Can a 90 be put in a vent line

[hookerp]

I am running a vent line for a sink that has a 1 1/2 in drain line.
When I come off the top of the tee and go about 4 feet up can I 90 the 1 1/2 vent over to a 2 inch vent line that reduces to a 1 1/2 that ties into the three inch vent line.

This would eliminate getting on a 12 12 pitch roof.

[LazyPup]

if you are under the International Residential Code in your jurisdiction you may do that, If you are under the Uniform Plumbing code ALL vents must terminate through the roof.

[hookerp]

Can a vent line be 90ed downward to make a tie in to another line?

[LazyPup]

The Uniform Plumbing Code prohibits horizontal venting. The UPC does permit offsetting but the offset may be no flatter than 45 deg from vertical.

The International Residential Code permits horizontal venting but the horizontal offset must be pitched upward in the direction of the vent.