So what is ice and water sheild? Simply put, it is a flexible and sticky membrane that is placed on the roof sheathing prior to installing the shingles. This membrane seals up leak-tight around roofing nails that puncture it. It is used mainly along the roof edges and in the valleys, and along walls or chimneys to protect against ice dams and water seeping underneath flashing. Typically one or two courses of it is installed along the roof eaves to achieve 3 to 6 feet of coverage. In the roof valleys it is usually positioned so that there is about 18 inches of coverage on either side of the valley. In short, EVERY sloped roof needs underlayments.