• Felt paper is the layer that goes below the roofing shingles.house image by jeancliclac from Fotolia.com
  
  When replacing the roof on a house or installing a new roof, you need to install more than one layer to protect the roof from the elements--most importantly, from the unwelcome penetration of water. One of the underlayments that has been used for more than a century is felt paper, a kind of tar paper designed to be water resistant as well as durable. When using felt paper, consider the cons of laying it over an entire roof, for it can affect the roof's integrity and longevity.
  
  

Drip Edge

• The drip edge must be installed before laying the felt paper. If you lay the felt paper over an entire roof without taking into account the possibility of ice damming, you could be inviting not only damage to the roof but the possibility of water infiltration as well. By installing the drip edge first, you create a direction for any water to travel from the roof--not only down, but away from the roof's edges. This helps to direct melting ice water from the roof and to the ground or gutter, rather then allowing it to remain on the roof where it could freeze and melt again, creating a pattern that could last for the length of the winter.
  
  

Ice and Water Shield

• The ice and water shield goes on top of the bare roof with one edge slightly overlapping the drip edge then extending up the roof toward the peak. This is another item that needs to be installed on the roof before the felt paper. If you lay the felt paper on the entire roof, you might not have a proper water-and-ice-resistant shield on the roof, thus running the risk of water infiltration issues. Some towns and cities require the installation of an ice and water shield beneath the final installation of the finish roofing product, such as asphalt shingles, wood shingles or even metal.
  
  

Patching of Holes

• If you lay down felt paper over the entire roof without first patching any holes, you can find yourself with damage caused by water penetrating these openings in the future. Adding a patch of tacked-down aluminum can save the cost of a major roof repair down the road.
  
  

Ridge Vent

• Part of keeping a house warm or cool is by having a well-ventilated attic. When your attic is well ventilated, water cannot collect in the form of condensation, which could create rot and decay of the room and framing members over time. By installing a roof vent before you lay down the felt paper, you help prolong the life of your house and roof.