Backyard low deck, help please!

cariasvar · Jan 05, 2021 10:01 am

Good afternoon guys,

First of all, Congratulate everybody here for all the info you guys share. It's amazing!

I'm completely new in terms of building a deck and a friend of mine with more idea than me will help me build it. So the plan is to completely deck the backyard area and build up some planter boxer.

The dimensions will be: 3600 x 7000 + 1500 x 3700 (kinda L shaped, except the cubby house area). The ground is uneven so the height will vary in between 250 mm (closer to the house) up to 350 mm

I have many many doubts and I've been reading so many posts here, even went to the calculator https://www.blocklayer.com/deckcalculator.aspx to see if the info i've been getting would be right or not.

My first question is:

BEARERS:
How far should I put the posts? Following another thread which is quite similar to my idea, he placed the bearers @1800mm from each other. Knowing that the width is 3600 mm should i place them at the distance that the calculator gave me: 3 bearers @ 1678mm or 4 bearers at 1118mm.
Should I go with 90x45 or 2/90x45? It would be a continuous span but i'm having trouble to read the tables you guys were posting.

Should I use gal stirrups or timber stumps? I like more the idea of stumps (I think it's cheaper)

JOISTS: For what i've seen: 90x45 H3 treatment pine @ 436 mm. Is it ok that separation in between joists or should I get them closer/more separated? I've also thought about using joist hangers and install joist between the bearers rather than on top, due to lack of clearance (only around 500 mm from floor level to ground), so I also appreciate your thoughts on this.

Any thoughts/ideas/advices will be highly appreciated. structuralbimguy?

Attached a sketch of the floorplan. Thank you!
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cariasvar · Jan 06, 2021 2:57 am

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Updated:
Info with some numbers so what you guys think? The pictures of that actual deck don't belong to me (the floorplans are my calculations with two options: stumps every 1715 or every 1372) but that deck is very similar to the one i'm planning to build. Same size and idea. The only difference will be that mine will go with stumps and that one is seat on stirrups. I'm still missing some info about prices on the Bearers and Joists

StructuralBIMGuy · Jan 07, 2021 8:37 am

Hi Cariasvar
Welcome to the forum.there are many sites,apps and posts on decks, but most don't pass the pub test..in that " can the information be certified in the Building Application" by either a Building Surveyor or Structural Engineer.
Hard and fast rule Is to follow the Design Recipe in the Australian Codes which incidentally are written by engineers

Deck Design Methodology
1.0 If you follow the tables in AS1684 Sup 1 , then a building surveyor (BS) can certify it as compliant as it's taken directly from the codes, Alternatively,
2.0 A structural engineer can use AS1720 Timber structures to design it from first principles.which is then certified by a BS as Equivalent of better than 1.0
See Image
The following has been borrowed from AS 1684 & Sup1
Residential Timber Framed Construction
Table 5 Floor Bearers ( Supporting floor loads only)
Floor Load width (FLW) 1800mm
single span 1800 , continuous span 2000mm
requires 2/ 120x 45mm Seasoned Softwood F5

Table 6 Floor Joists ( Supporting floor loads & Load Bearing walls)
Roof Load Width (RLW) 0mm
Single span 1900 Joist Spacings 450mm, Joists crippled over supports
requires 120x 45 Seasoned Softwood F5

From Experience 35mm wide timbers have a greater tendency to crack when nailed
hence my preference for 45mm thickness when reading the tables
Note.
Some Councils will require an engineer to check & stamp the
Drawings in 1.0 & 2.0 above, please follow this up with your Council Building dept.
Cheers
Chris
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