low level deck questions

[BradC]

Hi all,  looking to build a small deck 2800 long and 3500 wide. With only 100 between the ground and bottom of bearer at the lowest point and 250 at the highest. 

I want to use stirrups set in concrete and bolted directly to the bearer. Bearers are 2800 long and 2x90x45 glued and nailed together. 3 stirrups on the 2800 span so under 1400 between supports.  I have 300 and 450 long stirrups.

Looking at digging post holes 300x300 square and  400 or 500 deep.  100 of compacted gravel at the bottom them having the stirrup set with about 150 concrete above and below the base of the stirrup 

Does this all sound good so far?

I will use 90x45 joists with joist hangers to keep it as low profile as i can. 

Thanks for any help

[Jason]

Hi @BradC,

Welcome to the Bunnings Workshop community. It's great to have you join us. We have amazing community members sharing project advice and inspiration on the site every day, so we're sure you will receive lots of helpful information for all your projects. 

Have you seen the guide How to build a low-level deck by @Adam_W? I'm sure you will find it really helpful. Adam might also like to take a look at what you have shared above and give his thoughts. You are also encouraged to share some photos of the area so Adam and other community members can see what you are working with.

Please don't hesitate to post anytime you have something to share with the community or need a hand with a project around the house and garden. 

Thanks,

Jason

[MitchellMc]

Hi @BradC,

Everything you have mentioned sounds good to me. The footings you've mentioned sound more than adequate for the deck. Well done. 

How many bearers are you going to use? With joists of 90 x 45mm, I'd like to see at least 4 as per the diagram below.

Please let me know if you need further information or had any questions.

Mitchell

[BradC]

Thanks Mitchell!

I was only planning to have 3 bearers, I have re measured the space and it will only be 3200 wide so the joist spans will only be about 1450. I am basically copying an existing deck section only a step lower then the existing. Hopefully that will be ok. 

Also any idea how many bags of concrete I'll need per post hole? Using 300x300x450 as a guide? 

Thanks again!

[MitchellMc]

Hi @BradC,

According to span tables for single-span joists of 90 x 45mm H3 MGP10 with 450mm spacing, 1400mm is the maximum allowable distance between hangers. By using 90 x 45mm for joists at the maximum allowance there is always the potential of ending up with a somewhat spongey deck. Span tables are minimum requirements for building a deck and don't necessarily represent best practise. That's why I suggested the above layout to minimise any chance of significant deflection over the distances.  

If you are mirroring an existing deck, which used the same MPG10 softwood timber, then you already know what the new deck will end up like. If you are happy with the existing deck then there is no reason why you wouldn't go with your initial design specs.

For a concrete footer of 300x300x450mm that is 0.041m3 and would require 5 x 20kg bags of concrete.

Please let me know if you have any additional questions.

Mitchell