Small low level deck

[spicko]

Hi All,
I am about to start building a small low height deck 2.4m x 3.6m.
I have check span tables but just wanted to double check if all sizes sound reasonable.
I will have 9 posts 90x90 spaced roughly at 1.5m in one direction and 1m in the other.
The bearers will be in a continuous span of 3m with an intermediate post (2x1.5m). I get 2no 94*45 MGP10 for each of the 3 bearers.
For joists, I plan 90x45 MGP10 joists at 450mm.
So the total height of the bearers+joists+deck =200mm.
questions:
1. do these member sizes sound reasonable? Should I expect a bit of bounce?
2. i dont have a nailer so I was planning to build the framing with screws. I have read that nails are desirable for ductility, is this true?
3. As I plan this to be low level, what is the minimum length of post that would be acceptable outside the ground?
thanks in advance for your replies.
cheers
Leo

[jack620]

Should I expect a bit of bounce?


Yep. If you build to the minimum specified in the span tables you'll get bounce. Don't quote me, but I recall it says to go up to the next sized member to get a better outcome. Personally I don't think two 90x45 MGP10's nailed together makes a good bearer. If deck height is limiting consider two 90x45 F17's for the bearers.

[Skew ChiDAMN!!]

3. As I plan this to be low level, what is the minimum length of post that would be acceptable outside the ground?

If you want minimal height, have you given any thought to pouring concrete pads instead of posts?

[spicko]

thanks for your replies.
I will definitively change the bearers. NS timber have 90x45 lvl 13, I presume this would be better than pine.
as for the footings, I was going for 200mm structure with say +50mm off the ground. I imagine that as long as the bearers are not touching the ground, they should not be at risk of decay/ attack.
cheers
leo

[jack620]

I imagine that as long as the bearers are not touching the ground, they should not be at risk of decay/ attack.

I thought so too, until three of the pine joists in my pool deck completely rotted within 3 years. Somehow a length of untreated pine got mixed up with the treated pine I used on the rest of the deck. Those joists fell to bits while the surrounding treated pine joists were fine. Untreated hardwood might be OK. And I think LVLs are treated these days?

[droog]

The 90x45 lvl 13 that I could find was H2 rating, not what you want to use on a low level deck.

Treated Pine Timber Guide - Hazard Treatments H1 - H6 (https://www.atpine.com.au/treated-pine-timber-guide/)

H2 is also designed to minimise the risk of an attack by insects and termites, and can be used in well-ventilated places where the timber will be kept off the ground and isolated from weather conditions

[rwbuild]

I thought so too, until three of the pine joists in my pool deck completely rotted within 3 years. Somehow a length of untreated pine got mixed up with the treated pine I used on the rest of the deck. Those joists fell to bits while the surrounding treated pine joists were fine. Untreated hardwood might be OK. And I think LVLs are treated these days?

LVL can be treated against termite, not rot. The fine print specifies that LVL exposed to moister MUST be mechanically protected ie: flashing of some kind, suggest DPC or Protectadeck, the wider the better, before fixing any joists and even that close to the ground you will get water splashing onto the bottom edge so that is a real problem as well.
Personally I would use 90 SHS gal and fix joists to it with triple grips and it will be cheaper as well

[spicko]

what about using 90x90 H4 posts as bearers? would that work?

[droog]

what about using 90x90 H4 posts as bearers? would that work?


90x90 H4 is not rated, you will not find it in any span tables.

[spicko]

actually, found that these posts are rated. Any reason why these couldnt be used instead of 2No 90x45 bearers?

https://www.google.com/search?q=f7+deck+post+bunnings&rlz=1C1GCEJ_enAU1023AU1023&oq=f7+deck+post+bu&gs_lcrp=EgZjaHJvbWUqCQgBECEYChigATIGCAAQRRg5MgkIARAhGAoYoAEyCQgCECEYChigATIJCAMQIRgKGKABMgkIBBAhGAoYoAHSAQ0yODE1OTcyMWowajE1qAIAsAIA&sourceid=chrome&ie=UTF-8

[rwbuild]

As a post the load is vertical
Has to be F10 or higher F7 does not comply as a bearer

[jack620]

My 2006 version of AS1684 has span tables for F7 bearers. Has that been superseded?

[jack620]

The latest version (2010) has F7 bearer span tables too.

[virtuallyjohnny]

FWIW, bearers should be H3, with underside of decking boards being a minimum of 200mm above ground to allow air flow. You would want to have at least 50mm clearance between bearer and the ground. Although saying that, my friend built a covered verandah with what I think is H4 laying on the ground and the merbau boards are still looking fine 10+ years later (with no maintenance).