Toilets, Sinks, Showers, Tubs and Disposals - Replacing bathtub drain

I had an inspection and it was discovered the bathtub drain is leaking into the crawlspace.

How do I replace the tub drain? Is there a special tool I can buy?

Lets determine where the leak is coming from before we go replacing things. Quite often the sealing nuts just need tightening to stop the leaking. If not, then it is the seal between the drain itself and the body of the tub. If this is the case, there is a tool that looks like a tuning fork on one end and a slotted thingy on the other end. This is a universal type tool that unscrews the drain from the pipe below. Make sure the elbow under the tub is secured, or you will have to use two people to get it all back together when it falls down. Unscrew the drain from the pipe, clean the area really good, apply a generous bead of plumber's putty and reinstall the drain, screwing it securely, cleaning up the squeezed out putty.
Post back if we can help further.

Would a standard pipe wrench work to tighten from under the house? The same inspection revealed my heating duct had come loose, might as well do both while I'm under there.

Thanks.

Yes, as would channel lock pliers. I agree, make one trip do it.