In order to help prevent leak
caused by ice dams, an ice and water protection membrane should be installed
along all eaves, on all dormers, and in all valleys. Click on the following
picture for a full-sized view.
The next drawing shows
how far back the ice and water protection membrane should extend. It is
recommended by most material manufacturers that the membrane extend a minimum of
24 inches (30.5 cm) past the interior wall line on buildings with slopes equal
to or exceeding 4" in 12". For slopes less than 4" in 12", the underlayment
should extend 36 inches (91.4 cm) past the interior wall line. Once again,
you'll need to click on the picture to get a full sized view.