According to Richard J Kinch :
Chris Lewis writes:

On the contrary. Ice dams have _nothing_ to do with snow thickness,
and indeed, most ice dams occur with less snow rather than more.

If the eaves/attic is cold like it's supposed to be, you don't get ice
dams.

Nonsense.

The interior of the structure is above freezing, and assuming the
outside is below freezing, then somewhere in or around the structure, is
a boundary line between the two regions. Call this the freeze-thaw
boundary.

Whenever the freeze-thaw boundary moves out to the snow on the roof, you
get melting on the roof. Anywhere the melt runoff crosses the boundary
again, such as at an eave, refreezing will occur, and a dam will form.

Right. But, that's only if the freeze-thaw boundary _ever_ gets on the
roof.

On a properly ventilated roof, it never does (except during overall thaw) -
the attic is just as cold as the outside.

When it gets warm outside, if the roof has a uniform coverage and has
relatively uniform temperature across the whole thing (which is what
proper venting _does_), the eaves warm up slightly before the rest,
and the snow cover erodes from the perimeter in. More to the point,
proper venting ensures that the eaves are never colder than the outside
or the attic proper. Which means you don't get ice dams.

Certainly, there can be very unusual weather conditions that might trigger
some ice dam formation (rapid temperature excursions). But, with our house,
for example, we get uniform coverage up to 3 feet of snow by winter's end,
without a hint of ice damming or icicles throughout the whole time, including
throughout spring thaw.

Properly constructed/ventilated, the only time people around here consider
snow removal is if the overall weight gets too high (our codes specify 70 PSF
snow load).

If you think ice dam control is simply a matter of proper construction,
then consider the Space Shuttle. $Billions and lives lost to a similar
type of ice formation, despite the concerted effort of the best minds to
avoid it.

That's a rather different situation - we're not dealing with liquified
gases venting at absurdly low temperatures into very humid climate.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.