Most of the diuretic effect of dandelions is in the green parts, but it does have a mild effect from the roots. Saying that, I'm fairly sure that coffee is considered a moderate diuretic anyway, so you won't notice the difference when you drink this instead
To make, I dig the roots up, scrub em clean with a rough brush/wire scourer, but don't bother peeling them.
Put the oven on a low heat (100 deg C or below) and put them on a baking tray - bake for at least one hour - they should go hard, dry and look like twigs - it doesn't matter if they go a bit brown, as the roasting adds some flavour - too dry though and they'll taste a bit bitter.
Grind up in a coffee grinder, and use as you would coffee in a cafitiere (i.e around 1-2 dessertspoon per person, leave to steep for slightly longer than coffee.
It has a mild coffee-like taste, slightly nutty, slightly bitter like coffee.
My favourite thing to use it for is instead of coffee in mocha - mix it with hot chocolate and the nuttyness adds a whole new dimension!