Bathroom Drainage

[Double_DDs]

I helped a freind build his small house last summer. The bathtub and toilet is not draining. We have proper 4" drain with proper slope. The tub will drain for a minute, then back up. The toilet doesn't want to go down either.
I think that it might be a venting issue.... What can I check before I start opening up the walls???

 

[Fansplumbing]

It sounds like the main line is stopped up.