Probably not a great idea to actually suck it in this case, as all sorts of "stuff" will be washed off your garage roof into the butt (mainly from pigeons, which aren't called 'aerial rats' for nothing).
Materials:
- a decent length of clear 3/4" or 1" piping, say about 10ft, from a farm supplier,
- a funnel,
- an old wire tea-strainer or small kitchen sieve,
- some cable ties, and
- four basic coathooks (I assume it's close to a wall, otherwise you might nead a board as well to fix to):
It helps if the water butt is up on breeze blocks, otherwise you can't syphon out the bottom half.
Method:
Arrange the pipe so it reaches the bottom (minus about 3" to allow for the crud that always accumulates down there), fix one coathook close to the butt, very slightly above it, and tie off the pipe so that the end in the butt can't escape easily. Fix another one upside down near the bottom of the butt and a bit further away from it, and tie off the hose there next.
That's yer basic syphon, the next bit is the 'tap':
Take the pipe on, up again to a normal-way-up hook, close-to but slightly above the level of top of the tank, AND ABOVE THE FIRST HOOK slightly. Allow enough pipe length for it to go back down to the height at which you want to fill watering cans. Don't tie the pipe off on the hook this time as you need to unhook it in operation.
Finally fix a last coathook, upside-down (probably) and about four inches above the top of any watering can you want to fill.
Make a 1" loop with a cable tie. Tie this tightly onto the pipe, about 9" from the open end, as a hanging loop, and to keep it in place when you're filling cans (fourth coathook!).
To set it up you need the funnel: hook up the loose end of the hose to the highest hook. Pour tapwater in until it's as full as it will go. It should settle out with the U nearest the tank full and the level in the hose will be at the level of the water in the butt (It doesn't matter if the last loop ends up full or empty).
Operation is simple: put the end of the hose in the watering can. If it's been recently used, that will start things going. If it's dried up a bit, the last "n" may have emptied, so unhook that 'n' and lay it on the ground (which will restart the syphon without any extra effort).
To stop the flow, just lift up the end of the hose and hang it up, and replace the last loop on the same hook. It should stop when the end is level with the top of the water remaining in the water butt. The 'double-U' is so you can almost always restart the syphon without fuss.
No taps, almost no maintenance (occasional jug of water if the syphon does empty), and cheap.
Finessing it:
1. The tea strainer stops crud getting into the watering can and blocking the rose.
2. Depth gauge: cable-tie a split wine cork or a scrap of expanded polystyrene round a garden cane at the end as a float (should go through one of the holes) (you might need slightly more than one cork).
3. Anti-malarial: As ours is close to the house, I usually put a tablespoon of engine oil in our water butt at the start of the season as it floats, and suffocates mosquito larvae (I expect there's some cooking oil you could use instead).
I'm only slightly joking about Malaria (apparently it is heading north as the weather warms, and they do get occasional cases round Heathrow from mosquitoes that are carried in planes), but I hate having breeding sites near the house, as chasing them round the bedroom at 2AM is hopeless now I need glasses permanently.
