Please advise whether I need a liner underneath the shingles, how far up to bring the liner, and whether 25 year shingles are advisable. Where flashing meets side of new addition, there is a small leak into the addition, at the lowest point where both roofs meet. Should two layers of flashing be installed or is there a better alternative method. The roof of the original structure slopes off very slightly (perhaps 20-deg.), for about 20 feet - shingles are now curling (we estimate the roof is approx. 12 years old).
Answer:
The 20-year asphalt shingle is still the most cost effective product on the market and ice and water shield should go up high enough on the roof to be at least 9 inches higher than the exterior wall. From there on up to a baffled ridge vent that goes from gable to gable, you should have tar paper under it. When the old shingles are stripped off, back flashing needs to be installed and the surface flashings over that. This will prevent leaks of all kinds. Venting the roof cavities will normally prevent shingle curling.
