Deck joist, bearer and posts

Hi,
I'm looking to build a deck over an existing concrete alfresco. Due to the step down from the house of 140mm, I figure I can only use 90x45mm joists. I'd like the deck to extend out to 5m, which would mean the joists would need to go 1760mm beyond the concrete.



800mm out from the concrete, I have a storm water pipe, but either side of that is nothing. From what I understand, a continuous span of 90x45 of F7 H3 TP can span up to 1500mm with a 400mm cantilever. Does this mean that I can place my bearer at 1500mm and have up to 400mm of joist extending out beyond the bearer?

yes you can but there will be some bounce felt. For a more solid experience put an extra bearer in.

Saw a similar idea and the flex of the joists kept making the end cover board fall off, nails, glue and screws didnt work. Last option was bolting it on with angle brackets. That seemed to work.

More bearers = most posts. Where I am, I have rock everywhere. The chance of me hitting a rock that I cannot move or dig is pretty high, so the less holes I need to dig, the better.

What if I shift the bearer to 1400mm and use MGP10 joists with a 360mm overhang? I guess worst case, is to use double joists.

https://www.bunnings.com.au/deko-250-x- ... lsrc=aw.ds

good option?

I'm not sure. Where the storm water pipe is, it's about 300mm below the concrete and more so, the ground is loose and unstable.



However, because my step down from the internal floor to the alfresco is 140mm, I could probably use 120mm joists (the below pic I used a 90mm joist for reference).




The boards would then be pretty flush with the internal floor:

Just use stirrups in a blob of concrete, cheap and easy way to add a few stumps.

Been there when I owner built first time, specced timber at the limit with only a few stumps thinking how clever I was to read the span tables and save a few bucks, now I have a tiled deck which pools as the 1% slope is no longer and I needed to dig under the house to add a post mid bearer via a hole in the floor don’t feel smart at all.

Span tables allow deflection of 1/360 of the span and do not consider dynamic effects. You don’t want to step onto a cantilevered 90x45 joist, you will feel some spring. I won’t break, just won’t feel right.

So, I'll be going with a 120x45mm joist. That'll sit on a double 120x45mm bearer at 1400mm using 4 stirrups. If I find that space under the bearer is too tight, I could still use double 90x45mm for the bearer. To get my 5000mm length, the joists will only need to hang over the bearer by 360mm. A continuous span of F5 falls well within my requirements for both span and cantilever. I'd probably first dig the 4 holes for the stirrups, fix a few of the joists, hang the bearer off the joists, then hang the stirrups off the bearer. Then all I need is to fill the hole with concrete and all should be well.

Seconded; we just used these last weekend to build a small deck. They worked perfectly as we have bluestone in the area where we wanted to build so digging down was not really an option!