Every roof above a house has an underlayment material that sits between the shingles and the roof deck. Its main function is to protect what is underneath it from severe weathers and especially moisture. This roofing underlayment material comes in two major varieties: felt and synthetic.
Felt is the most common type of underlayment used and compared to other types, this material is thicker and can keep out moisture and water. The most basic idea of roofing felt is a felt paper bathed in asphalt and mixed with other water-resistant agents resulting in the final material. They come in 15 and 30 pounds. The material has been used for a long time and still being used due to its low cost.
Synthetic underlayments are starting to replace traditional roofing felts. It is simpler to work with due to the material being more lightweight and they are made in such a way so that it does not absorb moisture making it resistant to fungal growth. They either come as woven or spun from either polyethylene or polypropylene.
For colder locations, ice and water shield or I&W, is being used which is glued to the roof deck. After this shield is installed, a regular roof underlayment such as felt or synthetic can be placed on top. Laying the ice water barrier over the entire roof is not recommended as it can trap water under it leading to the formation of mold.
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