Hi!

First off, I am a complete dummy in plumbing but before I hire an expensive plumber I want to know if the job to be done is easy enough for me to do it myself.
I removed a piece of plywood underneath the toilet because it was damaged from constant leaking. Now if I put a few sheets of plywood to cover the gap and strengthen the floor, the floor level will rise and cover the waste pipe so I will have to extend the waste pipe. Can you look at the pic and tell me whether this pipe can be made longer and how it can be achieved. Which joint has to be dismantled, how to do it in order not to damage the bottom part of the pipe and how to connect the extension. (Solvent, glue?)
And more importantly, could I do it myself given that I have never done it before?
Any advice would be greatly appreciated.

This is a very simple job, but if you have never done it before you could really mess it up.

I am not a plumber, but I have done enough to know this is a 10 minute job, lots of working room, etc. Basically it involves cutting off the vertical pipe 2" (+/-) above the elbow, adding a coupling, and replacing the flange on a longer piece of pipe down to the coupling so that the under surface of the flange sits where you want it to be. You can't EVER take plastic plumbing apart at the joints. The glue is forever. If you have a friend who has done plumbing, buy a case of beer and invite him over to do it for you.

Second option would be to hire a plumber. They will do it for the minimum call-out fee. Maybe you will find one who will drop in on the way by and do it cheaper for cash. Use this photo to shop around for your best price.

If you really want to do it yourself, do you have the tools? Are you sure you could cut the existing pipe off SQUARE? If so, get back to us and someone will guide you through it.

It is easy. You can do it yourself:

Cut that 3" riser in the middle. Glue in ABS coupling. Lay your plywood . Don't nail it down yet. Test lay it only. Cut another piece of 3" pipe - longer piece that you need. Make sure you measure correctly. We usually glue closet ring to this pipe first. Then we insert it though the hole and measure exactly how much to cut off. Cut that excess length off. Apply glue to the coupling and the piece of pipe with closet ring you've prepared. Glue it in through the hole in the ply. Press all the way down so the closet ring touches ply. Screw closet ringh into the ply with brass screws. Nail ply.

Now, that 2"x 12" should be headed out and existing floor joist should be installed with joist hangers. You really should not cover it the way it is or it will flex and / or your toilet will never sit right.

Nail a 2x4 along the joist on the left. Put another on the far side of the one that has been cut for the plumbing spanning the cut out portion to reinforce that joist. Put one on the near side of the cut joist from the pipe to the edge of your flooring cutout, both sides of the pipe. Put another one on the joist on the right side of your cut out (the one that is out of the pic.)

Cut 2 pieces of ply wood. One to fit between the edge of the plywood flooring on the left and the pipe. The second to fit from the first piece to the joist on the right. Cut notch to fit around pipe, slip under flange and nail or screw down. No need to change height of pipe or screw up height of floor.