Hi I want to add a small sink next to my toilet, it is a cement slab floor, can I drain the sink into the vent stack of the toilet. A trap will be used for the sink. I really want to stay away from busting up the floor.

You may discharge into the toilet vent providing there is nothing tied in over it and you keep within code for distance. With a 1 1/2" drain pipe it's 3 1/2' from trap to vent. With a 2" drain pipe it's 5'. Cut a drainage tee into the stack at 17 3/4" from floor to the center of the tees branch. Make your lavatory stubout 18" to center from the floor. Use DWV drainage fittings throughout. Good luck, Tom

Thanks for the info... that would be less than 3.5' for the 1.5" pipe, right?

That would be less than 3.5' for the 1.5" pipe, right?

Right!

Great! One more question... is there a "formula" to determine the degree of slope in the drain?

I think it is about 3%. If you notice, a DVW tee isn't a full 90. The branch is meant to slope up slightly with the run verticle. That is the angle you want.

For your job the norm is 1/4" fall to the foot. A 5' run would give you 1 1/4" of fall. Tom