I noticed a small leak in my basement whenever my second floor toilet is flushed. The toilet is positioned near the outside wall of my house. A plumber indicated that the cause of the problem is most likely one or more holes in the PVC pipe which were caused by nail holes when our contractor nailed in siding on our house (he mentioned that PVC piping is often misdiagnosed as a stud by contractors). He believes the nails have rusted over time to the point where they fell off the PVC pipe causing the holes. He said the only solution was to rip the siding off the outside of the house and replace the pipe... to the tune of approximately $3,000-4,000. Might anyone know of another economical solution, such as inserting a slightly smaller flexible PVC pipe inside the existing 3" pipe? Any thoughts would be greatly appreciated. Many thanks.

I would not do anything without some better diagnose than that. One thing I would do if it is possible is to not use that commode for several days and see if the leak dries up. If it does then I would not use that commode but use any other fixtures in that bathroom that might drain in to that line. We are trying to see if what might be causing the leak to leak. It might be a bad glue joint somewhere on the pipe.
One question does this leak show up on the pipe under the commode or on a pipe that is coming out of the down the wall.
Give us more information and maybe we might be able to help. $4,000 is a lot to spend on just an idea.
One other thought is that there are cameral that con go into the pipe and see if there are nail holes in the pipe, this might cost you $100 or so but at least it might show the problem on the inside.

Thank you, Letmetellu, for your fast reply. As you suggested, I did isolate the problem by trying other fixtures in the bathroom as well as adjacent bathroom. There is no question that the problem is associated with the toilet in question... and not the other fixtures. Regarding the source of the leak, it is coming from the pipe which extends from the toilet to the basement. Specifically, the leak seems to be at a point approximately 8-10 feet below the toilet and then the water drips into the basement. Also, in addition to the leak appearing in the basement, the leak will also appear on the outside of the house. The PVC pipe is located at a section of the house where, on the outside of the house, the nailed shingles are about 8-10 inches long. Thus, the nail theory seems plausible. The camera idea is a good one... I might look into that. Lastly, I was trying to research if there is a liner or some other flexible plastic that I can insert into the existing PVC pipe that will direct the water below where the leak may be while at the same time not impacting the flow from the other fixtures upstairs... similar to the plastic diverters one places on the end of gutter leaders to divert the water away from the house.

Thank you, Letmetellu, for your fast reply. As you suggested, I did isolate the problem by trying other fixtures in the bathroom as well as adjacent bathroom. There is no question that the problem is associated with the toilet in question...and not the other fixtures. Regarding the source of the leak, it is coming from the pipe which extends from the toilet to the basement. Specifically, the leak seems to be at a point approximately 8-10 feet below the toilet and then the water drips into the basement. Also, in addition to the leak appearing in the basement, the leak will also appear on the outside of the house. The PVC pipe is located at a section of the house where, on the outside of the house, the nailed shingles are about 8-10 inches long. Thus, the nail theory seems plausible. The camera idea is a good one...I might look into that. Lastly, I was trying to research if there is a liner or some other flexible plastic that I can insert into the existing PVC pipe that will direct the water below where the leak may be while at the same time not impacting the flow from the other fixtures upstairs...similar to the plastic diverters one places on the end of gutter leaders to divert the water away from the house.

Yes there is a plastic liner that you can use to re-line the pipe that needs to be. It is a soft plastic that is sliped into the existing pipe and then filled with air expanding it to form against the pipe. After a time it sets up and then the plugs are removed and you virtually have a new pipe. This is a simplification of what really happens but it does work. Many sewer lines are re-lined in this way.