Harizontal vent, how far

[danO]

Hello
I am in the middle of a major remodeling project.
I want to join my vents together in my attic on a horizontal plane.
Toilet 2", shower/lavatory 1 1/2" and kitchen sink 1 1/2".
How far can the vents run horizontal?
I need to run 8' from shower/lav, join the toilet vent and continue another 16' before turning 90 degrees, going through a wall and than up.
Should I increase the size of the vent when the shower/lav joins the toilet vent to 3" ?
The kit sink would join the vent right before it exits the attic.
You probally have figured out I am trying not to put a hole in my new metal roof. There are no codes where I live but I would like to do it right.
Thanks

[hj]

If these are the only vents in the whole house, then your "final" pipe should be the same size as the one under the floor where it enters the house, and the size should start increasing as the separate vents are merged together. A "good" metal roof installer would have the engineered fittings to seal the vent through the roof.