When I remodeled my bathroom, I replaced the old cast Iron DWV with ABS. I left the end part intact (cast iron, which exits through the roof). When it rains, the ABS to Cast Iron coupling leaks water, mainly because the cast iron surface is not smooth like the ABS, so there are irregularities on the surface of the pipe, that creates tiny spaces. I tired two different couplings, and it still leaks. So I have decided to remove the end part and replace it with ABS. I've seen some people were saying that you cannot use ABS for venting outside, mainly because of the exposure to the sun. What's your opinion on the use of ABS in this situation? For those in the Seattle areas, does the code allow the use of ABS for venting outside? I know, PVC is definitely OK, but not sure if ABS is.

Thanks for your help,