The layers beneath shingles, Such as Ice and Water shield, provide against invasive rainwater and ice dams that can cause leaks or even lead to rotting plywood. Ice and Water shield is applied along all bottom eaves of each section of roof, In all valleys, Around chimneys, And around each skylight.
The mandatory application of Ice and Water shield along bottoms of roof is 3ft. However, we recommend at least 6ft on all lower pitch sections of roof, or if ice dams build up higher than 3ft. If left up to us we generally always apply 6ft.
Its been proven that GRACE ice and water shield is the best, (and all that is allowed on any of our roofs).