With the code recommended installation of the ice barrier over the drip edge water can back up at the eaves edge and get behind the vertical leg of the drip edge where it can end up flowing into the joint between the fascia and the roof sheathing.
Ice dam shiekld on roof.
But ice dams can form in the gutter as well as on the roof.
Ice and water protector must reach above the highest expected level of ice dams which will vary by region and the slope of your roof.
The water trickles down between the layer of snow and the shingles until it reaches the eave of the roof which stays cold because it extends beyond the side of the house.
This creates a sealed joint that prevent any water that got past the shingles from sitting on the edge of the wooden sheathing or dripping behind the wooden facia board.
If an ice dam occurs on your roof and water backs up under the shingles it will seep down into your home through the nail holes holding the shingles on and into your attic.
You should seriously consider using ice water shield if you live in ice dam country.
As an experienced licensed roofing contractor i know first hand what it should cost for various levels from basic better and of course the best.
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Ice and water shield is a thin rubberized asphalt membrane that goes beneath the shingles of your roof typically only along the overhangs and in the valleys.
Ice water shield is a thick sticky back underlayment also known as felt paper that will adhere to your roof deck and prevent water from seeping into your home if it gets under your shingles.
An ice dam forms when the roof over the attic gets warm enough to melt the underside of the layer of snow on the roof.
So in our area of the country we typically only put this up to the first three feet.
The minimum is typically 2 feet 24 inches or 0 60 m above the vertical projection of the inside of the exterior wall.
The ice and water shield must stick firmly over the top of the drip edge on the soffit edge of the roof.
In colder regions a good practice is to cover the first 3 feet 36 inches or 0 91 m of roof over heated space with ice and water protector.
Grace ice and water shield fully adhered smooth surface roofing underlayment provides best in class roof leak protection against wind driven rain and ice dams.