And modify your gutter so it drains into the top of the drum through the screen. You'll have to clean off the screen and top of the drum every once in a while if your gutters get dirty. Make sure the valve is closed and you're all done. Now you just need some rain!
Make up and glue a 1" overflow pipe into the fitting you installed earlier. You can direct it wherever you want.
Here's how I designed a system to gravity feed from the rain barrels down to my garden using gravity and some soaker hoses:
HOW I MADE A RAIN BARREL FROM A PLASTIC 55 GALLON DRUM
You'd be surprised at how much water you can collect off your roof after a good rain. When you have a garden, you know how valuable water can be when you go through a dry spell. Why waste all that water when you can save it for later? Sure, you can run your well pump or pay for city water, but why not make use of all that free water from mother nature?
If you have a few hand tools and some motivation, you can make a very functional and useful rain barrel out of a standard 55 gallon plastic drum. Sometimes you can find them on Craigslist or for sale in your local paper. Companies go through drums like these all the time. If you're able to get a couple, here's how to convert it into a rain collection barrel. You'll need the two bungs (plugs) that came with the drum. If you don't have them, fear not. They're easy to get on EBay or maybe a hardware store. One of the bungs usually comes with a 3/4" threaded knock-out hole (see a close-up below). If you're buying them, make sure one has a threaded hole in the middle. Here is how I made my rain barrel. Comments or suggestions are welcome!
For the opening that the gutter drains into, a 4" x 3" PVC reducer works great. The smaller end will go in the top of the drum.
Turn the drum over and set the reducer where the gutter will be. This is best done with the drum sitting on site so you can visualize how it will be set up and the easiest way to re-route the gutter downspout. Using a marker, trace the outline of the smaller end onto the drum.
Cut out the opening with some sort of saw. A jigsaw works great, just drill a hole first to get the blade started.
Here's what you should have so far. Your reducer should fit snugly into the hole. Now is a good time to hose out and clean the drum.
When the drum fills up, you'll need somewhere for the excess rainwater to exit. I guess you can let it run out the top and over the drum, but I prefer to direct the excess where I want it. Figure out what side of the drum you want it on. It helps to have the hole somewhere near the opening so you can reach your arm in and attach the fittings.
About 2" from the top drill a 1 1/4" hole for the overflow pipe.
You can use a 1" PVC to 1" thread adapter and a coupling to attach the overflow pipe.
The threads should screw right into the drum and then you can back it up on the inside with the coupling. The PVC threads won't go all the way, but it's enough to keep it from coming out. I think you can use an electrical conduit nut as it'll screw all the way on. I don't know, I haven't tried that.
The bung plugs will be on the bottom when you're done, and one of them will have the valve assembly screwed into it. For now, turn the drum right side up so you can get to them.
Find the bung that has the threads in it. Drill out the bottom of the plug so the hole goes right through. Be careful not to damage the threads.
Put Teflon tape around the threads of a 3/4" PVC to 3/4" NPT adapter.
Screw it into the bung and tighten it snugly. Don't over tighten - this is plastic, not steel and you can easily damage the threads. You don't need an 18" pipe wrench - a pair of channel locks works just fine.
Now put together the drain valve assembly. It's made up of two 3/4" PVC pipe sections about 3" long, a 3/4" elbow, and a 3/4" PVC pipe to 3/4" NPT coupling. The valve is up to you. You can get a PVC one, but I chose to get a bronze valve with a hose connection already on the end. Whatever you use, a hose adapter will be needed for your garden or soaker hose.
Tip: Don't get a valve that is hard to operate and takes a lot of effort to turn - the whole assembly is threaded into plastic and any excess force is going to loosen it. See below for a way to get around this.
Screw the bung back into the drum (don't forget the gasket) and then glue the PVC valve assembly in just like this. Remember, this all gets flipped upside down, so make sure your valve handle is on the right side.
The drum is almost done. Go back and get your large PVC reducer that you used to cut the hole earlier. Gather some vinyl screen material and a big hose clamp to go over the large opening. This is to keep any trash, bugs, leaves, or other gutter debris from getting into the drum and clogging up your valve and hoses.
Wrap the screen over the top and secure with the hose clamp like this.
Trim off any excess screen to make it nice and neat.
Level some cinder blocs to support and raise the barrel off the ground. Bring the blocks close up against the valve assembly to help hold it in place. You don't want it moving if you inadvertently pull on the hose.
Put the large PVC reducer with screen into it's opening...