On Sat, 31 Jan 2009 09:06:16 -0500, Van Chocstraw
wrote:

... Prevent it by raking off the snow 3 feet above the over hang,

I had some trouble with that: a new ice dam developed where the snow
cover was removed. The water ran down under the snow, then froze where
I'd raked it clear.

When I re-did the roof, I put membrane 3 courses up from the edge.

Re the general matter of venting: there was an article in 'Fine
Homebuilding' some years ago about this. They reviewed quite a lot of
research. One thing that really struck me was an (apparently thorough)
Canadian study, which concluded (paraphrasing), "if you're getting ice
dams and your vents are open, plug them. If you're getting ice dams and
your vents are closed, open them."

YMMV.

G