I have a rough in for a shower which is right beside the main stack in our basement. The question I have is do I need an s trap? Or can I install a shower directly into it?
You have to pour water down that pipe and with a flashlight confirm that a part of the water stays there, if the water goes straight thru that means you don't have a trap below the floor, you MUST have a trap below the floor, you CANNOT hook up a shower without one!
You need a trap between your new shower and the DWV stack. S-traps are to be avoided, and in many (all?) cases are illegal except in the case of grandfathered replacements. A coorect application would use a p-trap.
The trap should be correctly vented also, to "protect" the trap