"tom" wrote in message

You're right about keeping the roof as close to the outside temp as
possible. But if you insulate right up against the roof decking, you've
lost the ventilated air space that really does the job of keeping down
the ice dams.


You are correct about a normal heated house, but does this still apply to
a
detached garage or shed?

My house is detatched :-)

My garage has no venting at all. The roof is not
insulated, but the walls are. I run the heat out there maybe four hours
on
a weekend day. I may add some insulation on the roof also in the future.
but since it is a rather minimal use, I don't see any particular advantage
to venting unless I separate the area at the ceiling joists. The other
160
hours a week it is not heated at all.

Really, it's about heated vs. not heated. You heat just a little, so you
probably run a minimal risk of incurring roof damage.
It also has a lot to do with climate. The OP is in Ontario. Sub-zero days
(F) are to be expected. I live in that same climate band, you can't just
snap on the heater and run out into the shop in 10 minutes. You can warm up
the air, but touching 10-degree cast iron is no fun. I'll bet that the OP is
going to be doing more than 4 hours a week of heating.

-Steve