Sub Structure for Deck

[prodigyrf]

Re the double bearers a lot of tradeys and homeowners don't understand the value of engineered H2-S treated timber for serious load bearing when the alternative hardwood is more expensive and as straight as a dog's hind leg- https://www.chhwoodproducts.com.au/hyspan/
and manufacturers like CCH give you the design tools at your fingertips- https://www.chhsoftware.com/designit-for-houses/
Horsey, here plenty water, horsey no drinky, horsey go thirsty 

[GBC]

Supplement to AS1684 table 49. Specifies double bearers.
https://www.scribd.com/doc/309660524/AS-1684-2-2010-Supplements-Non-Cyclonic#

I didn't get a chance to get back to this. It look as though rather than being mandatory, they are specified for seasoned hardwoods. You can still use big section rhw, just probably less common these days so they have made allowances for laminating smaller seasoned sections as well. Happy to have someone with current experience give me a lesson here.

Timber QLD current doc attached still showing single and double bearer span tables.

http://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=9&ved=2ahUKEwjhlriOlYzdAhWFQd4KHTVUCA8QFjAIegQIAhAC&url=http%3A%2F%2Fwww.hyne.com.au%2Fdocuments%2FFeature%2520Documents%2FTQ_07_Timber-Deck_final.pdf&usg=AOvVaw2KzoVp2RnFNR4feLJj-l-R

[tryagain]

I didn't get a chance to get back to this. It look as though rather than being mandatory, they are specified for seasoned hardwoods. You can still use big section rhw, just probably less common these days so they have made allowances for laminating smaller seasoned sections as well. Happy to have someone with current experience give me a lesson here.

Timber QLD current doc attached still showing single and double bearer span tables.

http://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=9&ved=2ahUKEwjhlriOlYzdAhWFQd4KHTVUCA8QFjAIegQIAhAC&url=http%3A%2F%2Fwww.hyne.com.au%2Fdocuments%2FFeature%2520Documents%2FTQ_07_Timber-Deck_final.pdf&usg=AOvVaw2KzoVp2RnFNR4feLJj-l-R

What I suspect (haven't actually looked into it) is that the double bearers thing is a way of keeping everything simple/uniform. Basically at the ends, as there is only half of the tributaory area for the bearer, only half the strength is needed. So a way of keeping everything economical and simple, maintaining the same bearer size and spacing throughout is to just specify double bearers for all of the middle bearers.

[prodigyrf]

Similar discussion here- https://www.renovateforum.com/f198/reading-span-tables-lvl-vs-hardwood-110912/
We've cut down the decent timber years ago so the stuff that's left is pretty ordinary now so it's plantation pine to keep up but better still for serious load bearing is engineered timber that is straight and true with no weak knots or shakes in it. It's guaranteed along every part of its length which you can't say about any natural cut timber but old habits die hard.

[Fullberries]

If you are going to the effort to replace it make sure you put down some joist and bearer protection.

Look up g tape beat tape out there. Like a super masking tape that sticks to the joist so the water cant penetrate

[Fullberries]

https://youtu.be/JcXHgz6OFB0

[#jonesy]

I've used
this

[Rumpig]

I aways wonder how joist protector keeps water out, surely it has to track through the screws or nails to a degree. We used to have to put damp course under bottom plates of houses we built years ago, then they realised it traps the water, so no longer done. I thought you only needed joist protector on lvl's, personally I wouldn't use it on hardwood myself if you go that route.

[#jonesy]

I think it stops it sitting on top of the joist. In my last 2 houses I've had to replace the rocks. The top of the joist between the decking boards was rotten. Under the decking boards were okay. The bearers were pretty much rotten on the entire top. Both were hardwood, Vic Ash or similar which isn't any good for outside. We don't have good hardwood in Victoria.

[Rumpig]

I think it stops it sitting on top of the joist. In my last 2 houses I've had to replace the rocks. The top of the joist between the decking boards was rotten. Under the decking boards were okay. The bearers were pretty much rotten on the entire top. Both were hardwood, Vic Ash or similar which isn't any good for outside. We don't have good hardwood in Victoria.

yes it does supposedly protect them, but I do wonder how well really.....that is also why ripple deck was invented, allows airflow between the decking and joists to help dry them out and stop the rotting. As you say, Vic Ash is not suitable for doing such a job, we used it all the time for lintels inside house frames, but never outdoors.

[Fullberries]

The tape actually  adhears around the screw or nail. If you watch the visld you will see the tape actually sucks down into the timber so you can see the ripples it is great stuff and works like it is ment to. Look up doctor decks om instagram or yourtube. He is a massive deck builder and he swears by it