Bathtub Drain - How Many Elbows

I am doing a basement reno and need to tuck a p-trap up into the joists. The p-trap comes from a main floor bathroom . I can either drill a hole in the joist just above the hole that is already there from the existing p-trap or route the drain alway from the hole that is there and drill the new hole about 6 inches away from the current hole.

The joist is seven inches deep and I am running 1 1/2 inch plastic drain which means a 2 inch hole. The joists are true 2 inch fir joists - it is an older home.

I am planning on sistering the joist about 3 feet on either side of the hole - which is the best I can do given what is there.

I would prefer to run the drain away from the hole by turning the p-trap parallel to the joist and then doing a 90 degree into the joist which means I would have to follow with another 90 degree to get to the stack. This allows me to position the second hole away from the first. However, I think it may have some plumbing ramifications given I am doing two 90 degree turns - I would use long sweeps.

Otherwise I will have to cut a hole just above the existing hole and likely weaken the joist and am not sure of the structural implications of the combination of sister and joist with two 2 inch holes next to each other in the original joist. Note there would only be one hole in the sister.

I have also thought about routing the p-trap on a 45 but that would mean I have to cut a hole on a 45 and I think that takes more skill than I have - especially through a joist and a sister.

Does it matter that I am doing two 90 degree turns? I have included a diagram of the situation showing two views.

Roman

1) Are you sure you have to use 2 90s? I am pretty sure you could just do one 90 and then come through the joist perpendicular, and then turn the p-trap into an "L' shape...it could probably work if you measure where to cut the hole properly.

2) If you can't just get away with one 90 and the p-trap as you have proposed running the trap arm...why not just cut out the existing tee/trap arm and put in a new tee, and point it towards the joist so that you cut a hole in the joist right where the stack is and then only have to use one 90 and then have a straight run to the ptrap...all in the same joist space.

3) Why can't you reuse the existing hole? I certainly would not cut a new hole right next to the existing hole...and even your proposed hole seems too close to the existing hole. If you can't reuse the existing hole because the ptrap would extend down below the cieling...can't you just enlarge the existing hole up a bit with a saw? You need to be really carefull since you only have a 2x8 joist...you can't just go cutting in 2" holes anywhere you feel like!

The top and bottom of the joist are what provide most of the strength...you don't want to cut holes anywhere but the middle third of the height. There are other rules about where along the length and max diameter allowed.

Two nineties bad. Two forty-fives good. One ninety good.

Thanks to all of you for your suggestions

Wtih respect to one of the suggestions I can't really move the p trap closer tothe stack because the stack is cast iron, i have too much stuff around it to cut into it and the arm off the stack is on the other jost bay. It is a good idea I just can't do it.

I have another idea but will post it separately