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That's the problem with external heat tape and no insulation because in most cases with the heat loss when it's very cold there isn't enough heat to transfer into the pipe to melt the ice. That's why...

The simplest solution is to insulate the pipe where it passes through the unheated attic space. If this fails to solve the problem then heating cord or tape between the pipe and the insulation wrap...

I agree that the simplest solution is to insulate the pipe where it passes through the unheated attic space. If this fails to solve the problem then heating cord or tape between the pipe and the...

The reasons for the freeze up of the vent has many reasons and for most of these you can do very little. The 45° cut on the stack at the roof may help a little but I doubt it.

Different plumbing...

Nothing stops ice from expanding.
Regardless of the pipe material if it's full of water and freezes the water will expand and if the pipe is rigid like most are it will burst.

Here in Canada frozen vent stacks are very common. Lots of people climb onto their roofs and pour hot water into the stack. It's a little dangerous and a pain in the butt.

I found this info online...

Check out http//:Frozen Vent Stack Elimination for a permanent solution to frozen vent stacks