: There are at least three ways to make a loop vent. Usually only one of them is approved in any given area. Your building inspection department normally will have a sheet showing how they want it done.

hj is right.
With so many "correct" methods, you will need to check with the "locals" to see what is correct in your town. Terry

: : Where can I find information on how to construct a loop vent? Do the DIY books cover this?
: : How do they work so that sewer gases don't come back through?