Drain problem in jetted tub/shower.

I have done a complete remodel and put in a jetted tub and shower. The drain works fine with no leaks when shower is on, but when tub is filled up and then drained it leaks in the drain pipe in the crawlspace. The drain is 1 1/2 pvc. Any suggestions?

When you drain a tub full of water the drain may be under minor pressure and has more water in it than when you are just taking a shower...find where the leak is coming from and fix it...