I have a 1910 house and am converting a pantry to a 1/2 bath (toilet and sink only). The walls are 12 feet tall and are sitting on 8" X 8" timbers as foundation. Some walls have diagonal studs along with the vertical and make it's impossible for me to direct a 2" pipe through them, up to the attic and through the roof. I -can't -directly vent straight up from the fixtures but I can do down into the crawl space and then if I there is a wall without the diagonal obstruction run it back up and through to the roof. Toilet and sink will connect to 3 inch drain to septic tank.
1. How much can I stretch the critical distance from toilet to vent. I have read that it's 6'

2. Since I can't vent in the wall directly behind the sink - are there any problems that anyone has had with running the pipe in the crawl space to connect to the vent from the toilet?

Thank you.. any help is appreciated.

Why not simply connect the sink to the toilet branch and then vent the sink with a AAV,(see image) Good luck Tom

Tom, Thanks for responding so fast. The picture you provided is above the floor? Can that configuration be below the floor? In the crawl space? I have gone to a site to see AAV's which one would you recommend for a half bath like the one I've described?

"Diagonal Studs"? Never heard of such a construction. Do you mean fire-blocks, upper double plate or bottom plate ? Every house has those. You just drill hole through them...

Milo, Yes, to the diagonal studs in an old old house... along with the uprights which are not always 16" apart.

So then you can drill through them. Buy a drill sized for the pipe you want to run through it and drill through the horizontal studs.

Tom, Thanks for responding so fast. The picture you provided is above the floor? Can that configuration be below the floor? In the crawl space? I have gone to a site to see AAV's which one would you recommend for a half bath like the one I've described?

The trap for the lavatory's located under the floor? Why?

I have a 1910 house and am converting a pantry to a 1/2 bath (toilet and sink only). The walls are 12 feet tall and are sitting on 8" X 8" timbers as foundation. Some walls have diagonal studs along with the vertical and make it's impossible for me to direct a 2" pipe through them, up to the attic and through the roof.

It sounds like your house is 'Balloon Framed'.

Are the diagonal studs mortised into the vertical studs?

If they are, you can chop out a chase for the vent if you cut off the diagonal piece in the stud bay and then 'sister' a new stud onto the side of the left and right studs within the bay.

Wall that are Balloon Framed tended to sway when stood up, so the cross bracing was put in to reduce the sway while the joist/floor system was installed.

If you're concerned that cutting into the diagonal braces might weaken the vertical strength of the wall, you can always sheet it with 1/2" OSB and sheer nail it to the studs before putting up your drywall.