Combining Vent Pipes

I have four 1-1/2" vent pipes in my attic that I would like to combine into a single exit through the roof. What size should that pipe be, or how should I do it?

Do the pipe sizes need to increase as they are combined (as with drain pipes)?

Thanks

You can combine them into one 2in. vent. Do you have a larger vent in that area? if so you can run them into that if it's 3in. or larger.

you can join them all together but the size after joining depends on what is attached to the individual vents, depending on which code you are under. According to the UPC you can join them all together under a 2" vent as long as the overall fixture units do not exceed 24.