Making a bathroom in an old bedroom. Want to tie lavatory vent into an existing vent that exits the through attic but need to make a horizontal run to get there. How far above the sink must I be before I gor horizontal for several feet?

You can go horizontal starting 6" above the fixtures flood rim. You will give the vent slope back to the lavatory and connect to the vent using a inverted Sanitary Tee.
Good luck, Tom

Hi Dnm3skip...

The vent must be, at a minimum, six inches above the flood level rim of the highest fixture on the floor. 42-48" off the finish floor is most common. Be sure to pitch your vents back toward the drain pipe when going horizontal!

Good luck...

Mark.

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