Treated or non treated timber for framework of a shed?

[Deansplit]

I'm building a staircase to a raised roof garden and utilising the space underneath as a shed for kids stuff and garden bits etc. So would one normally use treated timber for the frame the framework and cross members of the shed or is non treated OK. Obviously the T&G outside timber will be treated but if the frameworks isn't getting wet is it necessary for the additional expense (plus I have hoards of 1.5x4" non treated lying around).

Thanks for any advice.

Dean

 

[Tigercubrider]

pressure treated is better but my shed has some shelves in it that I just put in and used preservative on.

Maybe buy a tin of treatment and pre-paint the timber before use?