Hi ,I want to move a 3" abs ventpipe in a floor space. The floor joists are 2X10 and near the supporting wall .Can I drill the joist? Do I need to use plywood or 2X10 to sister the floor joist to make up for the weakening .thanks bman55golf

The closer it is to a supporting wall the better. Is it only 1 joist you have to drill through? I am going to assume the joist's are 16" on center. If so you shouldnt have a problem. I would sister it and span 36" each way from the spot you wish to drill. Make sure and use some construction adhesive and nail it from both sides driving your nails in at a slight angle. Make sure not to nail where you are going to drill. A typical 2x10 is about 9 1/4 " so measure up 4 5/8 " to the center of your new hole. Drilling in the dead center will help you retain as much strength as possible. You should be good to go.
Mike

Tanks for your answer "the1luv". I am only going through 1 floor joist.I will know more once I remove the existing toilet and see what's there. Thanks again.

Holes bored in floor joist may not be closer than 2" of the edge. Holes may not be greater in diameter than 1/3 of the width of the joist. The hole you are going to have to have for a 3" pipe will technically exceed the 1/3 limit but is quite common. No reinforcing is required but you may if you wish.

The best strength will be a hole in the center of the joist.

Hi Guys:... borrowed this from law123.. good visual.

Notching a Joist | Framing | How-to | This Old House (http://www.thisoldhouse.com/toh/asktoh/question/0,,1582745,00.html)

Hope all is well. Talk soon.