Advice on plumbing leak

[Donald S.]

I'm finishing up the plumbing for a new kitchen double sink. The two tail pipes converge at a T-pipe, which connects to the P-trap, then to a 90° elbow that connects to a short extension and into the wall. 

I cannot for the life of me get the p trap nut to stop leaking. Where the p trap connects to the 90° elbow. The best I can get is a very slight leak, but I know that's wrong. What can I do? I've tried using 4 wraps of plumbing tape, and even laid a washer in the pipe (the elbow doesn't have a washer and the piece is designed that way). What trick am I missing? 

Thanks

Cheers!

[JD Martin]

^^I sometimes use Sch 40 PVC glue joints if it's a connection that lends itself to getting beat to death by tenant debris stuffed under the sink. Worst case scenario is you're cutting a piece of pipe down the road to clear a clog. Beyond that, you shouldn't need any kind of tape or anything else for threaded plastic drain connectors. 

P traps usually leak because:

1. There's a burr on the plastic form-fit piece that you don't notice, and the pieces are not sealed together.

2. The form-fit is cocked at an angle. Easy way around this is get that piece aligned and slightly snug first before snugging up the tailpiece.

3. The cheap plastic pipe is deformed.

You have to have a lot of patience with the cheap plastic that is used today for p-traps, tailpieces and the like. As I said, if I have any doubt as to whether it will leak if the tenant looks at it wrong, I just use a Sch 40 solvent-weld trap. Just leave some exposed pipe so you have something to cut (and couple) if you ever need to get in. Alternatively, use threaded fittings at the drain and tailpiece connection, and then you can remove the entire piece as an assembly. Or use a threaded union on a flat spot uphill from the trap. 

The cheap plastic connections work best in dry areas, i.e. there won't be any standing water. That's why you're having trouble, because it's the nature of the best.