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Hello @Redboy65
Welcome to the Bunnings Workshop community. It's sensational to have you join us and thanks for sharing your question about building an outdoor bench seat.
Would you happen to have a sketch of the seat you are building? If you can please share a picture of that sketch, it will give our members a better idea of its structure. We can then make recommendations on timber size. If it is a three-seater or larger, I would suggest using the 90mm x 45mm, but if it is just a two-seater it might be possible to use the 90mm x 35mm. You can even technically use the 70mmx45mm if the spotted gum can provide extra support to the frame.
If you need a hand posting an image, please let us know.
Eric
G'day Eric, thx for getting back in touch with me. Please find attached Plan and Elevation diagrams for the design. Basically the bench seat is in an L-shaped config at the end of our decking balcony. It's going to consist of two frames "Small Frame" and "Large Frame". The frames will be made up of treated pine and the cladding will be spotted gum decking board (19mm x 86mm x 1600mm).
Hope this helps and once again, thanks heaps for your help.
Cheers,
Redboy65
Hello @Redboy65
Thank you for providing those detailed plans of your bench seat. I was initially concerned that you would have a very long seat, but now that you've shown me the plans, I suggest using the 90 x 35mm Outdoor Framing H3 Treated Pine - 2.4m for framing. It's not too bulky like the 90mm x 45mm, but it is strong enough to hold the weight of people sitting on it. My best advice is to drill pilot holes on both the frame and your spotted gum to prevent splitting the timber.
Let me call on our experienced members @JoeAzza, @Dave-1 and @Nailbag for their opinion on timber frame sizing.
If you have any other questions we can help with, please let us know.
Eric
Afternoon @Redboy65
Love your plans 
Seriously nice and concise!
I prefer the 90 * 45mm pine 
Mostly because I figure you will have characters flopping down onto a bench seat and that little extra cannot hurt I also like @EricL's suggestion of pilot holes.
I built a bench seat with wheels and went with the larger sized supporting timber frame but looking similar (in my mind) to yours.
Outside timber lounge with wheels
If you "test" a smaller piece of timber in bunnings when you are purchasing what you want by having the span between two points and then slowly put your weight on it. if it bows go for a larger size if it dosnt then why not, problem solved
The last one id suggest is to oil your new seat as soon as possible and then set an aniversary/birthday on your ph (with a reminder the week before) to reoil it every year. We tend to forget that we should do it.
Dave
