poly under shingles - good or bad idea?

As I contemplate the need for a new roof (asphalt shingles), I'm wondering why it doesn't seem to be common to staple large, overlapping sheets of, say, 6 mil polyethylene to the plywood before putting the shingles on, as a second line of defense against water penetration. Or maybe it is and I don't realize it.
Comments?
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Grant wrote:

Shingles are nailed on. Nails make holes. Holes leak.
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Common in the roofing industry is felt paper. It is paper impregnated with asphalt. Nails go through it and paper seals around the holes.
I suggest you read up on common practices in your area before doing any work yourself. Each area of the world has a bit different weather and there are gotcha's that can be avoided with a little education
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>As I contemplate the need for a new roof (asphalt shingles), I'm

Let the roof deck breathe a little. The felt's there usually as a barrier for any ice dams that might happen, or as a temporary roof, because the roofers couldn't get to the job quickly enough. If you live in Wisc., you'd be wise to felt at least 3-4 feet beyond the inside wall of the house at the eaves. It's not necessary on the rakes or ridges. You local building codes could be different, if they're in cahoots with the asphalt industry.Tom Someday, it'll all be over....
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Poly used to be used in this area to help prevent ice dams from leaking through the roof into the drywall. This has all been replaced by Ice & Water Shields. Shingle manufacturers do recommend that the Ice & Water Shield be used on the entire roof in low slope situations where water could be driven up under the shingles. Otherwise the I&WS is used around the eaves of a sloped roof only.

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