I've got a tub/shower with a single turn valve for the water supply. When I turn the handle to fill up the tub I immediately get water coming out of the shower head. Any ideas?

The faucet may have been installed upside down or the diverter that is sometimes simply a copper piece may have been taken out or the shower riser may be too short or the flow restricter may have been removed from the shower head or you may simply have a defective tub valve.

One other cause could be excessive house pressure. If you have a Pressure Regulating Valve, (PRV) check the setting. Regards, Tom

the faucet may have been installed upside down or the diverter that is sometimes simply a copper piece may have been taken out or the shower riser may be too short or the flow restricter may have been removed from the shower head or you may simply have a defective tub valve.

Looks like it's an upside down valve. This is going to be fun :(

You can buy a valve for the shower arm that is about an inch long