About 5 years ago my wife and I had a new roofing system installed, which included Grace Ice & Water Shield 6 feet up from the roof's eaves. Our roof has recently begun leaking, due to the formation of ice dams causing water to back up around the eaves and somehow snaking it's way into the top of a bow window, where it then drips through cracks and onto the windowsill.

I contacted the contractor who installed the roofing system, suggesting to them I thought there was a hole in the Ice & Water shield, and he said that Grace Ice & Water Shield is not perfect and that in certain situations the membrane can be overwhelmed and the water can penetrate the membrane.

This sounds fishy to me. This fishy feeling was furthered by the fact that the contractor said he would help us out by installing - practically at cost - a continuous soffit vent under the eave of the leaky side of the roof.

Does anyone here have knowledge about Grace Ice & Water Shield? Can it really become overwhelmed?