About two inches should be good. When the barrel fills up, you need an overflow valve to let water run out in a controlled way instead of bubbling out the top.
 As with the spigot, use tape and washers to lock and seal it in place.
 This will keep your yard from flooding when water starts spewing out of your barrel.


You can also run the hose into a second rain barrel. This way when the first fills up, the water just flows into the next one. But eventually you will need to have a barrel that drains properly.
 A downspout diverter attaches to the downspout and the rain barrel. Once the rain barrel is full, water continues down the downspout, eliminating any chance of overflow,
Drill a hole in the side toward the top of the barrel.
Screw in and secure your valve.
Attach a hose to the valve and direct the hose to run into a ditch or other proper drainage near your house.
Alternatively you can simply use a diverter.
Finished.
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