my whole house  >:( >:( >:( >:(

Anyway my plans show that the builder had to options when he built, the above 2 x 245 x 45 in a treated pine or a 250 x 75 hardwood.

That may be where the double bearer come into your scenario.

I am simply replacing with a hardwood beam this time.